What are the new features added to Servlet 2.5?

• A new dependency on J2SE 5.0
• Support for annotations
• Loading the class
• Several web.xml conveniences
• A handful of removed restrictions
• Some edge case clarifications

What are the uses of Servlet?

     Typical uses for HTTP Servlets include:

• Processing and/or storing data submitted by an HTML form.
• Providing dynamic content, e.g. returning the results of a database query to the client.
• A Servlet can handle multiple request concurrently and be used to develop high performance system
• Managing state information on top of the stateless HTTP, e.g. for an online shopping cart system which manages shopping carts for many concurrent customers and maps every request to the right customer.

What are the advantages of Servlet over CGI ?

       Servlets have several advantages over CGI:

• A Servlet does not run in a separate process. This removes the overhead of creating a new process for each request.
• A Servlet stays in memory between requests. A CGI program (and probably also an extensive runtime system or interpreter) needs to be loaded and started for each CGI request.
• There is only a single instance which answers all requests concurrently. This saves memory and allows a Servlet to easily manage persistent data.
• Several web.xml conveniences
• A handful of removed restrictions
• Some edge case clarifications

   What is CGI ?

                  The Common Gateway Interface (CGI) is a standard for writing programs that can interact through a Web server with a client running a Web browser. These programs allow a Web developer to deliver dynamic information (usually in the form of HTML) via the browser. A CGI program can be written in any language, including Java, that can be executed by your Web server. CGI programs are commonly used to add search engines, guest-book applications, database-query engines, interactive-user forums, and other interactive applications to Web sites.

      In very basic terms, a CGI program must interpret the information sent to it, process the information in some way, and generate a response that will be sent back to the client.

     

       Most of the input to a CGI program is passed into it through environment variables. This article will demonstrate how to send these environment variables to a Java CGI program. The rest of the input (if any) is passed into a CGI program as standard input that can be read directly by your program.

    The processing can be as simple as appending information to a file or as complex as requesting data from a database.

Since a CGI program can return a myriad of document types, a CGI program must place a short header (ASCII text) on its output so that the client will know how to interpret the information it generates. Most commonly, CGI programs generate HTML. Below, you will find a library of functions including one that generates the appropriate header for HTML. Following the header, a CGI program simply generates the body of the output in its native form.

Passing the CGI environment into the Java program..

     Writing a CGI program in Java is fairly easy to do once you understand the issues. First and foremost, you need to wrap the execution of the Java program inside another script. So, the actual script invoked on your Web server will be a Unix shell script or a Windows batch file (or equivalent) that simply passes the CGI environment variables into your Java program.

Since Java no longer provides a method to access environment variables directly (theSystem.getenv() method has been disabled in the latest release of the JDK), I propose passing each CGI environment variable into the Java program using the -D command-line parameter on the Java interpreter. I will show you how to use the -D parameter below.

The library of functions I provide below assumes that you have used the approach described above; it uses theSystem.getProperty() method to access those command-line parameters. If your program needs to use any of the CGI environment variables, you can access them the same way. For example, if you want to access the SERVER_NAME environment variable, you could do so as follows:

                                      

                              String server_name =  

System.getProperty("cgi.server_name");  

 

Be aware that I am not passing all of the CGI environment variables into my Java program. I'm only passing the major ones. I'll leave the inclusion of the others as an exercise for the reader.  

 

The following example shows a Unix script file called hello.cgi invoking a Java program called hello. Note that the -D command-line parameter passes the CGI environment variables into the Java program:

#!/bin/sh
java     -Dcgi.content_type=$CONTENT_TYPE     -Dcgi.content_length=$CONTENT_LENGTH     -Dcgi.request_method=$REQUEST_METHOD     -Dcgi.query_string=$QUERY_STRING     -Dcgi.server_name=$SERVER_NAME     -Dcgi.server_port=$SERVER_PORT     -Dcgi.script_name=$SCRIPT_NAME     -Dcgi.path_info=$PATH_INFO   hello