Filter in Servlet

The filter is the interface in Java that will be executed before servlet execution, Filter will work as an intermediate from request object to Servlet Object.

If we want to apply authentication or Security before servlet response then we can use Filter Under Servlet.
Filter always will be created from a particular Servlet class.
Filter Interface contains three different methods
1 init  ---->  It is used to initialize data members etc.
2 doFilter()----> Write Logical Code under this method
3 destroy() ---->  it is used to destroy object or reset values.
Web.xml file use following tag to map with Servlet Class:-
 <filter>
        <filter-name>FilterExample</filter-name>
        <filter-class>Packagename.FilterExample</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterExample</filter-name>
        <servlet-name>Servlet1</servlet-name>
        <dispatcher>REQUEST</dispatcher>
    </filter-mapping>

import java.io.IOException;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.io.StringWriter;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
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 {
     PrintWriter out=response.getWriter(); 
    out.print("<script>alert('hello');</script>");         
    chain.doFilter(request, response);//sends request to next resource          
    out.print("filter is invoked after"); 
    }
    @Override
    public void destroy() {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
   
}

web.xml

<filter>

        <filter-name>FileterDemo</filter-name>

        <filter-class>packagename.FileterDemo</filter-class>

    </filter>

    <filter-mapping>

        <filter-name>FileterDemo</filter-name>

        <url-pattern>/Servlet2</url-pattern>

    </filter-mapping>

When we request to servlet2 class then the filter will apply and execute before servlet calls.

Complete code including servlet and filter

<servlet>

  <servlet-name>Demoser</servlet-name>

  <servlet-class>scs.Demoser</servlet-class>

  </servlet>

  <servlet-mapping>

  <servlet-name>Demoser</servlet-name>

  <url-pattern>/Demoser</url-pattern>

  </servlet-mapping>

  <filter>

    <filter-name>MyFilter</filter-name>

    <filter-class>scs.MyFilter</filter-class>

  </filter>

  <filter-mapping>

    <filter-name>MyFilter</filter-name>

    <url-pattern>/Demoser</url-pattern>

  </filter-mapping>

............................................................................................................

Another Example of Filter

First Create Servlet:

package com.javawebapp.basicpractice;

import java.io.IOException;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

/**

 * Servlet implementation class Welcome

 */

public class Welcome extends HttpServlet {

private static final long serialVersionUID = 1L;

       

    /**

     * @see HttpServlet#HttpServlet()

     */

    public Welcome() {

        super();

        // TODO Auto-generated constructor stub

    }

/**

* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

*/

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

// TODO Auto-generated method stub

response.getWriter().append("Served at: ").append(request.getContextPath());

}

}

Create Filter

package com.javawebapp.basicpractice;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.annotation.WebFilter;

import javax.servlet.http.HttpFilter;

/**

 * Servlet Filter implementation class MyFilter

 */

public class MyFilter extends HttpFilter implements Filter {

       

    /**

     * @see HttpFilter#HttpFilter()

     */

    public MyFilter() {

        super();

        // TODO Auto-generated constructor stub

    }

/**

* @see Filter#destroy()

*/

public void destroy() {

// TODO Auto-generated method stub

}

/**

* @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)

*/

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

PrintWriter out = response.getWriter();

out.print("<script>alert('Request welcome in filter')</script>");

chain.doFilter(request, response);

out.print("<script>alert('Response welcome in filter')</script>");

}

/**

* @see Filter#init(FilterConfig)

*/

public void init(FilterConfig fConfig) throws ServletException {

// TODO Auto-generated method stub

}

}

code of web.xml

<?xml version="1.0" encoding="UTF-8"?>

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

  <display-name>JavaWebApp</display-name>

  <welcome-file-list>

    

    <welcome-file>index.jsp</welcome-file>

   

  </welcome-file-list>

  

  <servlet>

  <servlet-name>Welcome</servlet-name>

  <servlet-class>com.javawebapp.basicpractice.Welcome</servlet-class>

  </servlet>

  <servlet-mapping>

  <servlet-name>Welcome</servlet-name>

  <url-pattern>/ram</url-pattern>

  </servlet-mapping>

  <filter>

        <filter-name>MyFilter</filter-name>

        <filter-class>com.javawebapp.basicpractice.MyFilter</filter-class>

    </filter>

    <filter-mapping>

        <filter-name>MyFilter</filter-name>

        <url-pattern>/ram</url-pattern>

    </filter-mapping>

</web-app>