1. What is Singleton Pattern?

In Java it is called a singleton Java class. In C# it is called a singleton C# class. Since it is a GOF pattern, it can be implemented in any OOP language (C++, Java, C#, Python, etc.).

Design patterns are defined by three parts

1. Problem: The recurring issue in software development
2. Solution: The best proven approach to solve it
3. Context: The situation where this solution applies

2. The Problem

In Java, object creation is a costly process in terms of memory and CPU utilization. Every new call allocates heap memory, runs the constructor, and initializes fields. If a class needs only one instance, creating more wastes resources.

Real-world analogy

Think of all employees in an office sharing a single printer. Every employee does not need their own printer. One printer serves everyone. If we design software for this office, the Printer class should be a singleton -- creating 100 Printer objects for 100 employees is wasteful when one object can serve all.

Three cases where singleton is needed

Case 1: Class with no state

A class that has no member variables, only methods. Creating multiple objects is pointless because every object behaves identically.

// No state -- all objects are identical
public class MathUtils {
    private MathUtils() {}

    private static MathUtils INSTANCE;

    public static MathUtils getInstance() {
        if (INSTANCE == null) INSTANCE = new MathUtils();
        return INSTANCE;
    }

    public int add(int a, int b) { return a + b; }
    public int multiply(int a, int b) { return a * b; }
}

// Can call methods for multiple times
// No need to create multiple objects
MathUtils.getInstance().add(10, 20);
MathUtils.getInstance().multiply(5, 6);

Case 2: Class with read-only state (final variables)

A class whose state is set once and never changes. Multiple objects all hold the same data.

// Read-only state -- all objects have same data
public class AppConfig {
    private static final String DB_URL = "jdbc:mysql://localhost:3306/bankdb";
    private static final int MAX_CONNECTIONS = 10;

    private static AppConfig INSTANCE;

    private AppConfig() {
        if (INSTANCE != null)
            throw new RuntimeException("Object already created");
    }

    public static AppConfig getInstance() {
        if (INSTANCE == null)
            INSTANCE = new AppConfig();
        return INSTANCE;
    }

    public String getDbUrl() { return DB_URL; }
    public int getMaxConnections() { return MAX_CONNECTIONS; }
}

Case 3: Class with sharable state (cache, buffer, pool)

A class that holds shared mutable state. Multiple objects would mean duplicate caches or connection pools, leading to inconsistency and wasted memory.

// Sharable state -- one connection pool serves all
public class ConnectionPool {
    private static ConnectionPool INSTANCE;
    private final List<Connection> pool = new ArrayList<>();
    private static final int POOL_SIZE = 10;

    private ConnectionPool() {
        for (int i = 0; i < POOL_SIZE; i++) {
            pool.add(createConnection());
        }
    }

    public static ConnectionPool getInstance() {
        if (INSTANCE == null)
            INSTANCE = new ConnectionPool();
        return INSTANCE;
    }

    public synchronized Connection getConnection() {
        if (pool.isEmpty()) throw new RuntimeException("Pool exhausted");
        return pool.remove(0);
    }

    public synchronized void releaseConnection(Connection conn) {
        pool.add(conn);
    }

    private Connection createConnection() {
        // create actual DB connection
        return DriverManager.getConnection(
            "jdbc:mysql://localhost:3306/bankdb", "root", "password");
    }
}

3. The Solution: Three Minimum Standards

Basic Singleton: Printer.java

package com.nt.sdp;

public class Printer {
    // Step 2: private static ref variable of class type
    private static Printer INSTANCE;

    // Step 1: private 0-param constructor
    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    // Step 3: static factory method having singleton logic
    public static Printer getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Printer();
        }
        return INSTANCE;
    }

    // business method
    public void printMessage(String msg) {
        System.out.println(msg);
    }
}

Client App: SingletonClassTest1.java

package com.nt.test;

import com.nt.sdp.Printer;

public class SingletonClassTest1 {
    public static void main(String[] args) {
        // Printer p1 = new Printer(); // INVALID - constructor is private

        // Get singleton object
        Printer p1 = Printer.getInstance();
        Printer p2 = Printer.getInstance();

        System.out.println("hash codes: " + p1.hashCode() + " " + p2.hashCode());
        System.out.println("p1 == p2? " + (p1 == p2));
    }
}

The constructor is called only once. Both p1 and p2 point to the same object (same hashCode, p1 == p2 is true).

4. Pre-defined Singleton Classes in Java

public class Test {
    Test t = new Test();  // Creates only one, but nothing stops creating more
    // ...
    // "Test" is NOT a singleton Java class
}

5. Multithreading Problem

Thread switching scenario

Web application scenario (BookMyShowApp)

Thread-unsafe demo code

// TicketBookingOperation.java
package com.nt.test;

import com.nt.sdp.Printer;

public class TicketBookingOperation implements Runnable {
    @Override
    public void run() {
        System.out.println("TicketBookingOperation.run()");
        Printer p = Printer.getInstance();
        System.out.println(p.hashCode());
    }
}

// SingletonClassTest_MultiThreaded.java
public class SingletonClassTest_MultiThreaded {
    public static void main(String[] args) {
        // create Threads with data
        TicketBookingOperation operation = new TicketBookingOperation();
        Thread th1 = new Thread(operation);
        Thread th2 = new Thread(operation);
        Thread th3 = new Thread(operation);

        // start threads
        th1.start();
        th2.start();
        th3.start();
    }
}

Three different hashCodes = three different objects. The singleton is broken in a multithreaded environment.

6. Six Thread-Safe Solutions

a) Synchronized static factory method

// In Printer.java
public synchronized static Printer getInstance() {
    // singleton logic
    if (INSTANCE == null) {
        try {
            Thread.sleep(10000); // simulate slow creation
        } catch (Exception e) {
            e.printStackTrace();
        }
        INSTANCE = new Printer();
    }
    return INSTANCE;
}

Since the synchronized method allows only one thread at a time inside the method, there is no problem of multithreading breaking singleton. However, every call acquires the lock, even after the object exists. This is slow.

Better: Place synchronized(){} block inside the method where the sensitive logic is, instead of making the whole method synchronized.

b) Synchronized block inside static factory method

public static Printer getInstance() {
    // singleton logic
    synchronized (Printer.class) {  // class level locking
        if (INSTANCE == null) {
            try {
                Thread.sleep(10000);
            } catch (Exception e) {
                e.printStackTrace();
            }
            INSTANCE = new Printer();
        }
    }
    return INSTANCE;
}

c) Double-checked locking (Best 1)

public class Printer {
    private static volatile Printer INSTANCE;

    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        if (INSTANCE == null) {  // 1st NULL check (no lock)
            System.out.println("Printer.getInstance()[outside sync block]: "
                + Thread.currentThread().getName());

            synchronized (Printer.class) {  // class level locking
                System.out.println("Printer.getInstance()[inside sync block]: "
                    + Thread.currentThread().getName());

                if (INSTANCE == null) {  // 2nd NULL check (with lock)
                    try {
                        Thread.sleep(10000);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    INSTANCE = new Printer();
                }
            }
        }
        return INSTANCE;
    }
}

d) Eager instantiation

public class Printer {
    // object created at class loading time itself (eager)
    private static final Printer INSTANCE = new Printer();

    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        return INSTANCE;  // no checking needed
    }
}

Since the object is the static reference variable, the JVM creates it at class loading time. The object is created as soon as the class is loaded, even if it is never used. Thread-safe because class loading is inherently thread-safe in JVM.

Drawback: Creates the object even if never needed (no lazy loading).

e) Inner class based lazy instantiation (Bill Pugh - Best 2)

public class Printer {
    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    // Inner class -- loaded only when getInstance() is called
    private static class InstanceHolder {
        private static final Printer INSTANCE = new Printer();
    }

    public static Printer getInstance() {
        return InstanceHolder.INSTANCE;
    }
}

f) ENUM based singleton (Best 3)

package com.nt.sdp;

public enum Printer3 {
    INSTANCE;  // enum constant -- internally: public static final Printer3 INSTANCE = new Printer3()

    public static Printer3 getInstance() {
        return INSTANCE;
    }

    // business method
    public void printMessage(String msg) {
        System.out.println("printing: " + msg);
    }
}

// Usage:
Printer3.INSTANCE.printMessage("Report #1234");

Why is Enum singleton the best?

1. No multithreading issues: Eager instantiation happens on enum constant like INSTANCE. There will not be any multi-threading issue.
2. Easy to develop: Writing less code with minimum coding standards.
3. Clone-safe: The clone() method is marked as protected final in the internal default super class java.lang.Enum<E>. So we cannot override clone() in our enum. clone() cannot be called on enum constants. Every enum already extends java.lang.Enum<E> class internally. It cannot be extended further (Java does not support multiple inheritance). So our enum is NOT Cloneable by default.
4. Serialization-safe: Every enum is Serializable by default. If we perform Serialization on enum constants, when we attempt to do Deserialization, we get already available enum obj/constant (i.e., some internal readResolve() method functionality is given).
5. Reflection-safe: For enums, we cannot create objects using Reflection API as they do not contain constructors.

Enum inheritance hierarchy

Enum singleton limitations

• a) We cannot apply few features of Java on enum like inheritance using "extends"
• b) We cannot place instance, static member variables in the enum representing the state of the object
• c) We cannot place serialVersionUID while working with Enum to perform serialization activity
• d) Eager instantiation based singleton is not ideal if the object is never used

7. Breaking Singleton (and How to Prevent)

a) Breaking via Cloning

If a singleton class implements Cloneable (directly or through inheritance), calling clone() creates a second object.

Prevention: Override clone()

// Option 1: Return existing instance (not recommended)
@Override
public Object clone() throws CloneNotSupportedException {
    return INSTANCE;
}

// Option 2: Throw exception (BEST solution)
@Override
public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException(
        "In this singleton class, cloning is restricted");
}

b) Breaking via Serialization and Deserialization

Prevention: readResolve() method

// Add this method in your singleton class
private Object readResolve() throws ObjectStreamException {
    return INSTANCE;
}

Full Serialization test code

package com.nt.test;

import java.io.*;
import com.nt.sdp.Printer;

public class SingletonSerializationTest {
    public static void doSerialization(Printer printer) {
        try (ObjectOutputStream oos = new ObjectOutputStream(
                new FileOutputStream("data.ser"))) {
            oos.writeObject(printer);
            oos.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("Object Serialization is done");
    }

    public static Printer doDeSerialization() {
        Printer p = null;
        try (ObjectInputStream ois = new ObjectInputStream(
                new FileInputStream("data.ser"))) {
            p = (Printer) ois.readObject();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return p;
    }

    public static void main(String[] args) {
        Printer p1 = Printer.getInstance();
        doSerialization(p1);

        System.out.println("--------DeSerialization--------");
        Printer p2 = doDeSerialization();
        Printer p3 = doDeSerialization();

        System.out.println(p1.hashCode() + " " + p2.hashCode() + " " + p3.hashCode());
    }
}

private static final long serialVersionUID = 1L;

java.io.InvalidClassException: com.nt.sdp.Printer;
local class incompatible: stream classdesc
serialVersionUID = 1104536345855,
local class serialVersionUID = 8110690610001600652

c) Breaking via Reflection API

Even if the constructor is private, the Reflection API can access it, set it accessible, and create multiple objects. This breaks the singleton guarantee.

package com.nt.test;

import java.lang.reflect.Constructor;

public class SingletonReflectionTest {
    public static void main(String[] args) throws Exception {
        // Normal way -- gives single object
        Printer p1 = Printer.getInstance();

        // Reflection API way -- breaks singleton
        Class c = Class.forName("com.nt.sdp.Printer");
        Constructor[] cons = c.getDeclaredConstructors();
        cons[0].setAccessible(true);  // bypass private access
        Printer p2 = (Printer) cons[0].newInstance();

        System.out.println(p1.hashCode() + " " + p2.hashCode());
        // Output: DIFFERENT hashCodes -- singleton is broken!
        System.out.println("p1 == p2? " + (p1 == p2));  // false
    }
}

Prevention: isInstantiated flag

public class Printer {
    private static Printer INSTANCE;
    private static boolean isInstantiated = false;

    private Printer() {
        // Stop reflection-based second object creation
        if (isInstantiated) {
            throw new RuntimeException(
                "Object is already created. Cannot create another.");
        }
        isInstantiated = true;
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        if (INSTANCE == null)
            INSTANCE = new Printer();
        return INSTANCE;
    }
}

8. Breaking Singleton via ClassLoader

JVM internally uses a ClassLoader subsystem to load Java classes. The system has three classloaders:

Notes on ClassLoader behavior

• If a class is loaded by parent classloader, the same class will NOT be loaded by child classloader (Principle of Delegation)
• The class loaded by parent classloader is visible to child classloader, but not the reverse (Principle of Visibility)
• When JVM gives a class to classloader subsystem for loading, it is delegated to all classloaders using the delegation principle

Breaking singleton using Custom ClassLoader

URLClassLoader is a readily available custom classloader which can load classes from specific jar files and also initialize them as needed.

