Servlet Life Cycle- Tcci.com

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.

• load or instantiation
• The servlet is initialized by calling the init()
• The servlet calls service()method to process a client's request.
• The servlet is terminated by calling the destroy()
• Finally, servlet is garbage collected by the garbage collector of the JVM.

Now let us discuss the life cycle methods in detail.

1.Instantiation: 

- At the time of starting the web container, it searches for the deployment descriptor (web.xml) for every web application.

- Once the servlet is found in the deployment descriptor, the container creates an instance of the Servlet class, which is considered as the loading of the servlet class.

The init() Method

The init method is called only once. It is called only when the servlet is created, and not called for any user requests afterwards. 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.

The init() method of GenericServlet class is invoked by the HttpServlet class which is the sub class of the GenericServlet, meaning that the HttpServlet inherits the init() method of GenericServlet.

- The init() method performs some specific actions like a constructor of a class which initializes an instance at the start up.

- It is automatically be invoked by the servlet container.

- This action causes the parsing by the application context (web.xml).

- The init() method is overloaded with zero parameters and with a parameter that takes a ServletConfig parameter.

- The init() method is invoked only once when the servlet is first loaded.

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 operations of a servlet is performed by the service () method.

- This method has HttpServletRequest and HttpServletResponse parameters.

- The service () method is invoked by the container and is called for each request processed.

- By default the method returns / dispatches the requests to the appropriate methods, usually a developer’s overridden helper methods such as doGet() and doPost().

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 doGet, 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 Diagram

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.

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