Today, we are going to create a wrapper class which extends ServletResponseWrapper class(which has implemented ServletResponse interface).
Using this wrapper class we will be able to wrap the ServletResponse object and hence we will be able to
manipulate, check any response sent by a Servlet to the user.
To do this, we will create a Filter class which will be associated with the requested Servlet and this
Filter class will also call the wrapper class to perform the check on any request sent to this Servlet by the user.
Creating a webpage which calls the Servlet
We are creating a webpage which asks the user to enter her/his name, city 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(
mentioned in the deployment descriptor file(web.xml).
Webpage1.jsp
<html>
<head>
<title> ServletRequest Wrapper Demo </title>
</head>
<body>
<b> Please enter your name : ? </b>
<br/>
<br/>
<br/>
<form action ="MyServ">
Name : <input type = "text" name = "username" />
City : <input type = "text" name = "cityname" />
<input type = "submit" name = "submit" />
</form>
</body>
</html>
Creating a Filter class
We are creating a Filter class by implementing the Filter interface and by implementing its three methods
This Filter class creates an object of ServletRequest wrapper class, which is wrapped around the object of ServletRequest. Eventually doFilter() method
of FilterChain is called and passed
with this request wrapper object as an argument, which makes sure there is a filtering over the request and response object, using the wrapper class.
MyFilter1.java
package decodejava;
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 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>Filter is filtering the response and passing it to Wrapper class</i> </b> <br/>");
//Calling the constructor of response wrapper class
ResponseWrapper1 responseWrapper = new ResponseWrapper1(response);
//This method calls the next filter in the chain
chain.doFilter(request,responseWrapper);
}
}
This wrapper class has extended the ServletResponseWrapper(which has implemented ServletResponse interface). This wrapper class
and has overridden its getLocale() method of ServletResponseWrapper class, which is
invoked when there is a call to getLocale() method of ServletResponse in the requested Servlet.
This class creates a ServletResponse wrapper object.
ResponseWrapper1.java
package decodejava;
import java.io.*;
import javax.servlet.*;
import java.util.*;
public class ResponseWrapper1 extends ServletResponseWrapper
{
public ResponseWrapper1(ServletResponse res)
{
//Calling the ServletResponseWrapper superclass constructor i.e. ServletResponse interface.
super(res);
}
public Locale getLocale()
{
System.out.println("Wrapper class called by Filter");
//Calling the superclass method i.e. ServletResponse's getLocale() method.
Locale loc = super.getLocale();
String country = loc.getCountry();
String language = loc.getLanguage();
if(!country.equals("US"))
{
Locale loc1 = new Locale("English, US");
loc = loc1;
}
System.out.println(country);
System.out.println(language);
return loc;
}
}
MyServlet1.java is a Servlet class, which is associated with the filter.
MyServlet1.java
package decodejava;
import javax.servlet.*;
import java.io.*;
import java.util.*;
public class MyServlet1 extends GenericServlet
{
public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
System.out.println("Servlet is called");
Locale loc = response.getLocale();
out.println("Locale set for this application is " + loc.toString());
}
}
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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 ServletResponseWrapperEx 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.
So, let's create one and this time, associate our filter class with our Servlet class -
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 decodejava.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 decodejava.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.
First we execute the webpage containing the form, asking the user to enter the name, city and press the submit button. Let's say this user does not enter the name and submits the form as shown below.
On submitting the form, 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.
This Filter calls the ServletResponseWrapper class, which checks if the language of user's system Locale
set to US English or not i.e. en,US. If not, the Locale is set to en,US
Finally, the request Servlet is executed, displaying the set Locale information.
To see the workflow of this web application or how the classes were called, please take a look at the Tomcat Web Server window. This proves the order of
operations we described earlier.
These messages are displayed in Tomcat window due to the call of System.out.println() method in Filter, Wrapper and Servlet class.
