Example of FilterConfig

In this example, if you change the param-value to no, request will be forwarded to the servlet otherwise filter will create the response with the message: this page is underconstruction.

1. index.html
2. MyFilter.java
3. HelloServlet.java
4. web.xml

index.html

<a href="servlet1">click here</a>

MyFilter.java

import java.io.IOException; 
import java.io.PrintWriter; 
 
import javax.servlet.*; 
 
public class MyFilter implements Filter{ 
FilterConfig config; 
 
public void init(FilterConfig config) throws ServletException { 
    this.config=config; 
} 
 
public void doFilter(ServletRequest req, ServletResponse resp, 
    FilterChain chain) throws IOException, ServletException { 
     
    PrintWriter out=resp.getWriter(); 
         
    String s=config.getInitParameter("construction"); 
         
    if(s.equals("yes")){ 
         out.print("This page is under construction"); 
    } 
    else{ 
         chain.doFilter(req, resp);//sends request to next resource 
    } 
         
} 
public void destroy() {} 
}   

HelloServlet.java

import java.io.IOException; 
import java.io.PrintWriter; 
 
import javax.servlet.ServletException; 
import javax.servlet.http.*; 
 
public class HelloServlet extends HttpServlet { 
public void doGet(HttpServletRequest request, HttpServletResponse response) 
        throws ServletException, IOException { 
 
        response.setContentType("text/html"); 
        PrintWriter out = response.getWriter(); 
     
        out.print("<br>welcome to servlet<br>"); 
         
    } 
 
}

web.xml

<web-app> 
 
 <servlet> 
    <servlet-name>HelloServlet</servlet-name> 
    <servlet-class>HelloServlet</servlet-class> 
  </servlet> 
 
  <servlet-mapping> 
    <servlet-name>HelloServlet</servlet-name> 
    <url-pattern>/servlet1</url-pattern> 
  </servlet-mapping> 
   
  <filter> 
  <filter-name>f1</filter-name> 
  <filter-class>MyFilter</filter-class> 
  <init-param> 
  <param-name>construction</param-name> 
  <param-value>no</param-value> 
  </init-param> 
  </filter> 
  <filter-mapping> 
  <filter-name>f1</filter-name> 
  <url-pattern>/servlet1</url-pattern> 
  </filter-mapping> 
   
   
</web-app>

Simple Example of Filter

1. index.html
2. MyFilter.java
3.  HelloServlet.java
4. web.xml

index.html

<a href="servlet1">click here</a>

MyFilter.java

import java.io.IOException; 
import java.io.PrintWriter; 
 
import javax.servlet.*; 
 
public class MyFilter implements Filter{ 
 
public void init(FilterConfig arg0) throws ServletException {} 
     
public void doFilter(ServletRequest req, ServletResponse resp, 
    FilterChain chain) throws IOException, ServletException { 
         
    PrintWriter out=resp.getWriter(); 
    out.print("filter is invoked before"); 
         
    chain.doFilter(req, resp);//sends request to next resource 
         
    out.print("filter is invoked after"); 
    } 
    public void destroy() {} 
} 

HelloServlet.java

import java.io.IOException; 
import java.io.PrintWriter; 
 
import javax.servlet.ServletException; 
import javax.servlet.http.*; 
 
public class HelloServlet extends HttpServlet { 
    public void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException { 
 
        response.setContentType("text/html"); 
        PrintWriter out = response.getWriter(); 
     
        out.print("<br>welcome to servlet<br>"); 
         
    } 
 
} 

web.xml

<web-app> 
 
<servlet> 
<servlet-name>s1</servlet-name> 
<servlet-class>HelloServlet</servlet-class> 
</servlet> 
 
<servlet-mapping> 
<servlet-name>s1</servlet-name> 
<url-pattern>/servlet1</url-pattern> 
</servlet-mapping> 
 
<filter> 
<filter-name>f1</filter-name> 
<filter-class>MyFilter</filter-class> 
</filter> 
  
<filter-mapping> 
<filter-name>f1</filter-name> 
<url-pattern>/servlet1</url-pattern> 
</filter-mapping> 
 
 
</web-app>

Servlet Filter

• A filter is an object that is invoked at the preprocessing and postprocessing of a request.
• It is mainly used to perform filtering tasks such as conversion, logging, compression, encryption and decryption, input validation etc.
• The servlet filter is pluggable, i.e. its entry is defined in the web.xml file, if we remove the entry of filter from the web.xml file, filter will be removed automatically and we don't need to change the servlet.
• So maintenance cost will be less.

Advantage of Filter

1. Filter is pluggable.
2. One filter don't have dependency onto another resource.
3. Less Maintenance

Filter API
The javax.servlet package contains the three interfaces of Filter API.

1. Filter
2. FilterChain
3. FilterConfig

Filter interface

For creating any filter, you must implement the Filter interface. Filter interface provides the life cycle methods for a filter.

Method	Description
public void init(FilterConfig config)	init() method is invoked only once. It is used to initialize the filter.
public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)	doFilter() method is invoked every time when user request to any resource, to which the filter is mapped.It is used to perform filtering tasks.
public void destroy()	This is invoked only once when filter is taken out of the service.

FilterChain interface

The object of FilterChain is responsible to invoke the next filter or resource in the chain.This object is passed in the doFilter method of Filter interface.The FilterChain interface contains only one method:

1. public void doFilter(HttpServletRequest request, HttpServletResponse response): it passes the control to the next filter or resource.

FilterConfig interface

An object of FilterConfig is created by the web container. This object can be used to get the configuration information from the web.xml file.

Methods of FilterConfig interface

There are following 4 methods in the FilterConfig interface.

