Servlet Filters

Servlet Filter is an object through which we can perform many filtering tasks within a web application such as - session validation, user authentication, verification, restricting access to a particular web resource, etc.

To create a Servlet Filter object, we can create a Filter class by implementing the Filter interface. This Filter class could be associated with a web resource such as - Servlet, JSP or a static html webpage.

How Servlet Filter works?

When a user makes a request to a web resource, the Filter class associated with the request web resource is executed in either of three cases:

To perform preprocessing i.e. to filter the request to the web resource.
To perform postprocessing i.e. to filter the response from the web resource.
To perform both preprocessing and postprocessing i.e. before and after the web resource requested by the user, hence, providing filtering to the user request and response.

Methods of Filter interface

In order to create a filter class we must implement the three methods of javax.servlet.Filter interface.

Methods	Description
void *doInit(FilterConfig config)*	This method initializes the Filter.
void *doFilter(ServletRequest request, ServletResponse, response, FilterChain chain)*	This method is called by the web container when a client requests for a web resource which could be a Servlet or a JSP page .
void destroy()	This method destroys the Filter instance.

Creating a webpage which calls the Servlet

In the upcoming example, we are creating a webpage which asks the user to enter his name and click the submit button, which when clicked, will call a Servlet, but before this Servlet is executed, a filter associated with it will be executed(specified in the deployment descriptor file(web.xml).

Webpage1.jsp

<html>

<head>
<title> Filters Demo </title>
</head>


<body>

<b> Please enter your name : ? </b>
<br/>
<br/>
<br/>

<form action ="MyServ">
Name : <input type = "text"  name = "name" />
<input type = "submit"  name = "submit" />
</form>

</body>
</html>

Creating Filter

Next, we are creating a Filter by implementing the Filter interface and by implementing its three methods -

init(FilterConfig)
doFilter(ServletRequest, ServletResponse, FilterChain )
destroy()

Within the doFilter(ServletRequest, ServletResponse, FilterChain) method, we must make a call to doFilter(request, response) method of FilterChain interface.

The call to doFilter(request, response) method calls any additional Filter attached to the Servlet.
If there is no more Filter attached to the Servlet then the requested Servlet is finally executed.
After the servlet has finished its executed, the doFilter(request,response) method returns, enabling you to handle Servlet output in the response object.

MyFilter1.java

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


public class MyFilter1 implements Filter
{


public void init(FilterConfig filterConfig) 
{
}

public void destroy()
{
}


//This method is called each time a client requests for a web resource 
// which could be a Servlet or a JSP page i.e. preprocessing request
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();


out.println("<b> <i>Filtering request</i> </b>");
out.println("Hello " + request.getParameter("name") + "!");

out.println("<br/>");
out.println("<br/>");
out.println("<br/>");


//Calling  doFilter() calls the next filter in the chain will execute or 
//if there is no filter then the requested web resource is executed.
chain.doFilter(request,response);

out.println("<br/>");
out.println("<br/>");
out.println("<br/>");


//post-processing the request and after the requested web resource is called.
out.println("<b> <i>Filtering response</i> </b>");
out.println("Bye " + request.getParameter("name") + "!");

}

}

MyServlet1.java is a Servlet class, which is executed after its associated filter is executed.

MyServlet1.java

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

public class MyServlet1 extends GenericServlet
{


public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException
{

response.setContentType("text/html");
PrintWriter out = response.getWriter();

out.println("The requested Servlet is being executed.");
}
}

Directory Structure of Servlet files

The diagram above depicts how to arrange the Servlet files in a specific directory structure, as per Java Servlet Specification-

Project Folder - We have created a project folder named FiltersEx within the webapps folder of Tomcat installation folder. This folder contains the Servlet class(.java) file and WEB-INF folder.

WEB-INF - Within your project folder, you create another folder named WEB-INF and this folder should contain the deployment descriptor file(web.xml) of your Java Servlet program.

classes - Within the WEB-INF folder you should create a folder named classes. This folder should contain the compiled(.class) form of your Java Servlet class.

Creating the Deployment Descriptor file

As per the Java Servlet specifications, every web application based on Servlet must have a Deployment Descriptor file(an XML file) named web.xml. Within this file, we can specify and associate our filter class with our Servlet class -

web.xml

<?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"
  metadata-complete="true">

  <display-name>Welcome tomcat</display-name>
  <description>
     Welcome tomcat
  </description>

  
<filter>
 	<filter-name>Filter1</filter-name>
	<filter-class>MyFilter1</filter-class>
</filter>

<filter-mapping>
	<filter-name>Filter1</filter-name>
	<url-pattern>/MyServ</url-pattern>
</filter-mapping>



<servlet>
 	<servlet-name>Servlet</servlet-name>
	<servlet-class>MyServlet1</servlet-class>
</servlet>

<servlet-mapping>
	<servlet-name>Servlet</servlet-name>
	<url-pattern>/MyServ</url-pattern>
</servlet-mapping>

</web-app>

In deployment descriptor file, The <filter> has two child tags <filter-name> and <filter-class>.

<filter-name> tag is used to specify a unique name for our filter class, we have given it a unique name Filter1.

<filter-class> tag is used to specify the full qualified name of the filter class and in our example our filter class is named MyFilter1.

<servlet> has two child tags <servlet-name> and <servlet-class> :

<servlet-name> tag is used to specify a unique name for our Servlet class, we have given it a unique name Servlet.

<servlet-class> tag is used to specify the full qualified name of the Servlet class and in our example our Servlet class is named MyServlet1.

Note

The child tag <filter-name> of <filter> tag is matched with the <filter-name> child tag of <filter-mapping>. The <url-pattern> child tag of <filter-mapping> should be the same as the <url-pattern> for our servlet i.e. MyServ.

The child tag <servlet-name> of <servlet> tag is matched with the <servlet-name> child tag of <servlet-mapping>. The <url-pattern> child tag is used to specify the URL to access Servlet and we have named this URL MyServ.

Setting the classpath

Much of the support for developing the web applications based on the Java Servlet technology does not come with the core Java. Hence, in order to compile the Servlet programs, we have to set the classpath to a jar file named servlet-api.jar.

This jar file provides all the classes that are required for the Servlet programming and it comes within the lib Folder of Tomcat installation folder.

For example, in our case we have installed Tomcat Web Server within the C: Drive, hence the path to our lib folder containing the servlet-api.jar is - C:\apache-tomcat-9.0.2\lib

There are two ways to set the classpath -

You could set the classpath by entering this command at the Command Prompt, but this only temporarily sets the classpath to the path of servlet-api.jar file. If you restart your system, you will have to set the classpath again.
```
set classpath=C:\apache-tomcat-9.0.2\lib\servlet-api.jar
```
In order to set the classpath permanently, you need to set your classpath system variables. A simple tutorial on how to permanently set the classpath system variables, you may read How to permanently set the classpath system variable

Compiling the Servlet class

After setting the classpath, you need to compile the filter and the Servlet class by entering the command at the folder where you've stored the Servlet class file.

javac -d WEB-INF/classes MyFilter1.java

javac -d WEB-INF/classes MyServlet1.java

Executing the Servlet

We are executing a webpage which asks the user to enter his name and click the submit button, First we execute the webpage containing the form, asking the user to enter the name and press submit button.

three activities takes place in the following order

The requested servlet is called but before this servlet is executed, a filter associated with this servlet will be executed, to filter the user request.
The request servlet is executed.
The filter associated with the requested servlet is executed again to filter the response.

Finally, the user is presented with the following messages.

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