1. What is Service Locator Pattern?

Local Services vs External Services

How external services are discovered

To make our logics/services as external services or distributed comps, we need to take the support of Distributed Technologies like EJB (old), RMI (old), WebServices (SOAP or REST based).

The distributed comps/external comps keep their comp details in special registries to expose the details to different local or remote client projects. The special registries are like:

• JNDI Registry (Java Naming and Directory Interface)
• RMI Registry
• UDDI Registry (for SOAP WebServices)

The remote or local Client Apps/Projects search for external services/distributed comp in JNDI registry or UDDI registry and get references from those registries in order to consume the services of external service or distributed comp.

Real-world examples of client-service relationships

• Flipkart, Amazon, Myntra, Snapdeal are Client Apps using GPay, PhonePe, BharatPe as external services (UPI payments)
• Sharekhan, Upstox, Zerodha, Paisa, Angel One are Client Apps using BSE, NSE Comps as external services
• CrickBuzz, CricInfo, Dream11, ESPNsports are Client Apps using ICC Score Comps as external services
• Flipkart, Amazon, Myntra are Client Apps using PayPal, PayTM, Razorpay as external services (card payment)

2. The Problem: JNDI Lookup in BusinessDelegate

3. The Solution: Service Locator with Cache

Advantages of Service Locator

1. Reusable lookup code: Separates the JNDI lookup from BusinessDelegate classes. Makes the code reusable across multiple BusinessDelegates.
2. Loose coupling with external comp: Any change in external comp location and technology, we just need to modify the code of Service Locator, not the BusinessDelegate classes.
3. Loose coupling with registry: Any change in JNDI registry location and technology, we just need to modify the code of Service Locator, not the BusinessDelegate classes.
4. Reduced network round trips: The buffer/cache maintained by Service Locator reduces the network round trips between client application and JNDI registry.

4. CrickBuzz Example: Consuming ICC Score Service

Different options for CrickBuzz.com to display updated scores:

1. Send a person to match area, gets up to scores minute by minute by phone and enter in the website (lots of manual parties, not recommended)
2. ICC organization provides its DB username, password details to CrickBuzz.com to connect to DB directly (gives unrestricted access to DB, can corrupt the DB data)
3. Best: ICC develops one external service (ICCScoreComp) and keeps its details in JNDI registry to expose to multiple clients. All client apps access the DB of ICC having scores mentioned.

If the external comp (ICCScoreComp) is developed in EJB or RMI, then the client app (CrickBuzz, CricInfo) should also be developed in Java language. If the external comp is developed as WebServices, then the client app can be developed in any language (Java, .NET, PHP, Python, etc.).

5. Full-Length Industry Project: Stock Trading System (ShareKhan)

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

ServiceLocator.java (Singleton with cache)

package com.sharekhan.locator;

import java.util.HashMap;
import java.util.Map;

public class ServiceLocator {
    private static volatile ServiceLocator INSTANCE;
    private final Map<String, Object> cache = new HashMap<>();

    private ServiceLocator() {
        System.out.println("[ServiceLocator] Initialized (singleton)");
    }

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

    @SuppressWarnings("unchecked")
    public <T> T getService(String jndiName, Class<T> serviceType) {
        // Check cache first
        if (cache.containsKey(jndiName)) {
            System.out.println("[ServiceLocator] Cache HIT for: " + jndiName);
            return (T) cache.get(jndiName);
        }

        // Cache miss: perform JNDI lookup (simulated)
        System.out.println("[ServiceLocator] Cache MISS. Looking up: " + jndiName);
        T service = performLookup(jndiName, serviceType);

        // Cache the reference
        cache.put(jndiName, service);
        System.out.println("[ServiceLocator] Cached: " + jndiName);

        return service;
    }

    @SuppressWarnings("unchecked")
    private <T> T performLookup(String jndiName, Class<T> serviceType) {
        // Simulated JNDI/registry lookup
        // In real app: InitialContext ctx = new InitialContext();
        //              return (T) ctx.lookup(jndiName);
        System.out.println("[ServiceLocator] JNDI lookup for: " + jndiName
            + " (network call to registry)");

        // Simulate creating the service based on JNDI name
        switch (jndiName) {
            case "stockFacade":
                return (T) new com.sharekhan.facade.StockFacadeImpl();
            default:
                throw new RuntimeException("Service not found in registry: " + jndiName);
        }
    }

    public int getCacheSize() {
        return cache.size();
    }
}

External Services (simulated)

// IGetStockId.java
package com.sharekhan.external;
public interface IGetStockId {
    int getStockId(String stockName);
}

