Java Reference
In-Depth Information
GenericServlet
Most servlets provide similar basic functionality through an abstract
javax.servlet.GenericServlet
class provided by the Servlet API. The
GenericServlet
class implementation is protocol-independent,
so it does not matter if it has to respond to HTTP or FTP requests. The
GenericServlet
abstract
class defines an
init()
method that is called by the default
init(ServletConfig)
method to execute
any application-specific servlet initialization.
HttpServlet
In a web application, lack of any protocol-dependent processing in a
GenericServlet
class signifies
that the developer has to write the code for this processing in any subclass she creates. Since
HTTP is the most well-known and widely used protocol on the Web, the Servlet API also includes
one more abstract subclass of
GenericServlet
:
javax.servlet.http.HttpServlet
. The
service()
method is implemented by
HttpServlet
, which inspects the incoming HTTP method and invokes the
appropriate method for that request type.
Note
HTTP/1.1 defines seven request methods. The
HttpServlet
class provides default implementations
for each of these methods that you can override in your servlets. However, most web application comprise
servlets that override only the
doGet()
and
doPost()
methods.
An HTTP request to an
HttpServlet
subclass goes through a number of steps:
1.
A call to the public
service (ServletRequest, ServletResponse)
method by
the container.
2.
Delegation of this call to
HttpServlet
's protected
service
(HttpServletRequest, HttpServletResponse)
method.
3.
The protected
service (HttpServletRequest, HttpServletResponse)
method then delegates to the appropriate
doXxx
method, depending on the
HTTP method used for the request.
HelloWorld Servlet
As mentioned in the previous section, the superclass of your servlet includes two versions of
init()
,
one that takes a
ServletConfig
and one that's a no-arg. The
init(ServletConfig)
method calls the
no-arg
init()
, so you need to override only the no-arg version.
init( )
The container calls
init()
on the servlet instance. You can override it in order to get database
connections registered with other objects. Otherwise, the
init()
method from the
Genericservlet
runs.
