Writing Java Servlets

What are Servlets?

While CGI programs can be written in Java, Java servlets add additional functionality including security, dynamic loading from remote servers, and the ability to add functionality to your web server.

Unlike CGI programs, servlets are loaded once and then stay resident. Future service calls to the servlet are faster since the servlet is already in memory.

Servlets can maintain state information between service calls.

The Servlet API is supported by most major web servers.

Servlets vs. Applets

They are often compared to "applets", which are client-side Java programs that are loaded and run with within the framework of a web browser.

Instead of subclassing the java.applet.Applet class, you write your servlets by subclassing a class that implements the javax.servlet.Servlet interface.

Servlets are called "faceless" in that do not have a GUI component. Note that the Applet class extends the java.awt.Panel class which necessitates an interactive interface.

Servlet Life-cycle

• Initialization: accepts configuration and initializes the state;
• Service: processes zero or more user requests and returns output; and
• Destruction: final preparations for shutdown and release of memory resources.

The corresponding life-cycle methods are

• init ( ServletConfig )
• service ( ServletRequest, ServletResponse )
• destroy ( )

Note that this differs slightly from the Applet life-cycle which includes init ( ), start ( ), stop ( ), and destroy ( ).

Hello, World!

It subclasses GenericServlet, a ready-made simple implementation of the Servlet interface. Note that I have included the init ( ) and destroy ( ) method calls to remind you that they can be overloaded.

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

public class HelloServlet extends GenericServlet {
//////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////

public void  init ( ServletConfig  config ) {
//////////////////////////////////////////////////////////////////////
  super.init ( config );
}

public void  service (
  ServletRequest  req, ServletResponse  res )
  throws ServletException, IOException {
//////////////////////////////////////////////////////////////////////
  PrintStream  out = new PrintStream ( res.getOutputStream ( ) );
  out.println ( "Hello, World!" );
}

public void  destroy ( ) {
//////////////////////////////////////////////////////////////////////
  super.destroy ( );
}

//////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////
}

Example: Random GIF

This servlet extends javax.servlet.http.HttpServlet. The HttpServlet class "understands" HTTP requests coming from web browsers and provides additional functionality.

In this example, the output is a dynamically generated graphics image file (GIF).

public class  RandomGIFServlet extends HttpServlet {
//////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////

/*********************************************************************
* Services a single request from a client.
*********************************************************************/
public void  doGet (
  HttpServletRequest   req,
  HttpServletResponse  res )
  throws ServletException, IOException {
//////////////////////////////////////////////////////////////////////
  short  w = 16;
  short  h = 16;
  int [ ]  values = new int [ w * h ];
  int  i = 0;
  for ( short  x = 0; x < w; x++ ) {
    for ( short  y = 0; y < h; y++ ) {
      values [ i++ ] = ( int ) ( Math.random ( ) * 0xFFFFFF );
    }
  }
  try {
    GIFEncoder  gifEncoder = new GIFEncoder ( w, h, values );
    res.setContentType ( "image/gif" );
    ServletOutputStream  out = res.getOutputStream ( );
    gifEncoder.Write ( out );
  } catch ( AWTException  e ) { }
}

//////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////
}

Post Example

<FORM ACTION="postservlet" METHOD=POST>
<INPUT TYPE=TEXT NAME="name" SIZE=40 VALUE="Enter your name here."><BR>
<INPUT TYPE=TEXT NAME="mail" SIZE=40 VALUE="Your e-mail address." ><BR>
<TEXTAREA ROWS=2 COLS=60 NAME="note">Send me a note here!</TEXTAREA><BR>
<INPUT TYPE="SUBMIT" VALUE="Send">
</FORM>

public void  doPost (
  HttpServletRequest  req, HttpServletResponse  res )
  throws ServletException, IOException {
//////////////////////////////////////////////////////////////////////
  res.setContentType ( "text/html" );
  ServletOutputStream  out = res.getOutputStream ( );

  try {

    out.println ( "<html><head><title>" + getTitle ( )
      + "</title></head>" );
    out.println ( "<body><center><H1>" + getTitle ( ) + "</H1>" );

    String  name = req.getParameter ( "name" );
    String  mail = req.getParameter ( "mail" );
    String  note = req.getParameter ( "note" );

    out.println ( "Name:  " + name + "<BR>" );
    out.println ( "Mail:  " + mail + "<BR>" );
    out.println ( "Note:  " + note + "<BR>" );

  } catch ( Throwable  t ) {
    out.println ( "<P></CENTER><PRE>" );
    t.printStackTrace ( new PrintStream ( out ) );
    out.println ( "</PRE><CENTER><P>" );
  }

  out.println ( "</center></body></html>" );
}

Java Web Server

It is free for non-commercial use and commercial licenses are inexpensive.

It runs on all platforms that support the JDK 1.1 but the least buggy versions run on Solaris, NT, and Win 95.

You can install it anywhere that you have command-line access, either remotely via telnet or on your desktop computer.

You do not need "root" access to install this program as you can hang it off of any port you choose. To hang it off of the default HTTP port 80, you will need root access. Otherwise, you will need to address your web pages using a URL of the form "http://www.domain.edu:8080/" where the port number is specifically included.

References

ANSER Java Agents and Extraneous (AJAX): a collection of servlet demos.