1. public void init(FilterConfig config): init() method is invoked only once it is used to initialize the filter.
2. public String getInitParameter(String parameterName): Returns the parameter value for the specified parameter name.
3. public java.util.Enumeration getInitParameterNames(): Returns an enumeration containing all the parameter names.
4. public ServletContext getServletContext(): Returns the ServletContext object.

How to define Filter

We can define filter same as servlet. Let's see the elements of filter and filter-mapping.


<web-app> 
 
<filter> 
<filter-name>...</filter-name> 
<filter-class>...</filter-class> 
</filter> 
  
<filter-mapping> 
<filter-name>...</filter-name> 
<url-pattern>...</url-pattern> 
</filter-mapping> 
 
</web-app>

HttpSessionActivationListener

Methods of HttpSessionActivationListener interface

1. public void sessionWillPassivate(HttpSessionEvent se): Notification that the session is about to be passivated.
2. public void sessionDidActivate(HttpSessionEvent se): Notification that the session has just been activated.

HttpSessionAttributeListener

Methods of HttpSessionAttributeListener interface

1. void attributeAdded(HttpSessionBindingEvent event): Receives notification that an attribute has been added to a session.
2.  void attributeRemoved(HttpSessionBindingEvent event): Receives notification that an attribute has been removed from a session.
3. void attributeReplaced(HttpSessionBindingEvent event): Receives notification that an attribute has been replaced in a session.

HttpSessionBindingEvent and HttpSessionBindingListener

Constructor of  HttpSessionBindingEvent class

1. public HttpSessionBindingEvent(HttpSession session, java.lang.String name)
2. public HttpSessionBindingEvent(HttpSession session, java.lang.String name, java.lang.Object value): Constructs an event that notifies an object that it has been bound to or unbound from a session

Methods of  HttpSessionBindingEvent class

1. public HttpSession getSession(): Return the session that changed.
   
2. public java.lang.String getName(): Returns the name with which the attribute is bound to or unbound from the session.
3. public java.lang.Object getValue(): Returns the value of the attribute that has been added, removed or replaced. If the attribute was added (or bound), this is the value of the attribute. If the attribute was removed (or unbound), this is the value of the removed attribute. If the attribute was replaced, this is the old value of the attribute.

Methods of  HttpSessionBindingListener interface

1. void valueBound(HttpSessionBindingEvent event): Notifies the object that it is being bound to a session and identifies the session.
2. void valueUnbound(HttpSessionBindingEvent event): Notifies the object that it is being unbound from a session and identifies the session.

HttpSessionEvent and HttpSessionListener

• The HttpSessionEvent is notified when session object is changed. The corresponding Listener interface for this event is HttpSessionListener.
• We can perform some operations at this event such as counting total and current logged-in users, maintaing a log of user details such as login time, logout time etc.

Constructor of HttpSessionEvent class

1. HttpSessionEvent(HttpSession source): Construct a session event from the given source.

Method of HttpSessionEvent class

1. public HttpSession getSession(): Return the session that changed

Methods of HttpSessionListener interface

1. public void sessionCreated(HttpSessionEvent e): is invoked when session object is created.
2. public void sessionDestroyed(ServletContextEvent e): is invoked when session is invalidated. 

In this example, are counting the total and current logged-in users. For this purpose, we have created three files:

    Login.html: to get input from the user.
    MyListener.java: A listener class that counts total and current logged-in users and stores this    
    information in ServletContext object as an attribute.
    First.java: A Servlet class that creates session and prints the total and current logged-in users.
    Logout.java: A Servlet class that invalidates session.

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
 
  <listener>
  <listener-class>CountUserListener</listener-class>
  </listener>
 
  <servlet>
     <servlet-name>First</servlet-name>
    <servlet-class>First</servlet-class>
  </servlet>
  <servlet>
      <servlet-name>LogoutServlet</servlet-name>
    <servlet-class>LogoutServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>First</servlet-name>
    <url-pattern>/servlet1</url-pattern>
  </servlet-mapping>

  <servlet-mapping>
    <servlet-name>LogoutServlet</servlet-name>
    <url-pattern>/logout</url-pattern>
  </servlet-mapping>
 
</web-app>

CountUserListener.java

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class CountUserListener implements HttpSessionListener{
    ServletContext ctx=null;
    static int total=0,current=0;
   
    public void sessionCreated(HttpSessionEvent e) {
    total++;
    current++;
   
    ctx=e.getSession().getServletContext();
    ctx.setAttribute("totalusers", total);
    ctx.setAttribute("currentusers", current);
   
    }

    public void sessionDestroyed(HttpSessionEvent e) {
            current--;
            ctx.setAttribute("currentusers",current);
    }

}
Login.html

<form action="servlet1">
Name:<input type="text" name="username"><br>
Password:<input type="password" name="userpass"><br>

<input type="submit" value="login"/>
</form>

First .java

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

public class First extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
   
        String n=request.getParameter("username");
        out.print("Welcome "+n);
      
        HttpSession session=request.getSession();
        session.setAttribute("uname",n);
      
        ServletContext ctx=getServletContext();
        int t=(Integer)ctx.getAttribute("totalusers");
        int c=(Integer)ctx.getAttribute("currentusers");
        out.print("<br>total users= "+t);
        out.print("<br>current users= "+c);
        out.print("<br><a href='logout'>logout</a>");
        out.close();
    }

}

LogoutServlet .java

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
 
public class LogoutServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
   
        HttpSession session=request.getSession(false);
        session.invalidate();
      
        out.print("You are successfully logged out");
        out.close();
    }

}