// GetStockIdImpl.java
package com.sharekhan.external;
public class GetStockIdImpl implements IGetStockId {
    public int getStockId(String stockName) {
        System.out.println("  [External Service 1] Looking up ID for: " + stockName);
        switch (stockName.toUpperCase()) {
            case "RELIANCE": return 500325;
            case "TCS": return 532540;
            case "INFOSYS": return 500209;
            case "HDFC": return 500180;
            default: return -1;
        }
    }
}

// IGetStockValue.java
package com.sharekhan.external;
public interface IGetStockValue {
    double getStockValue(int stockId);
}

// GetStockValueImpl.java
package com.sharekhan.external;
public class GetStockValueImpl implements IGetStockValue {
    public double getStockValue(int stockId) {
        System.out.println("  [External Service 2] Getting value for ID: " + stockId);
        switch (stockId) {
            case 500325: return 2485.50;  // Reliance
            case 532540: return 3678.25;  // TCS
            case 500209: return 1542.75;  // Infosys
            case 500180: return 1623.00;  // HDFC
            default: return 0.0;
        }
    }
}

// IGetStockFutureValue.java
package com.sharekhan.external;
public interface IGetStockFutureValue {
    double getFutureValue(int stockId);
}

// GetStockFutureValueImpl.java
package com.sharekhan.external;
public class GetStockFutureValueImpl implements IGetStockFutureValue {
    public double getFutureValue(int stockId) {
        System.out.println("  [External Service 3] Predicting future for ID: " + stockId);
        switch (stockId) {
            case 500325: return 2650.00;
            case 532540: return 3900.00;
            case 500209: return 1680.00;
            case 500180: return 1750.00;
            default: return 0.0;
        }
    }
}

StockDetailsDTO.java

package com.sharekhan.dto;

public class StockDetailsDTO {
    private final String stockName;
    private final int stockId;
    private final double currentValue;
    private final double futureValue;
    private final double changePercent;

    public StockDetailsDTO(String stockName, int stockId,
                            double currentValue, double futureValue) {
        this.stockName = stockName;
        this.stockId = stockId;
        this.currentValue = currentValue;
        this.futureValue = futureValue;
        this.changePercent = ((futureValue - currentValue) / currentValue) * 100;
    }

    public String getStockName() { return stockName; }
    public int getStockId() { return stockId; }
    public double getCurrentValue() { return currentValue; }
    public double getFutureValue() { return futureValue; }
    public double getChangePercent() { return changePercent; }

    @Override
    public String toString() {
        return String.format("%s (ID: %d) | Current: Rs %.2f | Future: Rs %.2f | Change: %.1f%%",
            stockName, stockId, currentValue, futureValue, changePercent);
    }
}

IStockFacade.java and StockFacadeImpl.java (Session Facade)

// IStockFacade.java
package com.sharekhan.facade;

import com.sharekhan.dto.StockDetailsDTO;

public interface IStockFacade {
    StockDetailsDTO getCompleteStockInfo(String stockName);
}

// StockFacadeImpl.java (aggregates 3 external services into ONE call)
package com.sharekhan.facade;

import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.external.*;

public class StockFacadeImpl implements IStockFacade {
    private final IGetStockId stockIdService;
    private final IGetStockValue stockValueService;
    private final IGetStockFutureValue stockFutureService;

    public StockFacadeImpl() {
        this.stockIdService = new GetStockIdImpl();
        this.stockValueService = new GetStockValueImpl();
        this.stockFutureService = new GetStockFutureValueImpl();
        System.out.println("[StockFacade] Initialized with 3 external services");
    }

    @Override
    public StockDetailsDTO getCompleteStockInfo(String stockName) {
        System.out.println("[StockFacade] Aggregating data for: " + stockName);

        // Call 3 external services (server-side, no extra network calls)
        int stockId = stockIdService.getStockId(stockName);
        if (stockId == -1) {
            throw new RuntimeException("Stock not found: " + stockName);
        }

        double currentValue = stockValueService.getStockValue(stockId);
        double futureValue = stockFutureService.getFutureValue(stockId);

        return new StockDetailsDTO(stockName, stockId, currentValue, futureValue);
    }
}

StockMgmt.java (BusinessDelegate)

package com.sharekhan.delegate;

import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.facade.IStockFacade;
import com.sharekhan.locator.ServiceLocator;

public class StockMgmt {

    public StockDetailsDTO getStockDetails(String stockName) {
        // Get StockFacade ref from ServiceLocator (cached after first lookup)
        IStockFacade facade = ServiceLocator.getInstance()
            .getService("stockFacade", IStockFacade.class);

