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Filters perform filtering in the
Every Filter has access to a FilterConfig object from which it can obtain
its initialization parameters, and 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.
Examples that have been identified for this design are:
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
doFiltermethod 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:
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.