Servlets HTTP Status Codes And Servlets Filters

The format of the HTTP request and HTTP response messages are similar and will have following structure −

• An initial status line + CRLF ( Carriage Return + Line Feed i.e. New Line )
• Zero or more header lines + CRLF
• A blank line, i.e., a CRLF
• An optional message body like file, query data or query output.

For example, a server response header looks as follows −

HTTP/1.1 200 OK
Content-Type: text/html
Header2: ...
...
HeaderN: ...
   (Blank Line)


   ...
   
      ...
   

The status line consists of the HTTP version (HTTP/1.1 in the example), a status code (200 in the example), and a very short message corresponding to the status code (OK in the example).

Following is a list of HTTP status codes and associated messages that might be returned from the Web Server −

Code	Message	Description
100	Continue	Only a part of the request has been received by the server, but as long as it has not been rejected, the client should continue with the request
101	Switching Protocols	The server switches protocol.
200	OK	The request is OK
201	Created	The request is complete, and a new resource is created
202	Accepted	The request is accepted for processing, but the processing is not complete.
203	Non-authoritative Information
204	No Content
205	Reset Content
206	Partial Content
300	Multiple Choices	A link list. The user can select a link and go to that location. Maximum five addresses
301	Moved Permanently	The requested page has moved to a new url
302	Found	The requested page has moved temporarily to a new url
303	See Other	The requested page can be found under a different url
304	Not Modified
305	Use Proxy
306	Unused	This code was used in a previous version. It is no longer used, but the code is reserved
307	Temporary Redirect	The requested page has moved temporarily to a new url.
400	Bad Request	The server did not understand the request
401	Unauthorized	The requested page needs a username and a password
402	Payment Required	You cannot use this code yet
403	Forbidden	Access is forbidden to the requested page
404	Not Found	The server cannot find the requested page.
405	Method Not Allowed	The method specified in the request is not allowed.
406	Not Acceptable	The server can only generate a response that is not accepted by the client.
407	Proxy Authentication Required	You must authenticate with a proxy server before this request can be served.
408	Request Timeout	The request took longer than the server was prepared to wait.
409	Conflict	The request could not be completed because of a conflict.
410	Gone	The requested page is no longer available.
411	Length Required	The "Content-Length" is not defined. The server will not accept the request without it.
412	Precondition Failed	The precondition given in the request evaluated to false by the server.
413	Request Entity Too Large	The server will not accept the request, because the request entity is too large.
414	Request-url Too Long	The server will not accept the request, because the url is too long. Occurs when you convert a "post" request to a "get" request with a long query information.
415	Unsupported Media Type	The server will not accept the request, because the media type is not supported.
417	Expectation Failed
500	Internal Server Error	The request was not completed. The server met an unexpected condition.
501	Not Implemented	The request was not completed. The server did not support the functionality required.
502	Bad Gateway	The request was not completed. The server received an invalid response from the upstream server.
503	Service Unavailable	The request was not completed. The server is temporarily overloading or down.
504	Gateway Timeout	The gateway has timed out.
505	HTTP Version Not Supported	The server does not support the "http protocol" version.

Methods to Set HTTP Status Code

The following methods can be used to set HTTP Status Code in your servlet program. These methods are available with HttpServletResponse object.

Sr.No.	Method & Description
1	public void setStatus ( int statusCode ) This method sets an arbitrary status code. The setStatus method takes an int (the status code) as an argument. If your response includes a special status code and a document, be sure to call setStatus before actually returning any of the content with the PrintWriter.
2	public void sendRedirect(String url) This method generates a 302 response along with a Locationheader giving the URL of the new document
3	public void sendError(int code, String message) This method sends a status code (usually 404) along with a short message that is automatically formatted inside an HTML document and sent to the client.

HTTP Status Code Example

Following is the example which would send a 407 error code to the client browser and browser would show you "Need authentication!!!" message.

// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// Extend HttpServlet class
public class showError extends HttpServlet {
 
   // Method to handle GET method request.
   public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      // Set error code and reason.
      response.sendError(407, "Need authentication!!!" );
   }
   
   // Method to handle POST method request.
   public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
      
      doGet(request, response);
   }
}

Now calling the above servlet would display the following result −

HTTP Status 407 - Need authentication!!!


type Status report

messageNeed authentication!!!

descriptionThe client must first authenticate itself with the proxy (Need authentication!!!).

Apache Tomcat/5.5.29







SERVLETS - WRITING FILTERS


Servlet Filters are Java classes that can be used in Servlet Programming for the following purposes −


To intercept requests from a client before they access a resource at back end.


To manipulate responses from server before they are sent back to the client.



There are various types of filters suggested by the specifications −

Authentication Filters.
Data compression Filters.
Encryption Filters.
Filters that trigger resource access events.
Image Conversion Filters.
Logging and Auditing Filters.
MIME-TYPE Chain Filters.
Tokenizing Filters .
XSL/T Filters That Transform XML Content.


Filters are deployed in the deployment descriptor file web.xml and then map to either servlet names or URL patterns in your application's deployment descriptor.

When the web container starts up your web application, it creates an instance of each filter that you have declared in the deployment descriptor. The filters execute in the order that they are declared in the deployment descriptor.

Servlet Filter Methods

A filter is simply a Java class that implements the javax.servlet.Filter interface. The javax.servlet.Filter interface defines three methods −

Sr.No.
Method & Description
1

public void doFilter (ServletRequest, ServletResponse, FilterChain)

This method is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.

2

public void init(FilterConfig filterConfig)

This method is called by the web container to indicate to a filter that it is being placed into service.

3

public void destroy()

This method is called by the web container to indicate to a filter that it is being taken out of service.



Servlet Filter − Example

Following is the Servlet Filter Example that would print the clients IP address and current date time. This example would give you basic understanding of Servlet Filter, but you can write more sophisticated filter applications using the same concept −
// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// Implements Filter class
public class LogFilter implements Filter  {
   public void  init(FilterConfig config) throws ServletException {
      
      // Get init parameter 
      String testParam = config.getInitParameter("test-param"); 

      //Print the init parameter 
      System.out.println("Test Param: " + testParam); 
   }
   
   public void  doFilter(ServletRequest request, ServletResponse response,
      FilterChain chain) throws java.io.IOException, ServletException {

      // Get the IP address of client machine.
      String ipAddress = request.getRemoteAddr();

      // Log the IP address and current timestamp.
      System.out.println("IP "+ ipAddress + ", Time " + new Date().toString());

      // Pass request back down the filter chain
      chain.doFilter(request,response);
   }

   public void destroy( ) {
      /* Called before the Filter instance is removed from service by the web container*/
   }
}

Compile LogFilter.java in usual way and put your class file in /webapps/ROOT/WEB-INF/classes

Servlet Filter Mapping in Web.xml

Filters are defined and then mapped to a URL or Servlet, in much the same way as Servlet is defined and then mapped to a URL pattern. Create the following entry for filter tag in the deployment descriptor file web.xml

   LogFilter

LogFilter test-param Initialization Paramter LogFilter /*

The above filter would apply to all the servlets because we specified /* in our configuration. You can specicy a particular servlet path if you want to apply filter on few servlets only.

Now try to call any servlet in usual way and you would see generated log in your web server log. You can use Log4J logger to log above log in a separate file.

Using Multiple Filters

Your web application may define several different filters with a specific purpose. Consider, you define two filters AuthenFilter and LogFilter. Rest of the process would remain as explained above except you need to create a different mapping as mentioned below −

   LogFilter

LogFilter test-param Initialization Paramter AuthenFilter AuthenFilter test-param Initialization Paramter LogFilter /* AuthenFilter /*

Filters Application Order

The order of filter-mapping elements in web.xml determines the order in which the web container applies the filter to the servlet. To reverse the order of the filter, you just need to reverse the filter-mapping elements in the web.xml file.

For example, above example would apply LogFilter first and then it would apply AuthenFilter to any servlet but the following example would reverse the order −

   AuthenFilter

/* LogFilter /*