javax.servlet
Interface Filter

public interface Filter

A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.

Filters perform filtering in the doFilter method. Every Filter has access to
a FilterConfig object from which it can obtain its initialization parameters, a
reference to the ServletContext which it can use, for example, to load resources
needed for filtering tasks.

Filters are configured in the deployment descriptor of a web application

destroy()
Called by the web container to indicate to a filter that it is being taken out of service.

void

doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
The doFilter method of the Filter is called by the container
each time a request/response pair is passed through the chain due
to a client request for a resource at the end of the chain.

void

init(FilterConfig filterConfig)
Called by the web container to indicate to a filter that it is being placed into
service.

Method Detail

init

Called by the web container to indicate to a filter that it is being placed into
service. The servlet container calls the init method exactly once after instantiating the
filter. The init method must complete successfully before the filter is asked to do any
filtering work.

The web container cannot place the filter into service if the init method either
1.Throws a ServletException
2.Does not return within a time period defined by the web container

doFilter

The doFilter method of the Filter is called by the container
each time a request/response pair is passed through the chain due
to a client request for a resource at the end of the chain. The FilterChain passed in to this
method allows the Filter to pass on the request and response to the next entity in the
chain.

A typical implementation of this method would follow the following pattern:-
1. Examine the request
2. Optionally wrap the request object with a custom implementation to
filter content or headers for input filtering
3. Optionally wrap the response object with a custom implementation to
filter content or headers for output filtering
4. a) Either invoke the next entity in the chain using the FilterChain object (chain.doFilter()),
4. b) or not pass on the request/response pair to the next entity in the filter chain to block the request processing
5. Directly set headers on the response after invocation of the next entity in the filter chain.

destroy

void destroy()

Called by the web container to indicate to a filter that it is being taken out of service. This
method is only called once all threads within the filter's doFilter method have exited or after
a timeout period has passed. After the web container calls this method, it will not call the
doFilter method again on this instance of the filter.

This method gives the filter an opportunity to clean up any resources that are being held (for
example, memory, file handles, threads) and make sure that any persistent state is synchronized
with the filter's current state in memory.