        // Invoke external comp method through facade
        return facade.getCompleteStockInfo(stockName);
    }
}

Application.java

package com.sharekhan;

import com.sharekhan.delegate.StockMgmt;
import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.locator.ServiceLocator;

public class Application {
    public static void main(String[] args) {
        StockMgmt delegate = new StockMgmt();

        // Scenario 1: First request (JNDI lookup + cache)
        System.out.println("=== Scenario 1: First Request (Reliance) ===");
        StockDetailsDTO reliance = delegate.getStockDetails("Reliance");
        System.out.println("Result: " + reliance);
        System.out.println();

        // Scenario 2: Second request (cache hit, no JNDI lookup)
        System.out.println("=== Scenario 2: Second Request (TCS) ===");
        StockDetailsDTO tcs = delegate.getStockDetails("TCS");
        System.out.println("Result: " + tcs);
        System.out.println();

        // Scenario 3: Third request (cache hit again)
        System.out.println("=== Scenario 3: Third Request (Infosys) ===");
        StockDetailsDTO infosys = delegate.getStockDetails("Infosys");
        System.out.println("Result: " + infosys);
        System.out.println();

        // Scenario 4: Verify ServiceLocator is singleton with cache
        System.out.println("=== Scenario 4: ServiceLocator Stats ===");
        System.out.println("Cache size: " + ServiceLocator.getInstance().getCacheSize());
        System.out.println("ServiceLocator hashCode: " + ServiceLocator.getInstance().hashCode());

        System.out.println();

        // Scenario 5: Invalid stock
        System.out.println("=== Scenario 5: Invalid Stock ===");
        try {
            delegate.getStockDetails("UNKNOWN_STOCK");
        } catch (RuntimeException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Expected Output

Key observations from the project

6. Service Locator Combined with Other Patterns

7. When to Use and When NOT to Use

When to use Service Locator

When NOT to use (modern alternatives)

8. Quick Reference Card

Service Locator Design Pattern in Java

Complete Tutorial | 10 Sections | Full Stock Trading Project with HLD and LLD | JNDI Lookup Caching | Local vs External Services | Combined with Session Facade | Multiple Scenarios | 12 Interview Questions

1. What is Service Locator Pattern?

Local Services vs External Services

How external services are discovered

To make our logics/services as external services or distributed comps, we need to take the support of Distributed Technologies like EJB (old), RMI (old), WebServices (SOAP or REST based).

The distributed comps/external comps keep their comp details in special registries to expose the details to different local or remote client projects. The special registries are like:

• JNDI Registry (Java Naming and Directory Interface)
• RMI Registry
• UDDI Registry (for SOAP WebServices)

The remote or local Client Apps/Projects search for external services/distributed comp in JNDI registry or UDDI registry and get references from those registries in order to consume the services of external service or distributed comp.

Real-world examples of client-service relationships

• Flipkart, Amazon, Myntra, Snapdeal are Client Apps using GPay, PhonePe, BharatPe as external services (UPI payments)
• Sharekhan, Upstox, Zerodha, Paisa, Angel One are Client Apps using BSE, NSE Comps as external services
• CrickBuzz, CricInfo, Dream11, ESPNsports are Client Apps using ICC Score Comps as external services
• Flipkart, Amazon, Myntra are Client Apps using PayPal, PayTM, Razorpay as external services (card payment)

2. The Problem: JNDI Lookup in BusinessDelegate

3. The Solution: Service Locator with Cache

Advantages of Service Locator

1. Reusable lookup code: Separates the JNDI lookup from BusinessDelegate classes. Makes the code reusable across multiple BusinessDelegates.
2. Loose coupling with external comp: Any change in external comp location and technology, we just need to modify the code of Service Locator, not the BusinessDelegate classes.
3. Loose coupling with registry: Any change in JNDI registry location and technology, we just need to modify the code of Service Locator, not the BusinessDelegate classes.
4. Reduced network round trips: The buffer/cache maintained by Service Locator reduces the network round trips between client application and JNDI registry.

4. CrickBuzz Example: Consuming ICC Score Service

Different options for CrickBuzz.com to display updated scores:

1. Send a person to match area, gets up to scores minute by minute by phone and enter in the website (lots of manual parties, not recommended)
2. ICC organization provides its DB username, password details to CrickBuzz.com to connect to DB directly (gives unrestricted access to DB, can corrupt the DB data)
3. Best: ICC develops one external service (ICCScoreComp) and keeps its details in JNDI registry to expose to multiple clients. All client apps access the DB of ICC having scores mentioned.

