Intercepting Filter Pattern

Filter Structure

Participant Components

Three Types of Filters

1. request filter: traps both request and response but contains only pre-request processing logic
2. response filter: traps both request and response but contains only post-response generation logic
3. request-response filter: traps both request and response and also contains both pre-request processing logic and post-response generation logics

Examples of Filters

• Network security, authentication (request filter)
• Auth-authorization and enabling into applications (request filter or response filter)
• Debugging (request, response filter)
• Pre-processing and monitoring specific clients (request filter, response filter)
• Convert input encoding streams (request filter)
• Is the client IP address from a locale source (request filter)
• Convert output encoding streams (response filter)
• Does the HTML Pass a valid validator (request filter)
• Body output or removed transparently in ciphertext and request (request filter, response filter)
• Data compression (request or response filter)
• Implement security using protocol or format (request filter)
• Performance Test Filter (request response filter)

Developing a Filter

To develop a filter comp in servlet 6.x version:

• option 1: Take a class implementing jakarta.servlet.Filter and provide implementation for 3 methods
• option 2: Take a class extending from jakarta.servlet.GenericFilter and provide implementation
• option 3: Take a class extending from jakarta.servlet.http.HttpFilter and provide implementation (init, doFilter, or destroy methods)

Session Check Filter Example

If user tries login to certain web comp without performing login activity by typing its URL, the request should not be allowed to get into that web comp. The user should get the login page to begin the login activity first. This is achieved by developing a Filter comp that checks whether the logged in user is having ValidSession or not.

// InvalidSessionCheckFilter.java
package com.nt.filter;

import jakarta.servlet.*;
import jakarta.servlet.annotation.WebFilter;
import jakarta.servlet.http.*;

@WebFilter("/*")
public class InvalidSessionCheckFilter implements Filter {

    @Override
    public void doFilter(ServletRequest req,
                          ServletResponse res,
                          FilterChain chain) throws Exception {
        System.out.println("InvalidSessionCheckFilter.doFilter()");

        // Get access to Session
        HttpSession ses = ((HttpServletRequest) req).getSession(false);

        if (ses != null && ses.getAttribute("userDetails") != null) {
            // Valid session: allow request to proceed
            chain.doFilter(req, res);
        } else {
            // Invalid session: redirect to login
            req.setAttribute("errorMsg", "You need to sign in first");
            RequestDispatcher rd = req.getRequestDispatcher("login.jsp");
            rd.forward(req, res);
        }
    }
}

View Helper Pattern

Limitations of placing Java code in JSP page

1. Makes the programmer mix presentation logic (HTML code) with business logic code (Java code). So any modification in presentation logic code will affect the business logic code.
2. Clear separation between presentation logic and business logics will not be there.
3. Becomes difficult for UI developer (no Java skills) to develop/maintain UI tags by mixing them with Java code.
4. Kills the readability of JSP page, i.e., reading JSP page that is having Java code, JSP tags and HTML tags becomes very much problematic for the programmers.
5. If same Java code placed in multiple JSP pages, there is no reusability for Java code.
6. JSP says develop the JSP pages as scriptless (Java code less) JSP pages. But placing Java code in JSP pages is against JSP programming principles.

Solution: Make JSP pages Java-code-less

To overcome all these problems, make JSP pages Java code less / scriptless JSP pages by using the following options:

1. HTML tags
2. JSP standard tags (scriptlet, expression, declaration)
3. Scripting tags (JSP actions)
4. EL code (Expression Language)
5. JSTL JSP tags (JSP Standard Tag Library)
6. Third-party JSP tags (Struts JSP page, Spring MVC JSP tags, etc.)
7. Custom JSP tags (View Helper pattern)

Except Custom JSP tags, the remaining all options are ready-made tags or facilities. Custom tags development by developing tag handler classes for custom JSP tags is called View Helper DP implementation.

For each custom JSP tag there will be one tag handler class defining the functionality, i.e., this tag handler class is acting as helper comp for view comp. So the process of developing custom tags falls under View Helper design pattern implementation.

