Servlet container create only one instance of each servlet but the request of each user is handled with a separate thread. Each thread then calls the doGet or doPost methods. This idea is shown in the above Figure-5.

1.The init method of the servlets is called when the servlets is first created and each time the server receives a request for a servlets, the server spawns a new thread calls service method.

2.The service method check the HTTP request type (GET,SET, PUT, DELETE, etc.) and calls doGet, doPost, doPut,doDelete, etc. method as appropriate.

3.Finally, the server may decide to remove a previous loaded servlet. In this case, the server calls destroy method of the servlets.

Before we can start writing the first Servlets, we need to know some basics of HTTP ("HyperText Transfer Protocol"), the protocol which is used by a WWW client (e.g. a browser) to send a request to a Web Server.

HTTP is the request-response oriented protocol. An HTTP request consist of a request method, a URI, header fields and a body (which can be empty). An HTTP response contain a result and again header fields and a body.

The service method of HttpServlet dispatch a request to different Java methods for different HTTP request methods. It recognize the standard HTTP/1.1 methods and should not be overridden in subclasses unless you need to implement additional methods. The recognized methods are GET, PUT,HEAD, POST, DELETE, OPTIONS and TRACE. Other methods are answered with a Bad Request HTTP error. An HTTP method XXX is dispatched to the Java method doXxx, e.g. GET -> doGet. All these method expect the parameters "(HttpServletRequest req, HttpServletResponse res)". The method doOptions and doTrace have suitable default implementations and are usually not overridden. The HEAD method (which is supposed to return the same header lines that a GET method would return, but doesn't include a body) is performed by calling the doGet and ignoring any output that is written by this method. That leaves us with the method doGet doPut doPost and doDelete whose default implementations in HttpServlet return a Bad Request HTTP error. A subclass of HttpServlet overrides one or more of these method to provide a meaningful implementation.

The request data is passed to all the methods through the first argument of type HttpServletRequest (which is a subclass of the more general ServletRequest class). The response can be created with the methods of the second argument of type HttpServletResponse (a subclass of ServletResponse).

When you request a URL in the Web Browser, the GET method is used for the request. A GET request does not have the body (i.e. the body is empty). The response should contain the body with the response data and header fields which describe the body (especially Content-Type and Content-Encoding). When you send an HTML form, either GET or POST action can be used. With the GET request the parameters are end in the URL, with a POST request they are transmited in the body. HTML editors and upload tools use PUT requests to the upload resources to a Web Server and DELETE requests to delete resources.

There are two types of package available in servlets which are as follows:

1.javax.servlet.*

2.javax.servlet.http.*

The <minOccurs> indicator specify the minimum number of time an element can occur:

There are four scope objects in servlets which enables the sharing information between web components. The scope objects and their corresponding Java classes are listed below:

You can get the attribute values from servlet scope objects using getAttribute method and set new values of attributes using setAttribute method. For example, you can get the client’s IP address by calling the getRemoteAddr method of HttpServletRequest class.

To be a servlet, a class should extend the HttpServlet and override doGet or doPost (or both), depending on whether the data is being sent by GET or by POST. These methods take two arguments: an HttpServletRequest and an HttpServletResponse. The HttpServletRequest has methods that let you find out about the incoming information such as FORM data, HTTP request headers, and the like.

The HttpServletResponse has method that lets you specify the HTTP response line (200, 404, etc.), response headers (Content-Type, Set-Cookie, etc.), and, most importantly, lets you obtain a PrintWriter used to send output back to the client. For simple servlet, most of the effort is spent in println statements that generate the desired page. Note that doGet and doPost throw two exception, so you are required to include them in the declaration. Also note that you have to import classes in the java.io (for PrintWriter, etc.), javax.servlet (for HttpServlet, etc.), and javax.servlet.http (for HttpServletRequest and HttpServletResponse). Finally, note that doGet and doPost are called by service method, and sometimes you may want to override service directly, e.g. for a servlet that handles both GET and POST request.

Note that the specific details for installing servlet vary from Web server to Web server. Please refer to your Web server documentation for the definitive directions. The on-line examples are running on Java Web Server (JWS) 2.0, where servlet are expected to be in a directory called servlets in the JWS installation hierarchy. However, I placed this servlets in a separate package (hall) to avoid conflicts with other servlets on this server; you'll want to do the same if you are using a Web server that is used by other people and doesn't have a good infrastructure for the "virtual servers" to prevent these conflicts automatically. Thus, HelloWorld.java actually goes in a subdirectory called hall in the servlets directory.

Note that setup on most other servers is similar, and the servlets and JSP examples in the tutorial have also been tested using BEA WebLogic and IBM WebSphere 3.0. WebSphere has an excellent mechanism for virtual servers, and it is not necessary to use packages solely to prevent name conflicts with other users.

If you've never used the packages before, there are two main ways to compile classes that are in packages.

One way is to set your CLASSPATH to point to directory above the one actually containing your servlets. You can them compile normally from within the directory. For example, if your base directory is C:\JavaWebServer\servlets and your package name (and thus subdirectory name) is the hall, and you were on Windows, you'd do:

  DOS> set CLASSPATH=C:\JavaWebServer\servlets;%CLASSPATH%
  DOS> cd C:\JavaWebServer\servlets\hall
  DOS> javac YourServlet.java

The first part, setting CLASSPATH, you probably want to do permanently, rather than each time you start a new DOS window. On Windows 95/98 you'd typically put the "set CLASSPATH=..." statement in your autoexec.bat file somewhere after the line that set CLASSPATH to point to servlet.jar and jsp.jar. On Windows NT, you'd go to Start menu, select Settings, select Control Panel, select System, select Environment, then enter the variable and value. Note also that if your package were of the form name1.name2.name3 rather than simply name1 as here, you'd still have the CLASSPATH point to the top-level directory of your package hierarchy (the one containing name1).

A second way to compile classes that are in packages is to go to directory above the one containing your servlets, and then do "javac directory\YourServlet.java" (Windows; note the backslash) or "javac directory/YourServlet.java" (Unix; note the forward slash). For example, suppose again that your base directory is C:\JavaWebServer\servlets and your package name (and thus subdirectory name) is hall, and you were on Windows. In that case, you'd do following:

  DOS> cd C:\JavaWebServer\servlets
  DOS> javac hall\YourServlet.java

Note that, on Windows, most JDK 1.1 versions of javac require a backslash, not a forward slash, after directory name. This is fixed in JDK 1.2, but since many Web servers are configured to use the JDK 1.1, many servlet authors stick with JDK 1.1 for portability.

Finally, another advanced option is to keep source code in a location distinct from the .class files, and use javac's "-d" option to install them in the location the Web server expects.

With the Java Web Server, servlet are placed in the servlets directory within the main JWS installation directory, and are invoked via http://host/servlet/ServletName. Note that the directory is servlets plural, while URL refers to servlet, singular. Since this example was placed in the hall package, it would be invoked via http://host/servlet/hall.HelloWorld. Other Web servers may have slightly different conventions on where to install servlets and how to invoke them. Most server also let you define aliases for servlets, so that a servlet can be invoked via http://host/any-path/any-file.html. The process for doing this is completely server-specific; check your server's documentation for the details.

Most servlet generate HTML, not plain text as in the previous example. To do that, you need two additional steps: tell the browser that you're sending back HTML, and modify the println statements to build a legal Web page. The first step is done by setting Content-Type response header. In general, headers can be set via the setHeader method of HttpServletResponse, but setting the content type is such a common task that there is also a special setContentType method just for this purpose. Note that you need to set the response headers before actually returning any of the content via the PrintWriter. Here's an example:

O/P: