Improve knowledge about Servlets - Life Cycle

Servlets - Life Cycle

A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet

·        The servlet is initialized by calling the init () method.

·        The servlet calls service() method to process a client's request.

·        The servlet is terminated by calling the destroy() method.

·        Finally, servlet is garbage collected by the garbage collector of the JVM.

Now let us discuss the life cycle methods in details.

The init() method :

The init method is designed to be called only once. It is called when the servlet is first created, and not called again for each user request. So, it is used for one-time initializations, just as with the init method of applets.

The servlet is normally created when a user first invokes a URL corresponding to the servlet, but you can also specify that the servlet be loaded when the server is first started.

When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost as appropriate. The init() method simply creates or loads some data that will be used throughout the life of the servlet.

The init method definition looks like this:

public void init() throws ServletException {

  // Initialization code...

}

The service() method :

The service() method is the main method to perform the actual task. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client( browsers) and to write the formatted response back to the client.

Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.

Here is the signature of this method:

public void service(ServletRequest request, 

                    ServletResponse response) 

      throws ServletException, IOException{

}

The service () method is called by the container and service method invokes doGe, doPost, doPut, doDelete, etc. methods as appropriate. So you have nothing to do with service() method but you override either doGet() or doPost() depending on what type of request you receive from the client.

The doGet() and doPost() are most frequently used methods with in each service request. Here is the signature of these two methods.

The doGet() Method

A GET request results from a normal request for a URL or from an HTML form that has no METHOD specified and it should be handled by doGet() method.

public void doGet(HttpServletRequest request,

                  HttpServletResponse response)

    throws ServletException, IOException {

    // Servlet code

}

The doPost() Method

A POST request results from an HTML form that specifically lists POST as the METHOD and it should be handled by doPost() method.

public void doPost(HttpServletRequest request,

                   HttpServletResponse response)

    throws ServletException, IOException {

    // Servlet code

}

The destroy() method :

The destroy() method is called only once at the end of the life cycle of a servlet. This method gives your servlet a chance to close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities.

After the destroy() method is called, the servlet object is marked for garbage collection. The destroy method definition looks like this:

  public void destroy() {

    // Finalization code...

  }

Architecture Digram:

The following figure depicts a typical servlet life-cycle scenario.

·        First the HTTP requests coming to the server are delegated to the servlet container.

·        The servlet container loads the servlet before invoking the service() method.

·        Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet.

Servlets - Examples

Servlets are Java classes which service HTTP requests and implement thejavax.servlet.Servlet interface. Web application developers typically write servlets that extend javax.servlet.http.HttpServlet, an abstract class that implements the Servlet interface and is specially designed to handle HTTP requests.

Sample Code for Hello World:

Following is the sample source code structure of a servlet example to write Hello World:

// Import required java libraries

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

// Extend HttpServlet class

public class HelloWorld extends HttpServlet {

  private String message;

  public void init() throws ServletException

  {

      // Do required initialization

      message = "Hello World";

  }

  public void doGet(HttpServletRequest request,

                    HttpServletResponse response)

            throws ServletException, IOException

  {

      // Set response content type

      response.setContentType("text/html");

      // Actual logic goes here.

      PrintWriter out = response.getWriter();

      out.println("<h1>" + message + "</h1>");

  }

  public void destroy()

  {

      // do nothing.

  }

}

Compiling a Servlet:

Let us put above code if HelloWorld.java file and put this file in C:\ServletDevel (Windows) or /usr/ServletDevel (Unix) then you would need to add these directories as well in CLASSPATH.

Assuming your environment is setup properly, go in ServletDevel directory and compile HelloWorld.java as follows:

$ javac HelloWorld.java

If the servlet depends on any other libraries, you have to include those JAR files on your CLASSPATH as well. I have included only servlet-api.jar JAR file because I'm not using any other library in Hello World program.

This command line uses the built-in javac compiler that comes with the Sun Microsystems Java Software Development Kit (JDK). For this command to work properly, you have to include the location of the Java SDK that you are using in the PATH environment variable.

If everything goes fine, above compilation would produce HelloWorld.class file in the same directory. Next section would explain how a compiled servlet would be deployed in production.

Servlet Deployment:

By default, a servlet application is located at the path <Tomcat-installation-directory>/webapps/ROOT and the class file would reside in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/classes.

If you have a fully qualified class name of com.myorg.MyServlet, then this servlet class must be located in WEB-INF/classes/com/myorg/MyServlet.class.

For now, let us copy HelloWorld.class into <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/classes and create following entries in web.xml file located in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/

<servlet>

   <servlet-name>HelloWorld</servlet-name>

   <servlet-class>HelloWorld</servlet-class>

</servlet>

<servlet-mapping>

   <servlet-name>HelloWorld</servlet-name>

   <url-pattern>/HelloWorld</url-pattern>

</servlet-mapping>

Above entries to be created inside <web-app>...</web-app> tags available in web.xml file. There could be various entries in this table already available, but never mind.

You are almost done, now let us start tomcat server using <Tomcat-installation-directory>\bin\startup.bat (on windows) or <Tomcat-installation-directory>/bin/startup.sh (on Linux/Solaris etc.) and finally type http://localhost:8080/HelloWorld in browser's address box. If everything goes fine, you would get following result: