http://www.jboss.org/jboss-servlet-api_4.0_spec: The Java Servlet 4.0 API classes (JBoss by Red Hat) 
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package javax.servlet;

import java.io.IOException;



Defines methods that all servlets must implement.

A servlet is a small Java program that runs within a Web server.
Servlets receive and respond to requests from Web clients,
usually across HTTP, the HyperText Transfer Protocol. 

To implement this interface, you can write a generic servlet
that extends
javax.servlet.GenericServlet or an HTTP servlet that
extends javax.servlet.http.HttpServlet.

This interface defines methods to initialize a servlet,
to service requests, and to remove a servlet from the server.
These are known as life-cycle methods and are called in the
following sequence:

    

  1. The servlet is constructed, then initialized with the init method.

  2. Any calls from clients to the service method are handled.

  3. The servlet is taken out of service, then destroyed with the 
destroy method, then garbage collected and finalized.



In addition to the life-cycle methods, this interface
provides the getServletConfig method, which the servlet 
can use to get any startup information, and the getServletInfo
method, which allows the servlet to return basic information about itself,
such as author, version, and copyright.

Author:	Various
See Also:
/**
 * Defines methods that all servlets must implement.
 *
 * <p>A servlet is a small Java program that runs within a Web server.
 * Servlets receive and respond to requests from Web clients,
 * usually across HTTP, the HyperText Transfer Protocol. 
 *
 * <p>To implement this interface, you can write a generic servlet
 * that extends
 * <code>javax.servlet.GenericServlet</code> or an HTTP servlet that
 * extends <code>javax.servlet.http.HttpServlet</code>.
 *
 * <p>This interface defines methods to initialize a servlet,
 * to service requests, and to remove a servlet from the server.
 * These are known as life-cycle methods and are called in the
 * following sequence:
 * <ol>
 * <li>The servlet is constructed, then initialized with the <code>init</code> method.
 * <li>Any calls from clients to the <code>service</code> method are handled.
 * <li>The servlet is taken out of service, then destroyed with the 
 * <code>destroy</code> method, then garbage collected and finalized.
 * </ol>
 *
 * <p>In addition to the life-cycle methods, this interface
 * provides the <code>getServletConfig</code> method, which the servlet 
 * can use to get any startup information, and the <code>getServletInfo</code>
 * method, which allows the servlet to return basic information about itself,
 * such as author, version, and copyright.
 *
 * @author 	Various
 *
 * @see 	GenericServlet
 * @see 	javax.servlet.http.HttpServlet
 *
 */


public interface Servlet {

    
Called by the servlet container to indicate to a servlet that the 
servlet is being placed into service.

The servlet container calls the init
method exactly once after instantiating the servlet.
The init method must complete successfully
before the servlet can receive any requests.

The servlet container cannot place the servlet into service
if the init method

    

  1. Throws a ServletException

  2. Does not return within a time period defined by the Web server



Params:
  • config – 			a ServletConfig object 
				containing the servlet's
					configuration and initialization parameters
Throws:
  • ServletException – 	if an exception has occurred that
				interferes with the servlet's normal
				operation
See Also:
/**
     * Called by the servlet container to indicate to a servlet that the 
     * servlet is being placed into service.
     *
     * <p>The servlet container calls the <code>init</code>
     * method exactly once after instantiating the servlet.
     * The <code>init</code> method must complete successfully
     * before the servlet can receive any requests.
     *
     * <p>The servlet container cannot place the servlet into service
     * if the <code>init</code> method
     * <ol>
     * <li>Throws a <code>ServletException</code>
     * <li>Does not return within a time period defined by the Web server
     * </ol>
     *
     *
     * @param config			a <code>ServletConfig</code> object 
     *					containing the servlet's
     * 					configuration and initialization parameters
     *
     * @exception ServletException 	if an exception has occurred that
     *					interferes with the servlet's normal
     *					operation
     *
     * @see 				UnavailableException
     * @see 				#getServletConfig
     *
     */

    public void init(ServletConfig config) throws ServletException;
    
    

    
Returns a ServletConfig object, which contains initialization and startup parameters for this servlet. The ServletConfig object returned is the one 
passed to the init method. 