// Using Application ClassLoader (default)
System.out.println("Using Application ClassLoader");
Printer p1 = Printer.getInstance();
Printer p2 = Printer.getInstance();
System.out.println(p1.hashCode() + " " + p2.hashCode());

System.out.println("---");

// Using Custom ClassLoader (URLClassLoader)
System.out.println("Using URLClassLoader");
URLClassLoader loader = new URLClassLoader(
    new URL[]{new URL("file:///c/myProj/jar5/1.jar")});
Class c1 = loader.loadClass("com.nt.sdp.Printer");
Constructor cons = c1.getDeclaredConstructors()[0];
cons.setAccessible(true);
Object obj = cons.newInstance();

System.out.println("object hash codes: " + obj.hashCode() + " " + p1.hashCode());

Here URLClassLoader is an independent classloader and not linked with JVM's classloader system. So one object is created using ApplicationClassLoader of ClassLoader sub system and another object will be using URLClassLoader (custom classloader).

9. Singleton Pattern vs Spring Singleton Scope

Spring IoC container creating one object for servlet comp class does not mean Spring is making classes as singletons. Spring is just having singleton behavior (one object for singleton scope spring bean), but the class is not a true singleton because there is no private constructor restriction. If you bypass the container, you could create more objects.

10. Full-Length Industry Project: BookMyShow Ticket Booking System

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

application.properties

# BookMyShow Configuration
app.name=BookMyShow
app.version=1.0.0

# Database
db.url=jdbc:mysql://localhost:3306/bookmyshow_db
db.username=bms_admin
db.password=encrypted_pwd
db.pool.size=20

# Cache
cache.default.ttl.seconds=300
cache.movies.ttl.seconds=600

# Booking
booking.max.seats.per.transaction=10
booking.payment.timeout.seconds=120

# Logging
log.file=bookmyshow.log
log.level=INFO

Connection.java (Simulated DB connection)

package com.bookmyshow.model;

import java.util.UUID;

public class Connection {
    private final String id;
    private final String url;
    private boolean inUse;

    public Connection(String url) {
        this.id = UUID.randomUUID().toString().substring(0, 8);
        this.url = url;
        this.inUse = false;
    }

    public String getId() { return id; }
    public String getUrl() { return url; }
    public boolean isInUse() { return inUse; }
    public void setInUse(boolean inUse) { this.inUse = inUse; }

    @Override
    public String toString() {
        return "Connection[" + id + "]";
    }
}

Movie.java

package com.bookmyshow.model;

public class Movie {
    private final String id;
    private final String title;
    private final String genre;
    private final int durationMinutes;

    public Movie(String id, String title, String genre, int durationMinutes) {
        this.id = id;
        this.title = title;
        this.genre = genre;
        this.durationMinutes = durationMinutes;
    }

    public String getId() { return id; }
    public String getTitle() { return title; }
    public String getGenre() { return genre; }
    public int getDurationMinutes() { return durationMinutes; }

    @Override
    public String toString() {
        return title + " (" + genre + ", " + durationMinutes + " min)";
    }
}

Show.java

package com.bookmyshow.model;

public class Show {
    private final String id;
    private final String movieId;
    private final String theatre;
    private final String time;
    private int availableSeats;
    private final double price;

    public Show(String id, String movieId, String theatre,
                String time, int availableSeats, double price) {
        this.id = id;
        this.movieId = movieId;
        this.theatre = theatre;
        this.time = time;
        this.availableSeats = availableSeats;
        this.price = price;
    }

    public String getId() { return id; }
    public String getMovieId() { return movieId; }
    public String getTheatre() { return theatre; }
    public String getTime() { return time; }
    public int getAvailableSeats() { return availableSeats; }
    public double getPrice() { return price; }

    public synchronized boolean reserveSeats(int count) {
        if (count > availableSeats) return false;
        availableSeats -= count;
        return true;
    }

    @Override
    public String toString() {
        return theatre + " | " + time + " | Seats: " + availableSeats
            + " | Rs " + price;
    }
}

Booking.java

package com.bookmyshow.model;

import java.time.LocalDateTime;
import java.util.UUID;

public class Booking {
    private final String id;
    private final String userId;
    private final String showId;
    private final int seats;
    private final double totalAmount;
    private final LocalDateTime bookedAt;
    private String status;

    public Booking(String userId, String showId, int seats, double totalAmount) {
        this.id = "BKG-" + UUID.randomUUID().toString().substring(0, 8).toUpperCase();
        this.userId = userId;
        this.showId = showId;
        this.seats = seats;
        this.totalAmount = totalAmount;
        this.bookedAt = LocalDateTime.now();
        this.status = "CONFIRMED";
    }

    public String getId() { return id; }
    public String getUserId() { return userId; }
    public String getShowId() { return showId; }
    public int getSeats() { return seats; }
    public double getTotalAmount() { return totalAmount; }
    public LocalDateTime getBookedAt() { return bookedAt; }
    public String getStatus() { return status; }
    public void setStatus(String status) { this.status = status; }

    @Override
    public String toString() {
        return "Booking{id=" + id + ", user=" + userId + ", show=" + showId
            + ", seats=" + seats + ", amount=Rs " + totalAmount
            + ", status=" + status + "}";
    }
}

ConfigManager.java (Enum Singleton)

package com.bookmyshow.singleton;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

/**
 * Application configuration manager.
 * Enum-based singleton: thread-safe, reflection-safe, serialization-safe.
 */
public enum ConfigManager {
    INSTANCE;

    private final Properties properties;

    ConfigManager() {
        this.properties = new Properties();
        try (InputStream stream = getClass().getClassLoader()
                .getResourceAsStream("application.properties")) {
            if (stream != null) {
                properties.load(stream);
            }
        } catch (IOException e) {
            throw new RuntimeException("Failed to load configuration", e);
        }
    }

    public String get(String key) {
        return properties.getProperty(key);
    }

    public String get(String key, String defaultValue) {
        return properties.getProperty(key, defaultValue);
    }

    public int getInt(String key, int defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        try {
            return Integer.parseInt(value.trim());
        } catch (NumberFormatException e) {
            return defaultValue;
        }
    }

    public boolean getBoolean(String key, boolean defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        return Boolean.parseBoolean(value.trim());
    }
}

AppLogger.java (Bill Pugh Singleton)

package com.bookmyshow.singleton;

import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

/**
 * Application logger.
 * Bill Pugh (inner class) singleton: lazy loaded, thread-safe, no synchronization overhead.
 */
public class AppLogger {
    private static final DateTimeFormatter FORMATTER =
        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");

    private final PrintWriter fileWriter;

    private AppLogger() {
        String logFile = ConfigManager.INSTANCE.get("log.file", "app.log");
        try {
            this.fileWriter = new PrintWriter(
                new FileWriter(logFile, true), true);
        } catch (IOException e) {
            throw new RuntimeException("Cannot open log file: " + logFile, e);
        }
    }

    // Inner class loaded only when getInstance() is first called
    private static class InstanceHolder {
        private static final AppLogger INSTANCE = new AppLogger();
    }

    public static AppLogger getInstance() {
        return InstanceHolder.INSTANCE;
    }

    public void info(String message) {
        log("INFO", message);
    }

    public void warn(String message) {
        log("WARN", message);
    }

    public void error(String message) {
        log("ERROR", message);
    }

    public void error(String message, Throwable throwable) {
        log("ERROR", message + " | " + throwable.getMessage());
    }

    private void log(String level, String message) {
        String timestamp = LocalDateTime.now().format(FORMATTER);
        String threadName = Thread.currentThread().getName();
        String entry = String.format("[%s] [%-5s] [%s] %s",
            timestamp, level, threadName, message);
        System.out.println(entry);
        fileWriter.println(entry);
    }
}

ConnectionPool.java (Double-Checked Locking Singleton)

package com.bookmyshow.singleton;

import com.bookmyshow.model.Connection;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Set;

/**
 * Database connection pool.
 * Double-checked locking singleton: lazy, thread-safe, minimal lock contention.
 */
public class ConnectionPool {
    private static volatile ConnectionPool INSTANCE;

    private final Queue<Connection> availableConnections;
    private final Set<Connection> usedConnections;
    private final int maxPoolSize;

    private ConnectionPool(int poolSize, String dbUrl) {
        this.maxPoolSize = poolSize;
        this.availableConnections = new LinkedList<>();
        this.usedConnections = new HashSet<>();

        for (int i = 0; i < poolSize; i++) {
            availableConnections.add(new Connection(dbUrl));
        }

        AppLogger.getInstance().info(
            "ConnectionPool initialized with " + poolSize + " connections");
    }

    public static ConnectionPool getInstance() {
        if (INSTANCE == null) {
            synchronized (ConnectionPool.class) {
                if (INSTANCE == null) {
                    String dbUrl = ConfigManager.INSTANCE.get("db.url");
                    int poolSize = ConfigManager.INSTANCE.getInt("db.pool.size", 10);
                    INSTANCE = new ConnectionPool(poolSize, dbUrl);
                }
            }
        }
        return INSTANCE;
    }

    public synchronized Connection getConnection() {
        if (availableConnections.isEmpty()) {
            AppLogger.getInstance().warn(
                "Connection pool exhausted. Used: " + usedConnections.size()
                + "/" + maxPoolSize);
            throw new RuntimeException("No available connections in pool");
        }
        Connection conn = availableConnections.poll();
        conn.setInUse(true);
        usedConnections.add(conn);
        return conn;
    }

    public synchronized void releaseConnection(Connection conn) {
        if (conn == null) return;
        conn.setInUse(false);
        usedConnections.remove(conn);
        availableConnections.offer(conn);
    }

    public int getAvailableCount() {
        return availableConnections.size();
    }

    public int getUsedCount() {
        return usedConnections.size();
    }
}

CacheManager.java (Inner Class Singleton with TTL)

package com.bookmyshow.singleton;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * In-memory cache with time-to-live (TTL) expiry.
 * Inner class singleton: lazy loaded, thread-safe.
 * Uses ConcurrentHashMap for thread-safe read/write.
 */
public class CacheManager {
    private final Map<String, CacheEntry> cache;
    private final long defaultTtlMs;

    private static class CacheEntry {
        final Object value;
        final long expiresAt;

        CacheEntry(Object value, long ttlMs) {
            this.value = value;
            this.expiresAt = System.currentTimeMillis() + ttlMs;
        }

        boolean isExpired() {
            return System.currentTimeMillis() > expiresAt;
        }
    }

    private CacheManager() {
        this.cache = new ConcurrentHashMap<>();
        int ttlSeconds = ConfigManager.INSTANCE.getInt(
            "cache.default.ttl.seconds", 300);
        this.defaultTtlMs = ttlSeconds * 1000L;

        AppLogger.getInstance().info(
            "CacheManager initialized with default TTL: " + ttlSeconds + "s");
    }

    private static class InstanceHolder {
        private static final CacheManager INSTANCE = new CacheManager();
    }

    public static CacheManager getInstance() {
        return InstanceHolder.INSTANCE;
    }

    public void put(String key, Object value) {
        put(key, value, defaultTtlMs);
    }

    public void put(String key, Object value, long ttlMs) {
        cache.put(key, new CacheEntry(value, ttlMs));
    }

    @SuppressWarnings("unchecked")
    public <T> T get(String key) {
        CacheEntry entry = cache.get(key);
        if (entry == null) return null;
        if (entry.isExpired()) {
            cache.remove(key);
            return null;
        }
        return (T) entry.value;
    }

    public void remove(String key) {
        cache.remove(key);
    }

    public void clear() {
        cache.clear();
    }

    public int size() {
        // Remove expired entries and return count
        cache.entrySet().removeIf(e -> e.getValue().isExpired());
        return cache.size();
    }
}

MovieService.java

package com.bookmyshow.service;

import com.bookmyshow.model.Connection;
import com.bookmyshow.model.Movie;
import com.bookmyshow.model.Show;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.Arrays;
import java.util.List;

public class MovieService {
    private final AppLogger logger = AppLogger.getInstance();
    private final CacheManager cache = CacheManager.getInstance();
    private final ConnectionPool pool = ConnectionPool.getInstance();

    public List<Movie> getAllMovies() {
        // Check cache first
        List<Movie> cached = cache.get("all_movies");
        if (cached != null) {
            logger.info("Movies loaded from cache");
            return cached;
        }

        // Cache miss: fetch from DB
        Connection conn = pool.getConnection();
        try {
            logger.info("Fetching movies from DB using " + conn);

            // Simulated DB query
            List<Movie> movies = Arrays.asList(
                new Movie("MOV001", "Pathaan", "Action", 146),
                new Movie("MOV002", "Jawan", "Action", 169),
                new Movie("MOV003", "Animal", "Drama", 201),
                new Movie("MOV004", "12th Fail", "Drama", 147),
                new Movie("MOV005", "Dunki", "Comedy-Drama", 161)
            );

            // Store in cache
            cache.put("all_movies", movies);
            logger.info("Movies cached. Count: " + movies.size());

            return movies;
        } finally {
            pool.releaseConnection(conn);
        }
    }

    public Movie getMovieById(String movieId) {
        Movie cached = cache.get("movie_" + movieId);
        if (cached != null) return cached;

        List<Movie> movies = getAllMovies();
        for (Movie m : movies) {
            if (m.getId().equals(movieId)) {
                cache.put("movie_" + movieId, m);
                return m;
            }
        }
        return null;
    }

    public List<Show> getShowsForMovie(String movieId) {
        List<Show> cached = cache.get("shows_" + movieId);
        if (cached != null) {
            logger.info("Shows for " + movieId + " loaded from cache");
            return cached;
        }

        Connection conn = pool.getConnection();
        try {
            logger.info("Fetching shows for " + movieId + " from DB using " + conn);

            // Simulated DB query
            List<Show> shows = Arrays.asList(
                new Show("SHW001", movieId, "PVR Phoenix Mall", "10:00 AM", 150, 250.0),
                new Show("SHW002", movieId, "PVR Phoenix Mall", "02:30 PM", 150, 300.0),
                new Show("SHW003", movieId, "INOX Lido", "06:00 PM", 200, 350.0),
                new Show("SHW004", movieId, "INOX Lido", "09:30 PM", 200, 400.0)
            );

            cache.put("shows_" + movieId, shows);
            return shows;
        } finally {
            pool.releaseConnection(conn);
        }
    }
}

BookingService.java

package com.bookmyshow.service;

import com.bookmyshow.model.Booking;
import com.bookmyshow.model.Connection;
import com.bookmyshow.model.Show;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConfigManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.List;

public class BookingService {
    private final AppLogger logger = AppLogger.getInstance();
    private final ConnectionPool pool = ConnectionPool.getInstance();
    private final CacheManager cache = CacheManager.getInstance();

    public Booking bookTicket(String userId, String showId, int seatCount) {
        int maxSeats = ConfigManager.INSTANCE.getInt(
            "booking.max.seats.per.transaction", 10);

        if (seatCount <= 0 || seatCount > maxSeats) {
            logger.error("Invalid seat count: " + seatCount
                + ". Max allowed: " + maxSeats);
            throw new IllegalArgumentException(
                "Seat count must be between 1 and " + maxSeats);
        }

        Connection conn = pool.getConnection();
        try {
            logger.info("Booking " + seatCount + " seats for user "
                + userId + " on show " + showId);

            // Find the show and reserve seats
            Show show = findShow(showId);
            if (show == null) {
                logger.error("Show not found: " + showId);
                throw new RuntimeException("Show not found: " + showId);
            }

            boolean reserved = show.reserveSeats(seatCount);
            if (!reserved) {
                logger.warn("Not enough seats for show " + showId
                    + ". Requested: " + seatCount
                    + ", Available: " + show.getAvailableSeats());
                throw new RuntimeException(
                    "Not enough seats. Available: " + show.getAvailableSeats());
            }

            double totalAmount = show.getPrice() * seatCount;
            Booking booking = new Booking(userId, showId, seatCount, totalAmount);

            // Invalidate show cache since seats changed
            cache.remove("shows_" + show.getMovieId());

            logger.info("Booking confirmed: " + booking.getId()
                + " | Amount: Rs " + totalAmount);

            return booking;
        } finally {
            pool.releaseConnection(conn);
        }
    }

    public boolean cancelBooking(Booking booking) {
        logger.info("Cancelling booking: " + booking.getId());
        booking.setStatus("CANCELLED");
        logger.info("Booking cancelled: " + booking.getId());
        return true;
    }

    private Show findShow(String showId) {
        // Search across all cached shows
        MovieService movieService = new MovieService();
        for (var movie : movieService.getAllMovies()) {
            List<Show> shows = movieService.getShowsForMovie(movie.getId());
            for (Show s : shows) {
                if (s.getId().equals(showId)) return s;
            }
        }
        return null;
    }
}

Application.java (Main Entry Point)

package com.bookmyshow;

import com.bookmyshow.model.Booking;
import com.bookmyshow.model.Movie;
import com.bookmyshow.model.Show;
import com.bookmyshow.service.BookingService;
import com.bookmyshow.service.MovieService;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConfigManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.List;

public class Application {
    public static void main(String[] args) throws InterruptedException {
        AppLogger logger = AppLogger.getInstance();
        logger.info("=== BookMyShow Application Started ===");
        logger.info("App: " + ConfigManager.INSTANCE.get("app.name")
            + " v" + ConfigManager.INSTANCE.get("app.version"));

        // ---- 1. VERIFY ALL FOUR SINGLETONS ----
        System.out.println("\n--- Singleton Verification ---");

        ConfigManager c1 = ConfigManager.INSTANCE;
        ConfigManager c2 = ConfigManager.INSTANCE;
        System.out.println("ConfigManager same?    " + (c1 == c2)
            + " | hashCode: " + c1.hashCode());

        AppLogger l1 = AppLogger.getInstance();
        AppLogger l2 = AppLogger.getInstance();
        System.out.println("AppLogger same?        " + (l1 == l2)
            + " | hashCode: " + l1.hashCode());

        ConnectionPool p1 = ConnectionPool.getInstance();
        ConnectionPool p2 = ConnectionPool.getInstance();
        System.out.println("ConnectionPool same?   " + (p1 == p2)
            + " | hashCode: " + p1.hashCode());

        CacheManager m1 = CacheManager.getInstance();
        CacheManager m2 = CacheManager.getInstance();
        System.out.println("CacheManager same?     " + (m1 == m2)
            + " | hashCode: " + m1.hashCode());

        // ---- 2. BROWSE MOVIES ----
        System.out.println("\n--- Available Movies ---");
        MovieService movieService = new MovieService();
        List<Movie> movies = movieService.getAllMovies();
        for (Movie m : movies) {
            System.out.println("  " + m);
        }

        // Call again -- should come from cache
        System.out.println("\n--- Movies (2nd call - from cache) ---");
        movieService.getAllMovies();

        // ---- 3. BROWSE SHOWS ----
        System.out.println("\n--- Shows for Pathaan ---");
        List<Show> shows = movieService.getShowsForMovie("MOV001");
        for (Show s : shows) {
            System.out.println("  " + s);
        }

        // ---- 4. BOOK TICKETS ----
        System.out.println("\n--- Booking Tickets ---");
        BookingService bookingService = new BookingService();

        Booking booking1 = bookingService.bookTicket("USER001", "SHW003", 4);
        System.out.println("  " + booking1);

        Booking booking2 = bookingService.bookTicket("USER002", "SHW003", 2);
        System.out.println("  " + booking2);

        // ---- 5. CANCEL BOOKING ----
        System.out.println("\n--- Cancel Booking ---");
        bookingService.cancelBooking(booking1);
        System.out.println("  " + booking1);

        // ---- 6. CONNECTION POOL STATUS ----
        System.out.println("\n--- Connection Pool Status ---");
        System.out.println("  Available: " + p1.getAvailableCount());
        System.out.println("  In use:    " + p1.getUsedCount());

        // ---- 7. CACHE STATUS ----
        System.out.println("\n--- Cache Status ---");
        System.out.println("  Cached items: " + m1.size());

        // ---- 8. MULTITHREADED BOOKING TEST ----
        System.out.println("\n--- Multithreaded Booking Test (5 users) ---");
        Thread[] threads = new Thread[5];
        for (int i = 0; i < 5; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                try {
                    BookingService svc = new BookingService();
                    Booking b = svc.bookTicket("USER_MT_" + idx, "SHW004", 2);
                    System.out.println("  Thread-" + idx + ": " + b.getId()
                        + " | Seats: " + b.getSeats()
                        + " | Amount: Rs " + b.getTotalAmount());
                } catch (Exception e) {
                    System.out.println("  Thread-" + idx + ": FAILED - "
                        + e.getMessage());
                }
            }, "BookingThread-" + i);
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        System.out.println("\n--- Final Pool Status ---");
        System.out.println("  Available: " + p1.getAvailableCount());
        System.out.println("  In use:    " + p1.getUsedCount());

        logger.info("=== BookMyShow Application Finished ===");
    }
}

SingletonThreadSafetyTest.java

package com.bookmyshow.singleton;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class SingletonThreadSafetyTest {

    @Test
    void configManager_isSingleton() {
        assertSame(ConfigManager.INSTANCE, ConfigManager.INSTANCE);
    }

    @Test
    void appLogger_isSingleton() {
        assertSame(AppLogger.getInstance(), AppLogger.getInstance());
    }

    @Test
    void connectionPool_isSingleton() {
        assertSame(ConnectionPool.getInstance(), ConnectionPool.getInstance());
    }

    @Test
    void cacheManager_isSingleton() {
        assertSame(CacheManager.getInstance(), CacheManager.getInstance());
    }

    @Test
    void allSingletons_threadSafe() throws InterruptedException {
        int threadCount = 100;
        AppLogger[] loggers = new AppLogger[threadCount];
        ConnectionPool[] pools = new ConnectionPool[threadCount];
        CacheManager[] caches = new CacheManager[threadCount];
        Thread[] threads = new Thread[threadCount];

        for (int i = 0; i < threadCount; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                loggers[idx] = AppLogger.getInstance();
                pools[idx] = ConnectionPool.getInstance();
                caches[idx] = CacheManager.getInstance();
            });
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        for (int i = 1; i < threadCount; i++) {
            assertSame(loggers[0], loggers[i],
                "AppLogger must be same across all threads");
            assertSame(pools[0], pools[i],
                "ConnectionPool must be same across all threads");
            assertSame(caches[0], caches[i],
                "CacheManager must be same across all threads");
        }
    }

    @Test
    void connectionPool_getAndRelease() {
        ConnectionPool pool = ConnectionPool.getInstance();
        int before = pool.getAvailableCount();

        var conn = pool.getConnection();
        assertEquals(before - 1, pool.getAvailableCount());
        assertEquals(1, pool.getUsedCount());

        pool.releaseConnection(conn);
        assertEquals(before, pool.getAvailableCount());
        assertEquals(0, pool.getUsedCount());
    }

    @Test
    void cacheManager_putAndGet() {
        CacheManager cache = CacheManager.getInstance();
        cache.put("test_key", "test_value");
        assertEquals("test_value", cache.get("test_key"));
    }

    @Test
    void cacheManager_expiredEntryReturnsNull() throws InterruptedException {
        CacheManager cache = CacheManager.getInstance();
        cache.put("expire_key", "value", 50); // 50ms TTL
        Thread.sleep(100);
        assertNull(cache.get("expire_key"));
    }

    @Test
    void configManager_readsProperties() {
        assertEquals("BookMyShow", ConfigManager.INSTANCE.get("app.name"));
        assertEquals(20, ConfigManager.INSTANCE.getInt("db.pool.size", 5));
    }
}

Expected Output

Key observations from the project

11. Smaller Example: Configuration Manager

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

application.properties

# Database Configuration
db.url=jdbc:mysql://localhost:3306/national_bank_db
db.username=nbi_admin
db.password=encrypted_pwd_here
db.pool.size=20
db.pool.timeout=30000

# API Configuration
api.base.url=https://api.nationalbankofIndia.com/v1
api.timeout=5000
api.retry.count=3

# Feature Flags
feature.upi.enabled=true
feature.international.transfer.enabled=false
feature.sms.notification.enabled=true

# Notification
notification.sms.gateway=https://sms.gateway.nbi.com
notification.email.from=noreply@nationalbankofIndia.com

ConfigManager.java

package com.nbi.banking.config;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.Properties;
import java.util.Set;

public class ConfigManager implements Serializable {
    private static final long serialVersionUID = 1L;
    private static volatile ConfigManager INSTANCE;
    private static boolean isInstantiated = false;

    private final Properties properties;

    private ConfigManager() {
        // Reflection protection
        if (isInstantiated) {
            throw new RuntimeException(
                "ConfigManager is a singleton. Use getInstance() instead.");
        }
        isInstantiated = true;

        this.properties = new Properties();
        loadProperties();
    }

    public static ConfigManager getInstance() {
        if (INSTANCE == null) {
            synchronized (ConfigManager.class) {
                if (INSTANCE == null) {
                    INSTANCE = new ConfigManager();
                }
            }
        }
        return INSTANCE;
    }

    public String get(String key) {
        return properties.getProperty(key);
    }

    public String get(String key, String defaultValue) {
        return properties.getProperty(key, defaultValue);
    }

    public int getInt(String key, int defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        try {
            return Integer.parseInt(value.trim());
        } catch (NumberFormatException e) {
            return defaultValue;
        }
    }

    public boolean getBoolean(String key, boolean defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        return Boolean.parseBoolean(value.trim());
    }

    public Set getAllKeys() {
        return properties.stringPropertyNames();
    }

    public synchronized void reload() {
        properties.clear();
        loadProperties();
        System.out.println("Configuration reloaded successfully");
    }

    private void loadProperties() {
        try (InputStream stream = getClass().getClassLoader()
                .getResourceAsStream("application.properties")) {
            if (stream == null) {
                throw new RuntimeException(
                    "application.properties not found on classpath");
            }
            properties.load(stream);
            System.out.println("Loaded " + properties.size()
                + " configuration properties");
        } catch (IOException e) {
            throw new RuntimeException(
                "Failed to load configuration", e);
        }
    }

    // Serialization protection
    private Object readResolve() throws ObjectStreamException {
        return INSTANCE;
    }

    // Clone protection
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException(
            "ConfigManager is a singleton. Cloning is not allowed.");
    }
}

AccountService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;

public class AccountService {
    private final ConfigManager config = ConfigManager.getInstance();

    public void getAccountDetails(String accountNumber) {
        String dbUrl = config.get("db.url");
        int poolSize = config.getInt("db.pool.size", 10);

        System.out.println("Fetching account " + accountNumber
            + " from database: " + dbUrl
            + " (pool size: " + poolSize + ")");
        // actual DB query logic here
    }
}

TransferService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;
import java.math.BigDecimal;

public class TransferService {
    private final ConfigManager config = ConfigManager.getInstance();

    public boolean transfer(String from, String to, BigDecimal amount) {
        boolean upiEnabled = config.getBoolean("feature.upi.enabled", false);
        int timeout = config.getInt("api.timeout", 5000);

        System.out.println("Transfer " + amount + " from " + from + " to " + to);
        System.out.println("UPI enabled: " + upiEnabled + ", timeout: " + timeout + "ms");

        // Verify same config instance across services
        System.out.println("Config instance hashCode: "
            + config.hashCode());
        return true;
    }
}

NotificationService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;

public class NotificationService {
    private final ConfigManager config = ConfigManager.getInstance();

    public void sendSmsNotification(String phone, String message) {
        boolean smsEnabled = config.getBoolean(
            "feature.sms.notification.enabled", false);

        if (!smsEnabled) {
            System.out.println("SMS notification is disabled");
            return;
        }

        String gateway = config.get("notification.sms.gateway");
        System.out.println("Sending SMS via " + gateway
            + " to " + phone + ": " + message);

        // Verify same config instance
        System.out.println("Config instance hashCode: "
            + config.hashCode());
    }
}

Application.java

package com.nbi.banking;

import com.nbi.banking.config.ConfigManager;
import com.nbi.banking.service.AccountService;
import com.nbi.banking.service.TransferService;
import com.nbi.banking.service.NotificationService;
import java.math.BigDecimal;

public class Application {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("=== National Bank of India - Banking Application ===");
        System.out.println();

        // 1. Verify singleton behavior
        ConfigManager config1 = ConfigManager.getInstance();
        ConfigManager config2 = ConfigManager.getInstance();
        System.out.println("Singleton verification:");
        System.out.println("  config1 hashCode: " + config1.hashCode());
        System.out.println("  config2 hashCode: " + config2.hashCode());
        System.out.println("  config1 == config2? " + (config1 == config2));
        System.out.println();

        // 2. Use services (all share same ConfigManager instance)
        AccountService accountService = new AccountService();
        TransferService transferService = new TransferService();
        NotificationService notificationService = new NotificationService();

        accountService.getAccountDetails("SAV2001");
        System.out.println();

        transferService.transfer("SAV2001", "CUR3001",
            new BigDecimal("50000.00"));
        System.out.println();

        notificationService.sendSmsNotification(
            "+91-9876543210",
            "Transfer of Rs 50,000 from SAV2001 to CUR3001 completed");
        System.out.println();

        // 3. Multithreaded access verification
        System.out.println("=== Multithreaded Access Test ===");
        Thread[] threads = new Thread[10];
        int[] hashCodes = new int[10];

        for (int i = 0; i < 10; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                ConfigManager cm = ConfigManager.getInstance();
                hashCodes[idx] = cm.hashCode();
                System.out.println("Thread-" + idx
                    + " config hashCode: " + cm.hashCode());
            });
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        // Verify all threads got the same instance
        boolean allSame = true;
        for (int i = 1; i < 10; i++) {
            if (hashCodes[i] != hashCodes[0]) {
                allSame = false;
                break;
            }
        }
        System.out.println("All threads got same instance? " + allSame);
        System.out.println();

        // 4. Read all configuration keys
        System.out.println("=== All Configuration Keys ===");
        for (String key : config1.getAllKeys()) {
            System.out.println("  " + key + " = " + config1.get(key));
        }
    }
}

ConfigManagerTest.java

package com.nbi.banking.config;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class ConfigManagerTest {

    @Test
    void getInstance_returnsSameInstance() {
        ConfigManager c1 = ConfigManager.getInstance();
        ConfigManager c2 = ConfigManager.getInstance();
        assertSame(c1, c2);
    }

    @Test
    void getInstance_isThreadSafe() throws InterruptedException {
        ConfigManager[] results = new ConfigManager[100];
        Thread[] threads = new Thread[100];

        for (int i = 0; i < 100; i++) {
            final int idx = i;
            threads[i] = new Thread(
                () -> results[idx] = ConfigManager.getInstance());
        }
        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        for (ConfigManager result : results) {
            assertSame(results[0], result);
        }
    }

    @Test
    void get_returnsConfigValue() {
        ConfigManager config = ConfigManager.getInstance();
        String dbUrl = config.get("db.url");
        assertNotNull(dbUrl);
        assertTrue(dbUrl.contains("national_bank_db"));
    }

    @Test
    void getInt_returnsIntValue() {
        ConfigManager config = ConfigManager.getInstance();
        int poolSize = config.getInt("db.pool.size", 5);
        assertEquals(20, poolSize);
    }

    @Test
    void getInt_returnsDefaultForMissingKey() {
        ConfigManager config = ConfigManager.getInstance();
        int value = config.getInt("nonexistent.key", 42);
        assertEquals(42, value);
    }

    @Test
    void getBoolean_returnsBooleanValue() {
        ConfigManager config = ConfigManager.getInstance();
        assertTrue(config.getBoolean("feature.upi.enabled", false));
        assertFalse(config.getBoolean(
            "feature.international.transfer.enabled", true));
    }

    @Test
    void clone_throwsException() {
        ConfigManager config = ConfigManager.getInstance();
        assertThrows(CloneNotSupportedException.class,
            () -> config.clone());
    }

    @Test
    void getAllKeys_returnsNonEmptySet() {
        ConfigManager config = ConfigManager.getInstance();
        assertFalse(config.getAllKeys().isEmpty());
    }
}

Expected Output

12. When to Use and When NOT to Use

When to use Singleton

When NOT to use Singleton

Singleton Design Pattern in Java

Complete Tutorial | 13 Sections | Full BookMyShow Project with HLD and LLD | 6 Thread-Safe Approaches | 3 Ways to Break Singleton | ClassLoader Attack | Spring vs Singleton | Multiple Scenarios | 15 Interview Questions

1. What is Singleton Pattern?

In Java it is called a singleton Java class. In C# it is called a singleton C# class. Since it is a GOF pattern, it can be implemented in any OOP language (C++, Java, C#, Python, etc.).

Design patterns are defined by three parts

1. Problem: The recurring issue in software development
2. Solution: The best proven approach to solve it
3. Context: The situation where this solution applies

2. The Problem

In Java, object creation is a costly process in terms of memory and CPU utilization. Every new call allocates heap memory, runs the constructor, and initializes fields. If a class needs only one instance, creating more wastes resources.

Real-world analogy

Think of all employees in an office sharing a single printer. Every employee does not need their own printer. One printer serves everyone. If we design software for this office, the Printer class should be a singleton -- creating 100 Printer objects for 100 employees is wasteful when one object can serve all.

Three cases where singleton is needed

Case 1: Class with no state

A class that has no member variables, only methods. Creating multiple objects is pointless because every object behaves identically.

// No state -- all objects are identical
public class MathUtils {
    private MathUtils() {}

    private static MathUtils INSTANCE;

    public static MathUtils getInstance() {
        if (INSTANCE == null) INSTANCE = new MathUtils();
        return INSTANCE;
    }

    public int add(int a, int b) { return a + b; }
    public int multiply(int a, int b) { return a * b; }
}

// Can call methods for multiple times
// No need to create multiple objects
MathUtils.getInstance().add(10, 20);
MathUtils.getInstance().multiply(5, 6);

Case 2: Class with read-only state (final variables)

A class whose state is set once and never changes. Multiple objects all hold the same data.

// Read-only state -- all objects have same data
public class AppConfig {
    private static final String DB_URL = "jdbc:mysql://localhost:3306/bankdb";
    private static final int MAX_CONNECTIONS = 10;

    private static AppConfig INSTANCE;

    private AppConfig() {
        if (INSTANCE != null)
            throw new RuntimeException("Object already created");
    }

    public static AppConfig getInstance() {
        if (INSTANCE == null)
            INSTANCE = new AppConfig();
        return INSTANCE;
    }

    public String getDbUrl() { return DB_URL; }
    public int getMaxConnections() { return MAX_CONNECTIONS; }
}

Case 3: Class with sharable state (cache, buffer, pool)

A class that holds shared mutable state. Multiple objects would mean duplicate caches or connection pools, leading to inconsistency and wasted memory.

// Sharable state -- one connection pool serves all
public class ConnectionPool {
    private static ConnectionPool INSTANCE;
    private final List<Connection> pool = new ArrayList<>();
    private static final int POOL_SIZE = 10;

    private ConnectionPool() {
        for (int i = 0; i < POOL_SIZE; i++) {
            pool.add(createConnection());
        }
    }

    public static ConnectionPool getInstance() {
        if (INSTANCE == null)
            INSTANCE = new ConnectionPool();
        return INSTANCE;
    }

    public synchronized Connection getConnection() {
        if (pool.isEmpty()) throw new RuntimeException("Pool exhausted");
        return pool.remove(0);
    }

    public synchronized void releaseConnection(Connection conn) {
        pool.add(conn);
    }

    private Connection createConnection() {
        // create actual DB connection
        return DriverManager.getConnection(
            "jdbc:mysql://localhost:3306/bankdb", "root", "password");
    }
}

3. The Solution: Three Minimum Standards

Basic Singleton: Printer.java

package com.nt.sdp;

public class Printer {
    // Step 2: private static ref variable of class type
    private static Printer INSTANCE;

    // Step 1: private 0-param constructor
    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    // Step 3: static factory method having singleton logic
    public static Printer getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Printer();
        }
        return INSTANCE;
    }

    // business method
    public void printMessage(String msg) {
        System.out.println(msg);
    }
}

Client App: SingletonClassTest1.java

package com.nt.test;

import com.nt.sdp.Printer;

public class SingletonClassTest1 {
    public static void main(String[] args) {
        // Printer p1 = new Printer(); // INVALID - constructor is private

        // Get singleton object
        Printer p1 = Printer.getInstance();
        Printer p2 = Printer.getInstance();

        System.out.println("hash codes: " + p1.hashCode() + " " + p2.hashCode());
        System.out.println("p1 == p2? " + (p1 == p2));
    }
}

The constructor is called only once. Both p1 and p2 point to the same object (same hashCode, p1 == p2 is true).

4. Pre-defined Singleton Classes in Java

public class Test {
    Test t = new Test();  // Creates only one, but nothing stops creating more
    // ...
    // "Test" is NOT a singleton Java class
}

5. Multithreading Problem

Thread switching scenario

Web application scenario (BookMyShowApp)

Thread-unsafe demo code

// TicketBookingOperation.java
package com.nt.test;

import com.nt.sdp.Printer;

public class TicketBookingOperation implements Runnable {
    @Override
    public void run() {
        System.out.println("TicketBookingOperation.run()");
        Printer p = Printer.getInstance();
        System.out.println(p.hashCode());
    }
}

// SingletonClassTest_MultiThreaded.java
public class SingletonClassTest_MultiThreaded {
    public static void main(String[] args) {
        // create Threads with data
        TicketBookingOperation operation = new TicketBookingOperation();
        Thread th1 = new Thread(operation);
        Thread th2 = new Thread(operation);
        Thread th3 = new Thread(operation);

        // start threads
        th1.start();
        th2.start();
        th3.start();
    }
}

Three different hashCodes = three different objects. The singleton is broken in a multithreaded environment.

6. Six Thread-Safe Solutions

a) Synchronized static factory method

// In Printer.java
public synchronized static Printer getInstance() {
    // singleton logic
    if (INSTANCE == null) {
        try {
            Thread.sleep(10000); // simulate slow creation
        } catch (Exception e) {
            e.printStackTrace();
        }
        INSTANCE = new Printer();
    }
    return INSTANCE;
}

Since the synchronized method allows only one thread at a time inside the method, there is no problem of multithreading breaking singleton. However, every call acquires the lock, even after the object exists. This is slow.

Better: Place synchronized(){} block inside the method where the sensitive logic is, instead of making the whole method synchronized.

b) Synchronized block inside static factory method

public static Printer getInstance() {
    // singleton logic
    synchronized (Printer.class) {  // class level locking
        if (INSTANCE == null) {
            try {
                Thread.sleep(10000);
            } catch (Exception e) {
                e.printStackTrace();
            }
            INSTANCE = new Printer();
        }
    }
    return INSTANCE;
}

c) Double-checked locking (Best 1)

public class Printer {
    private static volatile Printer INSTANCE;

    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        if (INSTANCE == null) {  // 1st NULL check (no lock)
            System.out.println("Printer.getInstance()[outside sync block]: "
                + Thread.currentThread().getName());

            synchronized (Printer.class) {  // class level locking
                System.out.println("Printer.getInstance()[inside sync block]: "
                    + Thread.currentThread().getName());

                if (INSTANCE == null) {  // 2nd NULL check (with lock)
                    try {
                        Thread.sleep(10000);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    INSTANCE = new Printer();
                }
            }
        }
        return INSTANCE;
    }
}

d) Eager instantiation

public class Printer {
    // object created at class loading time itself (eager)
    private static final Printer INSTANCE = new Printer();

    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        return INSTANCE;  // no checking needed
    }
}

Since the object is the static reference variable, the JVM creates it at class loading time. The object is created as soon as the class is loaded, even if it is never used. Thread-safe because class loading is inherently thread-safe in JVM.

Drawback: Creates the object even if never needed (no lazy loading).

e) Inner class based lazy instantiation (Bill Pugh - Best 2)

public class Printer {
    private Printer() {
        System.out.println("Printer:: 0-param constructor");
    }

    // Inner class -- loaded only when getInstance() is called
    private static class InstanceHolder {
        private static final Printer INSTANCE = new Printer();
    }

    public static Printer getInstance() {
        return InstanceHolder.INSTANCE;
    }
}

f) ENUM based singleton (Best 3)

package com.nt.sdp;

public enum Printer3 {
    INSTANCE;  // enum constant -- internally: public static final Printer3 INSTANCE = new Printer3()

    public static Printer3 getInstance() {
        return INSTANCE;
    }

    // business method
    public void printMessage(String msg) {
        System.out.println("printing: " + msg);
    }
}

// Usage:
Printer3.INSTANCE.printMessage("Report #1234");

Why is Enum singleton the best?

1. No multithreading issues: Eager instantiation happens on enum constant like INSTANCE. There will not be any multi-threading issue.
2. Easy to develop: Writing less code with minimum coding standards.
3. Clone-safe: The clone() method is marked as protected final in the internal default super class java.lang.Enum<E>. So we cannot override clone() in our enum. clone() cannot be called on enum constants. Every enum already extends java.lang.Enum<E> class internally. It cannot be extended further (Java does not support multiple inheritance). So our enum is NOT Cloneable by default.
4. Serialization-safe: Every enum is Serializable by default. If we perform Serialization on enum constants, when we attempt to do Deserialization, we get already available enum obj/constant (i.e., some internal readResolve() method functionality is given).
5. Reflection-safe: For enums, we cannot create objects using Reflection API as they do not contain constructors.

Enum inheritance hierarchy

Enum singleton limitations

• a) We cannot apply few features of Java on enum like inheritance using "extends"
• b) We cannot place instance, static member variables in the enum representing the state of the object
• c) We cannot place serialVersionUID while working with Enum to perform serialization activity
• d) Eager instantiation based singleton is not ideal if the object is never used

7. Breaking Singleton (and How to Prevent)

a) Breaking via Cloning

If a singleton class implements Cloneable (directly or through inheritance), calling clone() creates a second object.

Prevention: Override clone()

// Option 1: Return existing instance (not recommended)
@Override
public Object clone() throws CloneNotSupportedException {
    return INSTANCE;
}

// Option 2: Throw exception (BEST solution)
@Override
public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException(
        "In this singleton class, cloning is restricted");
}

b) Breaking via Serialization and Deserialization

Prevention: readResolve() method

// Add this method in your singleton class
private Object readResolve() throws ObjectStreamException {
    return INSTANCE;
}

Full Serialization test code

package com.nt.test;

import java.io.*;
import com.nt.sdp.Printer;

public class SingletonSerializationTest {
    public static void doSerialization(Printer printer) {
        try (ObjectOutputStream oos = new ObjectOutputStream(
                new FileOutputStream("data.ser"))) {
            oos.writeObject(printer);
            oos.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("Object Serialization is done");
    }

    public static Printer doDeSerialization() {
        Printer p = null;
        try (ObjectInputStream ois = new ObjectInputStream(
                new FileInputStream("data.ser"))) {
            p = (Printer) ois.readObject();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return p;
    }

    public static void main(String[] args) {
        Printer p1 = Printer.getInstance();
        doSerialization(p1);

        System.out.println("--------DeSerialization--------");
        Printer p2 = doDeSerialization();
        Printer p3 = doDeSerialization();

        System.out.println(p1.hashCode() + " " + p2.hashCode() + " " + p3.hashCode());
    }
}

private static final long serialVersionUID = 1L;

java.io.InvalidClassException: com.nt.sdp.Printer;
local class incompatible: stream classdesc
serialVersionUID = 1104536345855,
local class serialVersionUID = 8110690610001600652

c) Breaking via Reflection API

Even if the constructor is private, the Reflection API can access it, set it accessible, and create multiple objects. This breaks the singleton guarantee.

package com.nt.test;

import java.lang.reflect.Constructor;

public class SingletonReflectionTest {
    public static void main(String[] args) throws Exception {
        // Normal way -- gives single object
        Printer p1 = Printer.getInstance();

        // Reflection API way -- breaks singleton
        Class c = Class.forName("com.nt.sdp.Printer");
        Constructor[] cons = c.getDeclaredConstructors();
        cons[0].setAccessible(true);  // bypass private access
        Printer p2 = (Printer) cons[0].newInstance();

        System.out.println(p1.hashCode() + " " + p2.hashCode());
        // Output: DIFFERENT hashCodes -- singleton is broken!
        System.out.println("p1 == p2? " + (p1 == p2));  // false
    }
}

Prevention: isInstantiated flag

public class Printer {
    private static Printer INSTANCE;
    private static boolean isInstantiated = false;

    private Printer() {
        // Stop reflection-based second object creation
        if (isInstantiated) {
            throw new RuntimeException(
                "Object is already created. Cannot create another.");
        }
        isInstantiated = true;
        System.out.println("Printer:: 0-param constructor");
    }

    public static Printer getInstance() {
        if (INSTANCE == null)
            INSTANCE = new Printer();
        return INSTANCE;
    }
}

8. Breaking Singleton via ClassLoader

JVM internally uses a ClassLoader subsystem to load Java classes. The system has three classloaders:

Notes on ClassLoader behavior

• If a class is loaded by parent classloader, the same class will NOT be loaded by child classloader (Principle of Delegation)
• The class loaded by parent classloader is visible to child classloader, but not the reverse (Principle of Visibility)
• When JVM gives a class to classloader subsystem for loading, it is delegated to all classloaders using the delegation principle

Breaking singleton using Custom ClassLoader

URLClassLoader is a readily available custom classloader which can load classes from specific jar files and also initialize them as needed.

// Using Application ClassLoader (default)
System.out.println("Using Application ClassLoader");
Printer p1 = Printer.getInstance();
Printer p2 = Printer.getInstance();
System.out.println(p1.hashCode() + " " + p2.hashCode());

System.out.println("---");

// Using Custom ClassLoader (URLClassLoader)
System.out.println("Using URLClassLoader");
URLClassLoader loader = new URLClassLoader(
    new URL[]{new URL("file:///c/myProj/jar5/1.jar")});
Class c1 = loader.loadClass("com.nt.sdp.Printer");
Constructor cons = c1.getDeclaredConstructors()[0];
cons.setAccessible(true);
Object obj = cons.newInstance();

System.out.println("object hash codes: " + obj.hashCode() + " " + p1.hashCode());

Here URLClassLoader is an independent classloader and not linked with JVM's classloader system. So one object is created using ApplicationClassLoader of ClassLoader sub system and another object will be using URLClassLoader (custom classloader).

9. Singleton Pattern vs Spring Singleton Scope

Spring IoC container creating one object for servlet comp class does not mean Spring is making classes as singletons. Spring is just having singleton behavior (one object for singleton scope spring bean), but the class is not a true singleton because there is no private constructor restriction. If you bypass the container, you could create more objects.

10. Full-Length Industry Project: BookMyShow Ticket Booking System

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

application.properties

# BookMyShow Configuration
app.name=BookMyShow
app.version=1.0.0

# Database
db.url=jdbc:mysql://localhost:3306/bookmyshow_db
db.username=bms_admin
db.password=encrypted_pwd
db.pool.size=20

# Cache
cache.default.ttl.seconds=300
cache.movies.ttl.seconds=600

# Booking
booking.max.seats.per.transaction=10
booking.payment.timeout.seconds=120

# Logging
log.file=bookmyshow.log
log.level=INFO

Connection.java (Simulated DB connection)

package com.bookmyshow.model;

import java.util.UUID;

public class Connection {
    private final String id;
    private final String url;
    private boolean inUse;

    public Connection(String url) {
        this.id = UUID.randomUUID().toString().substring(0, 8);
        this.url = url;
        this.inUse = false;
    }

    public String getId() { return id; }
    public String getUrl() { return url; }
    public boolean isInUse() { return inUse; }
    public void setInUse(boolean inUse) { this.inUse = inUse; }

    @Override
    public String toString() {
        return "Connection[" + id + "]";
    }
}

Movie.java

package com.bookmyshow.model;

public class Movie {
    private final String id;
    private final String title;
    private final String genre;
    private final int durationMinutes;

    public Movie(String id, String title, String genre, int durationMinutes) {
        this.id = id;
        this.title = title;
        this.genre = genre;
        this.durationMinutes = durationMinutes;
    }

    public String getId() { return id; }
    public String getTitle() { return title; }
    public String getGenre() { return genre; }
    public int getDurationMinutes() { return durationMinutes; }

    @Override
    public String toString() {
        return title + " (" + genre + ", " + durationMinutes + " min)";
    }
}

Show.java

package com.bookmyshow.model;

public class Show {
    private final String id;
    private final String movieId;
    private final String theatre;
    private final String time;
    private int availableSeats;
    private final double price;

    public Show(String id, String movieId, String theatre,
                String time, int availableSeats, double price) {
        this.id = id;
        this.movieId = movieId;
        this.theatre = theatre;
        this.time = time;
        this.availableSeats = availableSeats;
        this.price = price;
    }

    public String getId() { return id; }
    public String getMovieId() { return movieId; }
    public String getTheatre() { return theatre; }
    public String getTime() { return time; }
    public int getAvailableSeats() { return availableSeats; }
    public double getPrice() { return price; }

    public synchronized boolean reserveSeats(int count) {
        if (count > availableSeats) return false;
        availableSeats -= count;
        return true;
    }

    @Override
    public String toString() {
        return theatre + " | " + time + " | Seats: " + availableSeats
            + " | Rs " + price;
    }
}

Booking.java

package com.bookmyshow.model;

import java.time.LocalDateTime;
import java.util.UUID;

public class Booking {
    private final String id;
    private final String userId;
    private final String showId;
    private final int seats;
    private final double totalAmount;
    private final LocalDateTime bookedAt;
    private String status;

    public Booking(String userId, String showId, int seats, double totalAmount) {
        this.id = "BKG-" + UUID.randomUUID().toString().substring(0, 8).toUpperCase();
        this.userId = userId;
        this.showId = showId;
        this.seats = seats;
        this.totalAmount = totalAmount;
        this.bookedAt = LocalDateTime.now();
        this.status = "CONFIRMED";
    }

    public String getId() { return id; }
    public String getUserId() { return userId; }
    public String getShowId() { return showId; }
    public int getSeats() { return seats; }
    public double getTotalAmount() { return totalAmount; }
    public LocalDateTime getBookedAt() { return bookedAt; }
    public String getStatus() { return status; }
    public void setStatus(String status) { this.status = status; }

    @Override
    public String toString() {
        return "Booking{id=" + id + ", user=" + userId + ", show=" + showId
            + ", seats=" + seats + ", amount=Rs " + totalAmount
            + ", status=" + status + "}";
    }
}

ConfigManager.java (Enum Singleton)

package com.bookmyshow.singleton;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

/**
 * Application configuration manager.
 * Enum-based singleton: thread-safe, reflection-safe, serialization-safe.
 */
public enum ConfigManager {
    INSTANCE;

    private final Properties properties;

    ConfigManager() {
        this.properties = new Properties();
        try (InputStream stream = getClass().getClassLoader()
                .getResourceAsStream("application.properties")) {
            if (stream != null) {
                properties.load(stream);
            }
        } catch (IOException e) {
            throw new RuntimeException("Failed to load configuration", e);
        }
    }

    public String get(String key) {
        return properties.getProperty(key);
    }

    public String get(String key, String defaultValue) {
        return properties.getProperty(key, defaultValue);
    }

    public int getInt(String key, int defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        try {
            return Integer.parseInt(value.trim());
        } catch (NumberFormatException e) {
            return defaultValue;
        }
    }

    public boolean getBoolean(String key, boolean defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        return Boolean.parseBoolean(value.trim());
    }
}

AppLogger.java (Bill Pugh Singleton)

package com.bookmyshow.singleton;

import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

/**
 * Application logger.
 * Bill Pugh (inner class) singleton: lazy loaded, thread-safe, no synchronization overhead.
 */
public class AppLogger {
    private static final DateTimeFormatter FORMATTER =
        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");

    private final PrintWriter fileWriter;

    private AppLogger() {
        String logFile = ConfigManager.INSTANCE.get("log.file", "app.log");
        try {
            this.fileWriter = new PrintWriter(
                new FileWriter(logFile, true), true);
        } catch (IOException e) {
            throw new RuntimeException("Cannot open log file: " + logFile, e);
        }
    }

    // Inner class loaded only when getInstance() is first called
    private static class InstanceHolder {
        private static final AppLogger INSTANCE = new AppLogger();
    }

    public static AppLogger getInstance() {
        return InstanceHolder.INSTANCE;
    }

    public void info(String message) {
        log("INFO", message);
    }

    public void warn(String message) {
        log("WARN", message);
    }

    public void error(String message) {
        log("ERROR", message);
    }

    public void error(String message, Throwable throwable) {
        log("ERROR", message + " | " + throwable.getMessage());
    }

    private void log(String level, String message) {
        String timestamp = LocalDateTime.now().format(FORMATTER);
        String threadName = Thread.currentThread().getName();
        String entry = String.format("[%s] [%-5s] [%s] %s",
            timestamp, level, threadName, message);
        System.out.println(entry);
        fileWriter.println(entry);
    }
}

ConnectionPool.java (Double-Checked Locking Singleton)

package com.bookmyshow.singleton;

import com.bookmyshow.model.Connection;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Set;

/**
 * Database connection pool.
 * Double-checked locking singleton: lazy, thread-safe, minimal lock contention.
 */
public class ConnectionPool {
    private static volatile ConnectionPool INSTANCE;

    private final Queue<Connection> availableConnections;
    private final Set<Connection> usedConnections;
    private final int maxPoolSize;

    private ConnectionPool(int poolSize, String dbUrl) {
        this.maxPoolSize = poolSize;
        this.availableConnections = new LinkedList<>();
        this.usedConnections = new HashSet<>();

        for (int i = 0; i < poolSize; i++) {
            availableConnections.add(new Connection(dbUrl));
        }

        AppLogger.getInstance().info(
            "ConnectionPool initialized with " + poolSize + " connections");
    }

    public static ConnectionPool getInstance() {
        if (INSTANCE == null) {
            synchronized (ConnectionPool.class) {
                if (INSTANCE == null) {
                    String dbUrl = ConfigManager.INSTANCE.get("db.url");
                    int poolSize = ConfigManager.INSTANCE.getInt("db.pool.size", 10);
                    INSTANCE = new ConnectionPool(poolSize, dbUrl);
                }
            }
        }
        return INSTANCE;
    }

    public synchronized Connection getConnection() {
        if (availableConnections.isEmpty()) {
            AppLogger.getInstance().warn(
                "Connection pool exhausted. Used: " + usedConnections.size()
                + "/" + maxPoolSize);
            throw new RuntimeException("No available connections in pool");
        }
        Connection conn = availableConnections.poll();
        conn.setInUse(true);
        usedConnections.add(conn);
        return conn;
    }

    public synchronized void releaseConnection(Connection conn) {
        if (conn == null) return;
        conn.setInUse(false);
        usedConnections.remove(conn);
        availableConnections.offer(conn);
    }

    public int getAvailableCount() {
        return availableConnections.size();
    }

    public int getUsedCount() {
        return usedConnections.size();
    }
}

CacheManager.java (Inner Class Singleton with TTL)

package com.bookmyshow.singleton;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * In-memory cache with time-to-live (TTL) expiry.
 * Inner class singleton: lazy loaded, thread-safe.
 * Uses ConcurrentHashMap for thread-safe read/write.
 */
public class CacheManager {
    private final Map<String, CacheEntry> cache;
    private final long defaultTtlMs;

    private static class CacheEntry {
        final Object value;
        final long expiresAt;

        CacheEntry(Object value, long ttlMs) {
            this.value = value;
            this.expiresAt = System.currentTimeMillis() + ttlMs;
        }

        boolean isExpired() {
            return System.currentTimeMillis() > expiresAt;
        }
    }

    private CacheManager() {
        this.cache = new ConcurrentHashMap<>();
        int ttlSeconds = ConfigManager.INSTANCE.getInt(
            "cache.default.ttl.seconds", 300);
        this.defaultTtlMs = ttlSeconds * 1000L;

        AppLogger.getInstance().info(
            "CacheManager initialized with default TTL: " + ttlSeconds + "s");
    }

    private static class InstanceHolder {
        private static final CacheManager INSTANCE = new CacheManager();
    }

    public static CacheManager getInstance() {
        return InstanceHolder.INSTANCE;
    }

    public void put(String key, Object value) {
        put(key, value, defaultTtlMs);
    }

    public void put(String key, Object value, long ttlMs) {
        cache.put(key, new CacheEntry(value, ttlMs));
    }

    @SuppressWarnings("unchecked")
    public <T> T get(String key) {
        CacheEntry entry = cache.get(key);
        if (entry == null) return null;
        if (entry.isExpired()) {
            cache.remove(key);
            return null;
        }
        return (T) entry.value;
    }

    public void remove(String key) {
        cache.remove(key);
    }

    public void clear() {
        cache.clear();
    }

    public int size() {
        // Remove expired entries and return count
        cache.entrySet().removeIf(e -> e.getValue().isExpired());
        return cache.size();
    }
}

MovieService.java

package com.bookmyshow.service;

import com.bookmyshow.model.Connection;
import com.bookmyshow.model.Movie;
import com.bookmyshow.model.Show;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.Arrays;
import java.util.List;

public class MovieService {
    private final AppLogger logger = AppLogger.getInstance();
    private final CacheManager cache = CacheManager.getInstance();
    private final ConnectionPool pool = ConnectionPool.getInstance();

    public List<Movie> getAllMovies() {
        // Check cache first
        List<Movie> cached = cache.get("all_movies");
        if (cached != null) {
            logger.info("Movies loaded from cache");
            return cached;
        }

        // Cache miss: fetch from DB
        Connection conn = pool.getConnection();
        try {
            logger.info("Fetching movies from DB using " + conn);

            // Simulated DB query
            List<Movie> movies = Arrays.asList(
                new Movie("MOV001", "Pathaan", "Action", 146),
                new Movie("MOV002", "Jawan", "Action", 169),
                new Movie("MOV003", "Animal", "Drama", 201),
                new Movie("MOV004", "12th Fail", "Drama", 147),
                new Movie("MOV005", "Dunki", "Comedy-Drama", 161)
            );

            // Store in cache
            cache.put("all_movies", movies);
            logger.info("Movies cached. Count: " + movies.size());

            return movies;
        } finally {
            pool.releaseConnection(conn);
        }
    }

    public Movie getMovieById(String movieId) {
        Movie cached = cache.get("movie_" + movieId);
        if (cached != null) return cached;

        List<Movie> movies = getAllMovies();
        for (Movie m : movies) {
            if (m.getId().equals(movieId)) {
                cache.put("movie_" + movieId, m);
                return m;
            }
        }
        return null;
    }

    public List<Show> getShowsForMovie(String movieId) {
        List<Show> cached = cache.get("shows_" + movieId);
        if (cached != null) {
            logger.info("Shows for " + movieId + " loaded from cache");
            return cached;
        }

        Connection conn = pool.getConnection();
        try {
            logger.info("Fetching shows for " + movieId + " from DB using " + conn);

            // Simulated DB query
            List<Show> shows = Arrays.asList(
                new Show("SHW001", movieId, "PVR Phoenix Mall", "10:00 AM", 150, 250.0),
                new Show("SHW002", movieId, "PVR Phoenix Mall", "02:30 PM", 150, 300.0),
                new Show("SHW003", movieId, "INOX Lido", "06:00 PM", 200, 350.0),
                new Show("SHW004", movieId, "INOX Lido", "09:30 PM", 200, 400.0)
            );

            cache.put("shows_" + movieId, shows);
            return shows;
        } finally {
            pool.releaseConnection(conn);
        }
    }
}

BookingService.java

package com.bookmyshow.service;

import com.bookmyshow.model.Booking;
import com.bookmyshow.model.Connection;
import com.bookmyshow.model.Show;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConfigManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.List;

public class BookingService {
    private final AppLogger logger = AppLogger.getInstance();
    private final ConnectionPool pool = ConnectionPool.getInstance();
    private final CacheManager cache = CacheManager.getInstance();

    public Booking bookTicket(String userId, String showId, int seatCount) {
        int maxSeats = ConfigManager.INSTANCE.getInt(
            "booking.max.seats.per.transaction", 10);

        if (seatCount <= 0 || seatCount > maxSeats) {
            logger.error("Invalid seat count: " + seatCount
                + ". Max allowed: " + maxSeats);
            throw new IllegalArgumentException(
                "Seat count must be between 1 and " + maxSeats);
        }

        Connection conn = pool.getConnection();
        try {
            logger.info("Booking " + seatCount + " seats for user "
                + userId + " on show " + showId);

            // Find the show and reserve seats
            Show show = findShow(showId);
            if (show == null) {
                logger.error("Show not found: " + showId);
                throw new RuntimeException("Show not found: " + showId);
            }

            boolean reserved = show.reserveSeats(seatCount);
            if (!reserved) {
                logger.warn("Not enough seats for show " + showId
                    + ". Requested: " + seatCount
                    + ", Available: " + show.getAvailableSeats());
                throw new RuntimeException(
                    "Not enough seats. Available: " + show.getAvailableSeats());
            }

            double totalAmount = show.getPrice() * seatCount;
            Booking booking = new Booking(userId, showId, seatCount, totalAmount);

            // Invalidate show cache since seats changed
            cache.remove("shows_" + show.getMovieId());

            logger.info("Booking confirmed: " + booking.getId()
                + " | Amount: Rs " + totalAmount);

            return booking;
        } finally {
            pool.releaseConnection(conn);
        }
    }

    public boolean cancelBooking(Booking booking) {
        logger.info("Cancelling booking: " + booking.getId());
        booking.setStatus("CANCELLED");
        logger.info("Booking cancelled: " + booking.getId());
        return true;
    }

    private Show findShow(String showId) {
        // Search across all cached shows
        MovieService movieService = new MovieService();
        for (var movie : movieService.getAllMovies()) {
            List<Show> shows = movieService.getShowsForMovie(movie.getId());
            for (Show s : shows) {
                if (s.getId().equals(showId)) return s;
            }
        }
        return null;
    }
}

Application.java (Main Entry Point)

package com.bookmyshow;

import com.bookmyshow.model.Booking;
import com.bookmyshow.model.Movie;
import com.bookmyshow.model.Show;
import com.bookmyshow.service.BookingService;
import com.bookmyshow.service.MovieService;
import com.bookmyshow.singleton.AppLogger;
import com.bookmyshow.singleton.CacheManager;
import com.bookmyshow.singleton.ConfigManager;
import com.bookmyshow.singleton.ConnectionPool;
import java.util.List;

public class Application {
    public static void main(String[] args) throws InterruptedException {
        AppLogger logger = AppLogger.getInstance();
        logger.info("=== BookMyShow Application Started ===");
        logger.info("App: " + ConfigManager.INSTANCE.get("app.name")
            + " v" + ConfigManager.INSTANCE.get("app.version"));

        // ---- 1. VERIFY ALL FOUR SINGLETONS ----
        System.out.println("\n--- Singleton Verification ---");

        ConfigManager c1 = ConfigManager.INSTANCE;
        ConfigManager c2 = ConfigManager.INSTANCE;
        System.out.println("ConfigManager same?    " + (c1 == c2)
            + " | hashCode: " + c1.hashCode());

        AppLogger l1 = AppLogger.getInstance();
        AppLogger l2 = AppLogger.getInstance();
        System.out.println("AppLogger same?        " + (l1 == l2)
            + " | hashCode: " + l1.hashCode());

        ConnectionPool p1 = ConnectionPool.getInstance();
        ConnectionPool p2 = ConnectionPool.getInstance();
        System.out.println("ConnectionPool same?   " + (p1 == p2)
            + " | hashCode: " + p1.hashCode());

        CacheManager m1 = CacheManager.getInstance();
        CacheManager m2 = CacheManager.getInstance();
        System.out.println("CacheManager same?     " + (m1 == m2)
            + " | hashCode: " + m1.hashCode());

        // ---- 2. BROWSE MOVIES ----
        System.out.println("\n--- Available Movies ---");
        MovieService movieService = new MovieService();
        List<Movie> movies = movieService.getAllMovies();
        for (Movie m : movies) {
            System.out.println("  " + m);
        }

        // Call again -- should come from cache
        System.out.println("\n--- Movies (2nd call - from cache) ---");
        movieService.getAllMovies();

        // ---- 3. BROWSE SHOWS ----
        System.out.println("\n--- Shows for Pathaan ---");
        List<Show> shows = movieService.getShowsForMovie("MOV001");
        for (Show s : shows) {
            System.out.println("  " + s);
        }

        // ---- 4. BOOK TICKETS ----
        System.out.println("\n--- Booking Tickets ---");
        BookingService bookingService = new BookingService();

        Booking booking1 = bookingService.bookTicket("USER001", "SHW003", 4);
        System.out.println("  " + booking1);

        Booking booking2 = bookingService.bookTicket("USER002", "SHW003", 2);
        System.out.println("  " + booking2);

        // ---- 5. CANCEL BOOKING ----
        System.out.println("\n--- Cancel Booking ---");
        bookingService.cancelBooking(booking1);
        System.out.println("  " + booking1);

        // ---- 6. CONNECTION POOL STATUS ----
        System.out.println("\n--- Connection Pool Status ---");
        System.out.println("  Available: " + p1.getAvailableCount());
        System.out.println("  In use:    " + p1.getUsedCount());

        // ---- 7. CACHE STATUS ----
        System.out.println("\n--- Cache Status ---");
        System.out.println("  Cached items: " + m1.size());

        // ---- 8. MULTITHREADED BOOKING TEST ----
        System.out.println("\n--- Multithreaded Booking Test (5 users) ---");
        Thread[] threads = new Thread[5];
        for (int i = 0; i < 5; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                try {
                    BookingService svc = new BookingService();
                    Booking b = svc.bookTicket("USER_MT_" + idx, "SHW004", 2);
                    System.out.println("  Thread-" + idx + ": " + b.getId()
                        + " | Seats: " + b.getSeats()
                        + " | Amount: Rs " + b.getTotalAmount());
                } catch (Exception e) {
                    System.out.println("  Thread-" + idx + ": FAILED - "
                        + e.getMessage());
                }
            }, "BookingThread-" + i);
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        System.out.println("\n--- Final Pool Status ---");
        System.out.println("  Available: " + p1.getAvailableCount());
        System.out.println("  In use:    " + p1.getUsedCount());

        logger.info("=== BookMyShow Application Finished ===");
    }
}

SingletonThreadSafetyTest.java

package com.bookmyshow.singleton;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class SingletonThreadSafetyTest {

    @Test
    void configManager_isSingleton() {
        assertSame(ConfigManager.INSTANCE, ConfigManager.INSTANCE);
    }

