Answer Posted / keerthi
The javax.servlet package
The javax.servlet package defines the contract between
container and the servlet class. It provides the flexibility
to container vendor to implement the API the way they want
so long as they follow the specification. To the developers,
it provides the library to develop the servlet based
applications.
the javax.servlet Interfaces
The javax.servlet package is composed of 14 interfaces.
Servlet interface
The Servlet Interface is the central abstraction of the Java
Servlet API. It defines the life cycle methods of a servlet.
All the servlet implementations must implement it either
directly or indirectly by extending a class which implements
the Servlet interface. The two classes in the servlet API
that implement the Servlet interface are GenericServlet and
HttpServlet. For most purposes, developers will extend
HttpServlet to implement their Servlets while implementing
web applications employing the HTTP protocol.
ServletRequest interface
ServletRequest interface provides an abstraction of client
request. The object of ServletRequest is used to obtain the
client request information such as request parameters,
content type and length, locale of the client, request
protocol etc. Developers don’t need to implement this
interface. The web container provides the implementation
this interface. The web container creates an instance of the
implementation class and passes it as an argument when
calling the service() method.
ServletResponse interface
The ServletResponse interface assists a servlet in sending a
response to the client. Developers don’t need to implement
this interface. Servlet container creates the
ServletResponse object and passes it as an argument to the
servlet’s service() method.
ServletConfig interface
The servlet container uses a ServletConfig object to pass
initialization information to the servlet. ServletConfig
object is most commonly used to read the initialization
parameters. Servlet intialialization parameters are
specified in deployment descriptor (web.xml) file. Servlet
container passes the ServletConfig object as an argument
when calling servlet’s init() method.
ServletContext interface
ServletContext object is used to communicate with the
servlet container. There is only one ServletContext object
per web application (per JVM). This is initialized when the
web application is started and destroyed only when the web
application is being shutdown.
ServletContext object is most commonly used to obtain the
MIME type of a file, dispatching a request, writing to
server’s log file, share the data across application, obtain
URL references to resources etc.
SingleThreadModel Interface
SingleThreadModel is a marker interface. It is used to
ensure that servlet handle only one request at a time.
Servlets can implement SingleThreadModel interface to inform
the container that it should make sure that only one thread
is executing the servlet’s service() method at any given moment.
RequestDispatcher Interface
The RequestDispatcher interface is used to dispatch requests
to other resources such a servlet, HTML file or a JSP file.
Filter Interface
Filter interface declares life cycle methods of a filter.
The life cycle methods include, init() doFilter() and destroy().
FilterChain Interface
The FilterChain object provides the filter with a handle to
invoke the rest of the filter chain. Each filter gets access
to a FilterChain object when its doFilter() method is
invoked. Using this object, the filter can invoke the next
filter in the chain.
FilterConfig interface
The FilterConfig interface is similar to ServletConfig
interface. It is used to get the initialization parameters.
ServletContextAttributeListner
Implementation of this interface receives notification
whenever an attribute is added, removed or replaced in
ServletContext. To receive the notifications, the
implementation class must be configured in the deployment
descriptor (web.xml file).
ServletContextListner Interface
Implementation of this interface receives notification when
the context is created and destroyed.
ServletRequestAttributeListner Interface
Implementation of this interface receives notification
whenever an attribute is added, removed or replaced in
ServletRequest.
ServletRequestListner Interface
Implementation of this interface receives notification
whenever the request is coming in and out of scope. A
request is defined as coming into scope when it is about to
enter the first servlet or filter in each web application,
as going out of scope when it exits the last servlet or the
first filter in the chain.
The javax.servlet Classes
The javax.servlet package contains 9 classes.
GenericServlet class
The GenericServlet abstract class defines the generic
protocol independent servlet. It can be extended to develop
our own protocol-based servlet.
ServletContextEvent class
This is the event class for notifications about changes to
the servlet context of a web application.
ServletInputStream class
Provides an input stream for reading binary data from a
client request.
ServletOutputStream class
Provides an output stream for sending binary data to the client.
ServletRequestAttributeEvent class
This is the event class for notifications of changes to the
attributes of ServletRequest in an application.
ServletRequestEvent class
This event class indicates life cycle events of a
ServletRequest.
ServletRequestWrapper class
ServletRequestWrapper class provides a convenient
implementation of ServletRequest interface.
ServletResponseWrapper class
ServletResponseWrapper class provides a convenient
implementation of ServletResponse interface.
javax.servlet Exception classes
The javax.servlet package defines 2 exception classes.
ServletException class
Defines a general exception a servlet can throw when it
encounters difficulty.
UnavailableException class
Defines an exception that a servlet or filter throws to
indicate that it is permanently or temporarily unavailable.
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