Java Servlet Filter Example1

Posted in: Servlet Filter

The filters in deployment descriptor:

<web-app>
	<filter>
		<filter-name>MyFilter</filter-name>
		<filter-class>subin.rnd.enterprise.filter.MyFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>MyFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>
	<filter-mapping>
		<filter-name>MyFilter</filter-name>
		<servlet-name>MyServlet</servlet-name>
		<dispatcher>REQUEST</dispatcher>
	</filter-mapping>
</web-app>

We can have a filter (or a chain of filters) processing the request/response. In the above descriptor, we are linking a filter for a URL pattern as well as for a specific servlet. So in the above case, for MyServlet, the filter will get executed twice as we have mapped the filter MyFilter to all URLs and for MyServlet. So the order of execution will be MyFilter.doFilter() > MyFilter.doFilter() > MyServlet.doGet().

MyServlet.java

01	import javax.servlet.ServletException;

02	import javax.servlet.http.HttpServlet;

03	import javax.servlet.http.HttpServletRequest;

04	import javax.servlet.http.HttpServletResponse;

05

06	public class MyServlet

07	extends HttpServlet

08

{

09

@Override

10	public void doGet(HttpServletRequest request,

11	HttpServletResponse response)

12	throws ServletException

13

{

14	System.out.println("doGet()");

15	response.setContentType("text/plain");

16	ServletUtility.write(response, "Hello world!".getBytes());

17

}

18

}

MyFilter.java

01	import java.io.IOException;

02	import javax.servlet.Filter;

03	import javax.servlet.FilterChain;

04	import javax.servlet.FilterConfig;

05	import javax.servlet.ServletException;

06	import javax.servlet.ServletRequest;

07	import javax.servlet.ServletResponse;

08	import javax.servlet.http.HttpServletResponse;

09

10	public class MyFilter

11	implements Filter

12

{

13	public void destroy()

14

{

15

}

16

17	public void doFilter(ServletRequest request, ServletResponse response,

18	FilterChain filterChain)

19	throws IOException, ServletException

20

{

21	System.out.println("doFilter()");

22

23	ByteStreamResponseWrapper responseWrapper = null;

24	byte[] responseAsBytes = null;

25	String processedResponse = null;

26

27	// Creating a response wrapper

28	responseWrapper = new ByteStreamResponseWrapper(

29	(HttpServletResponse) response);

30	filterChain.doFilter(request, responseWrapper);

31

32	// Process the response

33	processedResponse = responseWrapper.toString();

34	if (null != processedResponse)

35

{

36	processedResponse = processedResponse.toUpperCase();

37	responseAsBytes = processedResponse.getBytes();

38

}

39

40	// Writing the response (as bytes) to the servlet output stream

41	ServletUtility.write(response, responseAsBytes);

42

}

43

44	public void init(FilterConfig filterConfig)

45	throws ServletException

46

{

47	// We can initialize a filter using the init-params here

48	// (which we defined in the deployment descriptor - web.xml)

49

}

50

}

Other filesThere are two more Java files – ByteStreamResponseWrapper & ServletUtility.ByteStreamResponseWrapper is an extension ofjavax.servlet.http.HttpServletResponseWrapper with some methods – toString(),getOutputStream() & getWriter() – overriden. ServletUtility is a utility class to write a byte stream to the servlet output stream.

OutputThe “Hello world!” from MyServlet is filtered to “HELLO WORLD!”  by MyFilter.

Fig: Output in browser

Fig: Jboss console output