Learn Java for Web Development

62 CHAPTER 2: Building Web Applications Using Servlets and JSP

1. When a servlet instance is created, its init() method is invoked. The init()
   method allows a servlet to initialize itself before the first request is processed.
   You can specify init() parameters to the servlet in the web.xml file or via
   annotations.


2. The web container calls the init() method of the servlet (only once in the
   servlet’s life), and the init() method must complete before the container can
   call service() method.

service( )

The container calls the service() method of the servlet. This method looks at the request,
determines the type of HTTP method, and invokes the matching doGet() or doPost() on the servlet.
You never override it. Your job is to override doGet() or doPost() and let the service implementation
from HTTPServlet worry about calling the right one.

   When the service() method is called on your servlet, it will be handed

references to HttpServletRequest and HttpServletResponse objects that

implement the HttpServletRequest and HttpServletResponse interfaces.

The container implements these interfaces.

   For every request received to the servlet, the servlet’s service() method

is called. For HttpServlet subclasses, one of the doGet(), doPost(), and

so on, methods is typically called. The container creates two objects:

HTTPServletRequest and HttpServletResponse.

   The service() method can be called multiple times as long as the servlet is

active in the servlet container.

   The service() methods call doGet()/doPost(). You always override at least one

of them in your servlet.

destroy( )

The servlet container calls the destroy() method on the servlet, and this method is called only
once. This method provides the servlet with an opportunity to release acquired resources before
it is destroyed.

ServletContext and ServletConfig

After the init() method is called, the servlet gets one ServletConfig object per servlet and one
ServletContext per web application. In a distributed environment, you will have one ServletContext
per JVM. A ServletContext is a means by which a servlet can connect with both the container
and the other parts of the web application. In the servlet, the only time you have to go through the
ServletConfig to get the ServletContext is if your servlet is not an HttpServlet or a GenericServlet.