Custom JSP Tag Library Development Steps

1. Design JSP tagLibrary having JSP tags (define tags: <abc>, <xyz>)
2. Develop tag handler classes (AbcTag.java extends TagSupport, XyzTag.java extends TagSupport)
3. Clip JSP tags in TLD file (tag library descriptor file) in WEB-INF folder - maps tags to tag handler classes
4. Import JSP tagLibrary to JSP page and use the JSP tags in JSP page

Front Controller Pattern

Problem: Multiple Controller Servlets

Solution: Single Front Controller

Front Controller Responsibilities

Implementation

// FrontController pattern using Command pattern

// Command interface
public interface Command {
    String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception;
}

// Concrete commands
public class RegistrationCommand implements Command {
    public String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception {
        // registration logic
        return "registration_success.jsp";
    }
}

public class LoginCommand implements Command {
    public String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception {
        String uname = req.getParameter("uname");
        String pwd = req.getParameter("pwd");
        // validate credentials
        if (isValid(uname, pwd)) {
            return "home.jsp";
        }
        req.setAttribute("errorMsg", "Invalid Credentials");
        return "login.jsp";
    }
}

// CommandFactory
public class CommandFactory {
    private static Map<String, Command> commands = new HashMap<>();
    static {
        commands.put("/register", new RegistrationCommand());
        commands.put("/login", new LoginCommand());
        commands.put("/search", new SearchCommand());
    }

    public static Command getCommand(String path) {
        Command cmd = commands.get(path);
        if (cmd == null) throw new RuntimeException("No command for: " + path);
        return cmd;
    }
}

// FrontControllerServlet
@WebServlet("/*")
public class FrontControllerServlet extends HttpServlet {

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        processRequest(req, res);
    }

    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        processRequest(req, res);
    }

    private void processRequest(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        // 1. Log the request
        System.out.println("Request: " + req.getServletPath());

        try {
            // 2. Get the appropriate command
            Command command = CommandFactory.getCommand(req.getServletPath());

            // 3. Execute the command
            String viewPage = command.execute(req, res);

            // 4. Forward to the view
            RequestDispatcher rd = req.getRequestDispatcher(viewPage);
            rd.forward(req, res);

        } catch (Exception e) {
            req.setAttribute("errorMsg", e.getMessage());
            req.getRequestDispatcher("error.jsp").forward(req, res);
        }
    }
}

Front Controller flow in our architecture

Comparison Table

Intercepting Filter and Front Controller Patterns

Two JEE Presentation Tier Patterns | Filter Chain Architecture | Central Entry Point | View Helper | Session Check Filter | Complete Web Application Example | 12 Interview Questions

Intercepting Filter Pattern

Filter Structure

Participant Components

Three Types of Filters

1. request filter: traps both request and response but contains only pre-request processing logic
2. response filter: traps both request and response but contains only post-response generation logic
3. request-response filter: traps both request and response and also contains both pre-request processing logic and post-response generation logics

Examples of Filters

• Network security, authentication (request filter)
• Auth-authorization and enabling into applications (request filter or response filter)
• Debugging (request, response filter)
• Pre-processing and monitoring specific clients (request filter, response filter)
• Convert input encoding streams (request filter)
• Is the client IP address from a locale source (request filter)
• Convert output encoding streams (response filter)
• Does the HTML Pass a valid validator (request filter)
• Body output or removed transparently in ciphertext and request (request filter, response filter)
• Data compression (request or response filter)
• Implement security using protocol or format (request filter)
• Performance Test Filter (request response filter)

Developing a Filter

To develop a filter comp in servlet 6.x version:

• option 1: Take a class implementing jakarta.servlet.Filter and provide implementation for 3 methods
• option 2: Take a class extending from jakarta.servlet.GenericFilter and provide implementation
• option 3: Take a class extending from jakarta.servlet.http.HttpFilter and provide implementation (init, doFilter, or destroy methods)

Session Check Filter Example

If user tries login to certain web comp without performing login activity by typing its URL, the request should not be allowed to get into that web comp. The user should get the login page to begin the login activity first. This is achieved by developing a Filter comp that checks whether the logged in user is having ValidSession or not.