If the external comp (ICCScoreComp) is developed in EJB or RMI, then the client app (CrickBuzz, CricInfo) should also be developed in Java language. If the external comp is developed as WebServices, then the client app can be developed in any language (Java, .NET, PHP, Python, etc.).

5. Full-Length Industry Project: Stock Trading System (ShareKhan)

High-Level Design (HLD)

Low-Level Design (LLD)

Project Structure

ServiceLocator.java (Singleton with cache)

package com.sharekhan.locator;

import java.util.HashMap;
import java.util.Map;

public class ServiceLocator {
    private static volatile ServiceLocator INSTANCE;
    private final Map<String, Object> cache = new HashMap<>();

    private ServiceLocator() {
        System.out.println("[ServiceLocator] Initialized (singleton)");
    }

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

    @SuppressWarnings("unchecked")
    public <T> T getService(String jndiName, Class<T> serviceType) {
        // Check cache first
        if (cache.containsKey(jndiName)) {
            System.out.println("[ServiceLocator] Cache HIT for: " + jndiName);
            return (T) cache.get(jndiName);
        }

        // Cache miss: perform JNDI lookup (simulated)
        System.out.println("[ServiceLocator] Cache MISS. Looking up: " + jndiName);
        T service = performLookup(jndiName, serviceType);

        // Cache the reference
        cache.put(jndiName, service);
        System.out.println("[ServiceLocator] Cached: " + jndiName);

        return service;
    }

    @SuppressWarnings("unchecked")
    private <T> T performLookup(String jndiName, Class<T> serviceType) {
        // Simulated JNDI/registry lookup
        // In real app: InitialContext ctx = new InitialContext();
        //              return (T) ctx.lookup(jndiName);
        System.out.println("[ServiceLocator] JNDI lookup for: " + jndiName
            + " (network call to registry)");

        // Simulate creating the service based on JNDI name
        switch (jndiName) {
            case "stockFacade":
                return (T) new com.sharekhan.facade.StockFacadeImpl();
            default:
                throw new RuntimeException("Service not found in registry: " + jndiName);
        }
    }

    public int getCacheSize() {
        return cache.size();
    }
}

External Services (simulated)

// IGetStockId.java
package com.sharekhan.external;
public interface IGetStockId {
    int getStockId(String stockName);
}

// GetStockIdImpl.java
package com.sharekhan.external;
public class GetStockIdImpl implements IGetStockId {
    public int getStockId(String stockName) {
        System.out.println("  [External Service 1] Looking up ID for: " + stockName);
        switch (stockName.toUpperCase()) {
            case "RELIANCE": return 500325;
            case "TCS": return 532540;
            case "INFOSYS": return 500209;
            case "HDFC": return 500180;
            default: return -1;
        }
    }
}

// IGetStockValue.java
package com.sharekhan.external;
public interface IGetStockValue {
    double getStockValue(int stockId);
}

// GetStockValueImpl.java
package com.sharekhan.external;
public class GetStockValueImpl implements IGetStockValue {
    public double getStockValue(int stockId) {
        System.out.println("  [External Service 2] Getting value for ID: " + stockId);
        switch (stockId) {
            case 500325: return 2485.50;  // Reliance
            case 532540: return 3678.25;  // TCS
            case 500209: return 1542.75;  // Infosys
            case 500180: return 1623.00;  // HDFC
            default: return 0.0;
        }
    }
}

// IGetStockFutureValue.java
package com.sharekhan.external;
public interface IGetStockFutureValue {
    double getFutureValue(int stockId);
}

// GetStockFutureValueImpl.java
package com.sharekhan.external;
public class GetStockFutureValueImpl implements IGetStockFutureValue {
    public double getFutureValue(int stockId) {
        System.out.println("  [External Service 3] Predicting future for ID: " + stockId);
        switch (stockId) {
            case 500325: return 2650.00;
            case 532540: return 3900.00;
            case 500209: return 1680.00;
            case 500180: return 1750.00;
            default: return 0.0;
        }
    }
}

StockDetailsDTO.java

package com.sharekhan.dto;

public class StockDetailsDTO {
    private final String stockName;
    private final int stockId;
    private final double currentValue;
    private final double futureValue;
    private final double changePercent;

    public StockDetailsDTO(String stockName, int stockId,
                            double currentValue, double futureValue) {
        this.stockName = stockName;
        this.stockId = stockId;
        this.currentValue = currentValue;
        this.futureValue = futureValue;
        this.changePercent = ((futureValue - currentValue) / currentValue) * 100;
    }

    public String getStockName() { return stockName; }
    public int getStockId() { return stockId; }
    public double getCurrentValue() { return currentValue; }
    public double getFutureValue() { return futureValue; }
    public double getChangePercent() { return changePercent; }

    @Override
    public String toString() {
        return String.format("%s (ID: %d) | Current: Rs %.2f | Future: Rs %.2f | Change: %.1f%%",
            stockName, stockId, currentValue, futureValue, changePercent);
    }
}

IStockFacade.java and StockFacadeImpl.java (Session Facade)

// IStockFacade.java
package com.sharekhan.facade;

import com.sharekhan.dto.StockDetailsDTO;

public interface IStockFacade {
    StockDetailsDTO getCompleteStockInfo(String stockName);
}

// StockFacadeImpl.java (aggregates 3 external services into ONE call)
package com.sharekhan.facade;

import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.external.*;

public class StockFacadeImpl implements IStockFacade {
    private final IGetStockId stockIdService;
    private final IGetStockValue stockValueService;
    private final IGetStockFutureValue stockFutureService;

    public StockFacadeImpl() {
        this.stockIdService = new GetStockIdImpl();
        this.stockValueService = new GetStockValueImpl();
        this.stockFutureService = new GetStockFutureValueImpl();
        System.out.println("[StockFacade] Initialized with 3 external services");
    }

    @Override
    public StockDetailsDTO getCompleteStockInfo(String stockName) {
        System.out.println("[StockFacade] Aggregating data for: " + stockName);

        // Call 3 external services (server-side, no extra network calls)
        int stockId = stockIdService.getStockId(stockName);
        if (stockId == -1) {
            throw new RuntimeException("Stock not found: " + stockName);
        }

        double currentValue = stockValueService.getStockValue(stockId);
        double futureValue = stockFutureService.getFutureValue(stockId);

        return new StockDetailsDTO(stockName, stockId, currentValue, futureValue);
    }
}

StockMgmt.java (BusinessDelegate)

package com.sharekhan.delegate;

import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.facade.IStockFacade;
import com.sharekhan.locator.ServiceLocator;

public class StockMgmt {

    public StockDetailsDTO getStockDetails(String stockName) {
        // Get StockFacade ref from ServiceLocator (cached after first lookup)
        IStockFacade facade = ServiceLocator.getInstance()
            .getService("stockFacade", IStockFacade.class);

        // Invoke external comp method through facade
        return facade.getCompleteStockInfo(stockName);
    }
}

Application.java

package com.sharekhan;

import com.sharekhan.delegate.StockMgmt;
import com.sharekhan.dto.StockDetailsDTO;
import com.sharekhan.locator.ServiceLocator;

public class Application {
    public static void main(String[] args) {
        StockMgmt delegate = new StockMgmt();

        // Scenario 1: First request (JNDI lookup + cache)
        System.out.println("=== Scenario 1: First Request (Reliance) ===");
        StockDetailsDTO reliance = delegate.getStockDetails("Reliance");
        System.out.println("Result: " + reliance);
        System.out.println();

        // Scenario 2: Second request (cache hit, no JNDI lookup)
        System.out.println("=== Scenario 2: Second Request (TCS) ===");
        StockDetailsDTO tcs = delegate.getStockDetails("TCS");
        System.out.println("Result: " + tcs);
        System.out.println();

        // Scenario 3: Third request (cache hit again)
        System.out.println("=== Scenario 3: Third Request (Infosys) ===");
        StockDetailsDTO infosys = delegate.getStockDetails("Infosys");
        System.out.println("Result: " + infosys);
        System.out.println();

        // Scenario 4: Verify ServiceLocator is singleton with cache
        System.out.println("=== Scenario 4: ServiceLocator Stats ===");
        System.out.println("Cache size: " + ServiceLocator.getInstance().getCacheSize());
        System.out.println("ServiceLocator hashCode: " + ServiceLocator.getInstance().hashCode());

        System.out.println();

        // Scenario 5: Invalid stock
        System.out.println("=== Scenario 5: Invalid Stock ===");
        try {
            delegate.getStockDetails("UNKNOWN_STOCK");
        } catch (RuntimeException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Expected Output

Key observations from the project

6. Service Locator Combined with Other Patterns

7. When to Use and When NOT to Use

When to use Service Locator

When NOT to use (modern alternatives)

8. Quick Reference Card