Filter (Servlet 3.1 API Documentation - Apache Tomcat 8.5.57)

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

  Examples that have been identified for this design are
  1) Authentication Filters
  2) Logging and Auditing Filters
  3) Image conversion Filters
  4) Data compression Filters
  5) Encryption Filters
  6) Tokenizing Filters
  7) Filters that trigger resource access events
  8) XSL/T filters
  9) Mime-type chain Filter

  Since:
    Servlet 2.3

- - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `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

          void init(FilterConfig filterConfig)
                    throws ServletException

      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:

      - Throws a ServletException
      - Does not return within a time period defined by the web container
      Parameters:
        `filterConfig` - The configuration information associated with the filter instance being initialised
      Throws:
        `ServletException` - if the initialisation fails

    - #### doFilter

          void doFilter(ServletRequest request,
                      ServletResponse response,
                      FilterChain chain)
                        throws java.io.IOException,
                               ServletException

      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.

      Parameters:
        `request` - The request to process
        `response` - The response associated with the request
        `chain` - Provides access to the next filter in the chain for this filter to pass the request and response to for further processing
      Throws:
        `java.io.IOException` - if an I/O error occurs during this filter's processing of the request
        `ServletException` - if the processing fails for any other reason

    - #### 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.

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