// InvalidSessionCheckFilter.java
package com.nt.filter;

import jakarta.servlet.*;
import jakarta.servlet.annotation.WebFilter;
import jakarta.servlet.http.*;

@WebFilter("/*")
public class InvalidSessionCheckFilter implements Filter {

    @Override
    public void doFilter(ServletRequest req,
                          ServletResponse res,
                          FilterChain chain) throws Exception {
        System.out.println("InvalidSessionCheckFilter.doFilter()");

        // Get access to Session
        HttpSession ses = ((HttpServletRequest) req).getSession(false);

        if (ses != null && ses.getAttribute("userDetails") != null) {
            // Valid session: allow request to proceed
            chain.doFilter(req, res);
        } else {
            // Invalid session: redirect to login
            req.setAttribute("errorMsg", "You need to sign in first");
            RequestDispatcher rd = req.getRequestDispatcher("login.jsp");
            rd.forward(req, res);
        }
    }
}

View Helper Pattern

Limitations of placing Java code in JSP page

1. Makes the programmer mix presentation logic (HTML code) with business logic code (Java code). So any modification in presentation logic code will affect the business logic code.
2. Clear separation between presentation logic and business logics will not be there.
3. Becomes difficult for UI developer (no Java skills) to develop/maintain UI tags by mixing them with Java code.
4. Kills the readability of JSP page, i.e., reading JSP page that is having Java code, JSP tags and HTML tags becomes very much problematic for the programmers.
5. If same Java code placed in multiple JSP pages, there is no reusability for Java code.
6. JSP says develop the JSP pages as scriptless (Java code less) JSP pages. But placing Java code in JSP pages is against JSP programming principles.

Solution: Make JSP pages Java-code-less

To overcome all these problems, make JSP pages Java code less / scriptless JSP pages by using the following options:

1. HTML tags
2. JSP standard tags (scriptlet, expression, declaration)
3. Scripting tags (JSP actions)
4. EL code (Expression Language)
5. JSTL JSP tags (JSP Standard Tag Library)
6. Third-party JSP tags (Struts JSP page, Spring MVC JSP tags, etc.)
7. Custom JSP tags (View Helper pattern)

Except Custom JSP tags, the remaining all options are ready-made tags or facilities. Custom tags development by developing tag handler classes for custom JSP tags is called View Helper DP implementation.

For each custom JSP tag there will be one tag handler class defining the functionality, i.e., this tag handler class is acting as helper comp for view comp. So the process of developing custom tags falls under View Helper design pattern implementation.

Custom JSP Tag Library Development Steps

1. Design JSP tagLibrary having JSP tags (define tags: <abc>, <xyz>)
2. Develop tag handler classes (AbcTag.java extends TagSupport, XyzTag.java extends TagSupport)
3. Clip JSP tags in TLD file (tag library descriptor file) in WEB-INF folder - maps tags to tag handler classes
4. Import JSP tagLibrary to JSP page and use the JSP tags in JSP page

Front Controller Pattern

Problem: Multiple Controller Servlets

Solution: Single Front Controller

Front Controller Responsibilities

Implementation

// FrontController pattern using Command pattern

// Command interface
public interface Command {
    String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception;
}

// Concrete commands
public class RegistrationCommand implements Command {
    public String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception {
        // registration logic
        return "registration_success.jsp";
    }
}

public class LoginCommand implements Command {
    public String execute(HttpServletRequest req, HttpServletResponse res)
        throws Exception {
        String uname = req.getParameter("uname");
        String pwd = req.getParameter("pwd");
        // validate credentials
        if (isValid(uname, pwd)) {
            return "home.jsp";
        }
        req.setAttribute("errorMsg", "Invalid Credentials");
        return "login.jsp";
    }
}

// CommandFactory
public class CommandFactory {
    private static Map<String, Command> commands = new HashMap<>();
    static {
        commands.put("/register", new RegistrationCommand());
        commands.put("/login", new LoginCommand());
        commands.put("/search", new SearchCommand());
    }

    public static Command getCommand(String path) {
        Command cmd = commands.get(path);
        if (cmd == null) throw new RuntimeException("No command for: " + path);
        return cmd;
    }
}

// FrontControllerServlet
@WebServlet("/*")
public class FrontControllerServlet extends HttpServlet {

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        processRequest(req, res);
    }

    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        processRequest(req, res);
    }

    private void processRequest(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
        // 1. Log the request
        System.out.println("Request: " + req.getServletPath());

        try {
            // 2. Get the appropriate command
            Command command = CommandFactory.getCommand(req.getServletPath());

            // 3. Execute the command
            String viewPage = command.execute(req, res);

            // 4. Forward to the view
            RequestDispatcher rd = req.getRequestDispatcher(viewPage);
            rd.forward(req, res);

        } catch (Exception e) {
            req.setAttribute("errorMsg", e.getMessage());
            req.getRequestDispatcher("error.jsp").forward(req, res);
        }
    }
}

Front Controller flow in our architecture

Comparison Table