Java Servlet Life Cycle

A servlet is a Java programming language class that is used to extend the capabilities of servers that host applications accessed by means of a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by web servers. For such applications, Java Servlet technology defines HTTP-specific servlet classes.

The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines life-cycle methods. When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

Servlet Life Cycle

The life cycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.

1.      If an instance of the servlet does not exist, the web container

a.      Loads the servlet class.

b.      Creates an instance of the servlet class.

c.       Initializes the servlet instance by calling the init method.

2.      Invokes the service method, passing request and response objects.

3.      If the container needs to remove the servlet, it finalizes the servlet by calling the servlet’s destroy method.

Lets try to understand the Servlet Life Cycle in details

1.      A web browser sends an HTTP request to a web server by any one of the following ways.

a.      A user clicks on a hyperlink displayed in an HTML page.

b.      A user fills out a form in an HTML page and submits it.

c.       A user enters a URL in the browser’s address field and presses Enter.

2.      The container "sees" that the request is for a servlet, so the web container creates two objects.

a.      HttpServletRequest

b.      HttpServletResponse

3.      The Web Container finds the correct servlet based on the URL in the request , the container then creates or allocate a thread for that request and calls the service() method of the servlet, passing the request and response objects as arguments.

4.      The service() method figures out which servlet method to call based on HTTP method sent by the client.If the client sent GET , then corresponding doGet() method is called.

5.      The servlet uses the response object to write the response to the client.

6.      The service() method completes, the threads dies or returns to  back to the pool.The request and response object are garbage collected.The client gets the response.

Servlet Life Cycle