Implementations of this interface are responsible for storing the 
ServletConfig object so that this method can return it. The GenericServlet class, which implements this interface, already does this. 
See Also:
Returns:		the ServletConfig object
		that initializes this servlet
/**
     *
     * Returns a {@link ServletConfig} object, which contains
     * initialization and startup parameters for this servlet.
     * The <code>ServletConfig</code> object returned is the one 
     * passed to the <code>init</code> method. 
     *
     * <p>Implementations of this interface are responsible for storing the 
     * <code>ServletConfig</code> object so that this 
     * method can return it. The {@link GenericServlet}
     * class, which implements this interface, already does this.
     *
     * @return		the <code>ServletConfig</code> object
     *			that initializes this servlet
     *
     * @see 		#init
     *
     */

    public ServletConfig getServletConfig();
    
    

    
Called by the servlet container to allow the servlet to respond to 
a request.

This method is only called after the servlet's init()
method has completed successfully.

  The status code of the response always should be set for a servlet 
that throws or sends an error.

Servlets typically run inside multithreaded servlet containers
that can handle multiple requests concurrently. Developers must 
be aware to synchronize access to any shared resources such as files,
network connections, and as well as the servlet's class and instance 
variables. 
More information on multithreaded programming in Java is available in 

the Java tutorial on multi-threaded programming.

Params:
  • req – 	the ServletRequest object that contains
		the client's request
  • res – 	the ServletResponse object that contains
		the servlet's response
Throws:
  • ServletException – 	if an exception occurs that interferes
				with the servlet's normal operation
  • IOException – 		if an input or output exception occurs
/**
     * Called by the servlet container to allow the servlet to respond to 
     * a request.
     *
     * <p>This method is only called after the servlet's <code>init()</code>
     * method has completed successfully.
     * 
     * <p>  The status code of the response always should be set for a servlet 
     * that throws or sends an error.
     *
     * 
     * <p>Servlets typically run inside multithreaded servlet containers
     * that can handle multiple requests concurrently. Developers must 
     * be aware to synchronize access to any shared resources such as files,
     * network connections, and as well as the servlet's class and instance 
     * variables. 
     * More information on multithreaded programming in Java is available in 
     * <a href="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">
     * the Java tutorial on multi-threaded programming</a>.
     *
     *
     * @param req 	the <code>ServletRequest</code> object that contains
     *			the client's request
     *
     * @param res 	the <code>ServletResponse</code> object that contains
     *			the servlet's response
     *
     * @exception ServletException 	if an exception occurs that interferes
     *					with the servlet's normal operation 
     *
     * @exception IOException 		if an input or output exception occurs
     *
     */

    public void service(ServletRequest req, ServletResponse res)
	throws ServletException, IOException;
	
	

    
Returns information about the servlet, such
as author, version, and copyright.

The string that this method returns should
be plain text and not markup of any kind (such as HTML, XML,
etc.).

Returns:		a String containing servlet information
/**
     * Returns information about the servlet, such
     * as author, version, and copyright.
     * 
     * <p>The string that this method returns should
     * be plain text and not markup of any kind (such as HTML, XML,
     * etc.).
     *
     * @return 		a <code>String</code> containing servlet information
     *
     */

    public String getServletInfo();
    
    

    
Called by the servlet container to indicate to a servlet that the
servlet is being taken out of service.  This method is
only called once all threads within the servlet's
service method have exited or after a timeout
period has passed. After the servlet container calls this 
method, it will not call the service method again
on this servlet.

This method gives the servlet an opportunity 
to clean up any resources that are being held (for example, memory,
file handles, threads) and make sure that any persistent state is
synchronized with the servlet's current state in memory.

/**
     *
     * Called by the servlet container to indicate to a servlet that the
     * servlet is being taken out of service.  This method is
     * only called once all threads within the servlet's
     * <code>service</code> method have exited or after a timeout
     * period has passed. After the servlet container calls this 
     * method, it will not call the <code>service</code> method again
     * on this servlet.
     *
     * <p>This method gives the servlet an opportunity 
     * to clean up any resources that are being held (for example, memory,
     * file handles, threads) and make sure that any persistent state is
     * synchronized with the servlet's current state in memory.
     *
     */

    public void destroy();
}