Implementing filters in web applications follows a three-phase approach:

1. Create backend resources including static assets (HTML, CSS, etc.) and dynamic components (Servlets, JSPs).
2. Implemetn the Filter interface.
3. Register the filter in web.xml to specify which resources it should intercept.

Creating Servlets

Two servlets are defined below:

package com.msb.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class MyServlet1 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("myServlet1 executed service method");
        resp.setContentType("text/html;charset=UTF-8");
        resp.setCharacterEncoding("UTF-8");
        resp.getWriter().print("Data from myServlet1");
    }
}

package com.msb.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class MyServlet2 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("myServlet2 executed service method");
        resp.setContentType("text/html;charset=UTF-8");
        resp.setCharacterEncoding("UTF-8");
        resp.getWriter().print("Data from myServlet2");
    }
}

Implementnig the Filter

The filter logic is implemented as follows:

package com.msb.filter;

import javax.servlet.*;
import java.io.IOException;

public class MyFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        System.out.println("Filter processing request");
        chain.doFilter(request, response);
        System.out.println("Filter processing response");
        response.getWriter().print("Additional data from filter");
    }

    @Override
    public void destroy() {
    }
}

Configuring the Filter

The filter is registered in the deployment descriptor:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>myServlet1</servlet-name>
        <servlet-class>com.msb.servlet.MyServlet1</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>myServlet1</servlet-name>
        <url-pattern>/myServlet1.do</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>myServlet2</servlet-name>
        <servlet-class>com.msb.servlet.MyServlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>myServlet2</servlet-name>
        <url-pattern>/myServlet2.do</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>filter1</filter-name>
        <filter-class>com.msb.filter.MyFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>filter1</filter-name>
        <servlet-name>myServlet1</servlet-name>
        <servlet-name>myServlet2</servlet-name>
    </filter-mapping>
</web-app>