    @Test
    void appLogger_isSingleton() {
        assertSame(AppLogger.getInstance(), AppLogger.getInstance());
    }

    @Test
    void connectionPool_isSingleton() {
        assertSame(ConnectionPool.getInstance(), ConnectionPool.getInstance());
    }

    @Test
    void cacheManager_isSingleton() {
        assertSame(CacheManager.getInstance(), CacheManager.getInstance());
    }

    @Test
    void allSingletons_threadSafe() throws InterruptedException {
        int threadCount = 100;
        AppLogger[] loggers = new AppLogger[threadCount];
        ConnectionPool[] pools = new ConnectionPool[threadCount];
        CacheManager[] caches = new CacheManager[threadCount];
        Thread[] threads = new Thread[threadCount];

        for (int i = 0; i < threadCount; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                loggers[idx] = AppLogger.getInstance();
                pools[idx] = ConnectionPool.getInstance();
                caches[idx] = CacheManager.getInstance();
            });
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        for (int i = 1; i < threadCount; i++) {
            assertSame(loggers[0], loggers[i],
                "AppLogger must be same across all threads");
            assertSame(pools[0], pools[i],
                "ConnectionPool must be same across all threads");
            assertSame(caches[0], caches[i],
                "CacheManager must be same across all threads");
        }
    }

    @Test
    void connectionPool_getAndRelease() {
        ConnectionPool pool = ConnectionPool.getInstance();
        int before = pool.getAvailableCount();

        var conn = pool.getConnection();
        assertEquals(before - 1, pool.getAvailableCount());
        assertEquals(1, pool.getUsedCount());

        pool.releaseConnection(conn);
        assertEquals(before, pool.getAvailableCount());
        assertEquals(0, pool.getUsedCount());
    }

    @Test
    void cacheManager_putAndGet() {
        CacheManager cache = CacheManager.getInstance();
        cache.put("test_key", "test_value");
        assertEquals("test_value", cache.get("test_key"));
    }

    @Test
    void cacheManager_expiredEntryReturnsNull() throws InterruptedException {
        CacheManager cache = CacheManager.getInstance();
        cache.put("expire_key", "value", 50); // 50ms TTL
        Thread.sleep(100);
        assertNull(cache.get("expire_key"));
    }

    @Test
    void configManager_readsProperties() {
        assertEquals("BookMyShow", ConfigManager.INSTANCE.get("app.name"));
        assertEquals(20, ConfigManager.INSTANCE.getInt("db.pool.size", 5));
    }
}

Expected Output

Key observations from the project

11. Smaller Example: Configuration Manager

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

application.properties

# Database Configuration
db.url=jdbc:mysql://localhost:3306/national_bank_db
db.username=nbi_admin
db.password=encrypted_pwd_here
db.pool.size=20
db.pool.timeout=30000

# API Configuration
api.base.url=https://api.nationalbankofIndia.com/v1
api.timeout=5000
api.retry.count=3

# Feature Flags
feature.upi.enabled=true
feature.international.transfer.enabled=false
feature.sms.notification.enabled=true

# Notification
notification.sms.gateway=https://sms.gateway.nbi.com
notification.email.from=noreply@nationalbankofIndia.com

ConfigManager.java

package com.nbi.banking.config;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.Properties;
import java.util.Set;

public class ConfigManager implements Serializable {
    private static final long serialVersionUID = 1L;
    private static volatile ConfigManager INSTANCE;
    private static boolean isInstantiated = false;

    private final Properties properties;

    private ConfigManager() {
        // Reflection protection
        if (isInstantiated) {
            throw new RuntimeException(
                "ConfigManager is a singleton. Use getInstance() instead.");
        }
        isInstantiated = true;

        this.properties = new Properties();
        loadProperties();
    }

    public static ConfigManager getInstance() {
        if (INSTANCE == null) {
            synchronized (ConfigManager.class) {
                if (INSTANCE == null) {
                    INSTANCE = new ConfigManager();
                }
            }
        }
        return INSTANCE;
    }

    public String get(String key) {
        return properties.getProperty(key);
    }

    public String get(String key, String defaultValue) {
        return properties.getProperty(key, defaultValue);
    }

    public int getInt(String key, int defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        try {
            return Integer.parseInt(value.trim());
        } catch (NumberFormatException e) {
            return defaultValue;
        }
    }

    public boolean getBoolean(String key, boolean defaultValue) {
        String value = properties.getProperty(key);
        if (value == null) return defaultValue;
        return Boolean.parseBoolean(value.trim());
    }

    public Set getAllKeys() {
        return properties.stringPropertyNames();
    }

    public synchronized void reload() {
        properties.clear();
        loadProperties();
        System.out.println("Configuration reloaded successfully");
    }

    private void loadProperties() {
        try (InputStream stream = getClass().getClassLoader()
                .getResourceAsStream("application.properties")) {
            if (stream == null) {
                throw new RuntimeException(
                    "application.properties not found on classpath");
            }
            properties.load(stream);
            System.out.println("Loaded " + properties.size()
                + " configuration properties");
        } catch (IOException e) {
            throw new RuntimeException(
                "Failed to load configuration", e);
        }
    }

    // Serialization protection
    private Object readResolve() throws ObjectStreamException {
        return INSTANCE;
    }

    // Clone protection
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException(
            "ConfigManager is a singleton. Cloning is not allowed.");
    }
}

AccountService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;

public class AccountService {
    private final ConfigManager config = ConfigManager.getInstance();

    public void getAccountDetails(String accountNumber) {
        String dbUrl = config.get("db.url");
        int poolSize = config.getInt("db.pool.size", 10);

        System.out.println("Fetching account " + accountNumber
            + " from database: " + dbUrl
            + " (pool size: " + poolSize + ")");
        // actual DB query logic here
    }
}

TransferService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;
import java.math.BigDecimal;

public class TransferService {
    private final ConfigManager config = ConfigManager.getInstance();

    public boolean transfer(String from, String to, BigDecimal amount) {
        boolean upiEnabled = config.getBoolean("feature.upi.enabled", false);
        int timeout = config.getInt("api.timeout", 5000);

        System.out.println("Transfer " + amount + " from " + from + " to " + to);
        System.out.println("UPI enabled: " + upiEnabled + ", timeout: " + timeout + "ms");

        // Verify same config instance across services
        System.out.println("Config instance hashCode: "
            + config.hashCode());
        return true;
    }
}

NotificationService.java

package com.nbi.banking.service;

import com.nbi.banking.config.ConfigManager;

public class NotificationService {
    private final ConfigManager config = ConfigManager.getInstance();

    public void sendSmsNotification(String phone, String message) {
        boolean smsEnabled = config.getBoolean(
            "feature.sms.notification.enabled", false);

        if (!smsEnabled) {
            System.out.println("SMS notification is disabled");
            return;
        }

        String gateway = config.get("notification.sms.gateway");
        System.out.println("Sending SMS via " + gateway
            + " to " + phone + ": " + message);

        // Verify same config instance
        System.out.println("Config instance hashCode: "
            + config.hashCode());
    }
}

Application.java

package com.nbi.banking;

import com.nbi.banking.config.ConfigManager;
import com.nbi.banking.service.AccountService;
import com.nbi.banking.service.TransferService;
import com.nbi.banking.service.NotificationService;
import java.math.BigDecimal;

public class Application {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("=== National Bank of India - Banking Application ===");
        System.out.println();

        // 1. Verify singleton behavior
        ConfigManager config1 = ConfigManager.getInstance();
        ConfigManager config2 = ConfigManager.getInstance();
        System.out.println("Singleton verification:");
        System.out.println("  config1 hashCode: " + config1.hashCode());
        System.out.println("  config2 hashCode: " + config2.hashCode());
        System.out.println("  config1 == config2? " + (config1 == config2));
        System.out.println();

        // 2. Use services (all share same ConfigManager instance)
        AccountService accountService = new AccountService();
        TransferService transferService = new TransferService();
        NotificationService notificationService = new NotificationService();

        accountService.getAccountDetails("SAV2001");
        System.out.println();

        transferService.transfer("SAV2001", "CUR3001",
            new BigDecimal("50000.00"));
        System.out.println();

        notificationService.sendSmsNotification(
            "+91-9876543210",
            "Transfer of Rs 50,000 from SAV2001 to CUR3001 completed");
        System.out.println();

        // 3. Multithreaded access verification
        System.out.println("=== Multithreaded Access Test ===");
        Thread[] threads = new Thread[10];
        int[] hashCodes = new int[10];

        for (int i = 0; i < 10; i++) {
            final int idx = i;
            threads[i] = new Thread(() -> {
                ConfigManager cm = ConfigManager.getInstance();
                hashCodes[idx] = cm.hashCode();
                System.out.println("Thread-" + idx
                    + " config hashCode: " + cm.hashCode());
            });
        }

        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        // Verify all threads got the same instance
        boolean allSame = true;
        for (int i = 1; i < 10; i++) {
            if (hashCodes[i] != hashCodes[0]) {
                allSame = false;
                break;
            }
        }
        System.out.println("All threads got same instance? " + allSame);
        System.out.println();

        // 4. Read all configuration keys
        System.out.println("=== All Configuration Keys ===");
        for (String key : config1.getAllKeys()) {
            System.out.println("  " + key + " = " + config1.get(key));
        }
    }
}

ConfigManagerTest.java

package com.nbi.banking.config;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class ConfigManagerTest {

    @Test
    void getInstance_returnsSameInstance() {
        ConfigManager c1 = ConfigManager.getInstance();
        ConfigManager c2 = ConfigManager.getInstance();
        assertSame(c1, c2);
    }

    @Test
    void getInstance_isThreadSafe() throws InterruptedException {
        ConfigManager[] results = new ConfigManager[100];
        Thread[] threads = new Thread[100];

        for (int i = 0; i < 100; i++) {
            final int idx = i;
            threads[i] = new Thread(
                () -> results[idx] = ConfigManager.getInstance());
        }
        for (Thread t : threads) t.start();
        for (Thread t : threads) t.join();

        for (ConfigManager result : results) {
            assertSame(results[0], result);
        }
    }

    @Test
    void get_returnsConfigValue() {
        ConfigManager config = ConfigManager.getInstance();
        String dbUrl = config.get("db.url");
        assertNotNull(dbUrl);
        assertTrue(dbUrl.contains("national_bank_db"));
    }

    @Test
    void getInt_returnsIntValue() {
        ConfigManager config = ConfigManager.getInstance();
        int poolSize = config.getInt("db.pool.size", 5);
        assertEquals(20, poolSize);
    }

    @Test
    void getInt_returnsDefaultForMissingKey() {
        ConfigManager config = ConfigManager.getInstance();
        int value = config.getInt("nonexistent.key", 42);
        assertEquals(42, value);
    }

    @Test
    void getBoolean_returnsBooleanValue() {
        ConfigManager config = ConfigManager.getInstance();
        assertTrue(config.getBoolean("feature.upi.enabled", false));
        assertFalse(config.getBoolean(
            "feature.international.transfer.enabled", true));
    }

    @Test
    void clone_throwsException() {
        ConfigManager config = ConfigManager.getInstance();
        assertThrows(CloneNotSupportedException.class,
            () -> config.clone());
    }

    @Test
    void getAllKeys_returnsNonEmptySet() {
        ConfigManager config = ConfigManager.getInstance();
        assertFalse(config.getAllKeys().isEmpty());
    }
}

Expected Output

12. When to Use and When NOT to Use

When to use Singleton

When NOT to use Singleton