javax.servlet.Filter

public interface Filter {

    /**
     * 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. <br>
     * <br>
     * The web container cannot place the filter into service if the init method
     * either<br>
     * 1.Throws a ServletException <br>
     * 2.Does not return within a time period defined by the web container
     */
    public void init(FilterConfig filterConfig) throws ServletException;

    /**
     * The <code>doFilter</code> 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.
     * <p>
     * A typical implementation of this method would follow the following
     * pattern:- <br>
     * 1. Examine the request<br>
     * 2. Optionally wrap the request object with a custom implementation to
     * filter content or headers for input filtering <br>
     * 3. Optionally wrap the response object with a custom implementation to
     * filter content or headers for output filtering <br>
     * 4. a) <strong>Either</strong> invoke the next entity in the chain using
     * the FilterChain object (<code>chain.doFilter()</code>), <br>
     * 4. b) <strong>or</strong> not pass on the request/response pair to the
     * next entity in the filter chain to block the request processing<br>
     * 5. Directly set headers on the response after invocation of the next
     * entity in the filter chain.
     **/
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException;

    /**
     * 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. <br>
     * <br>
     * 
     * 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.
     */
    public void destroy();

}

javax.servlet.FilterChain

public interface FilterChain {

    /**
     * Causes the next filter in the chain to be invoked, or if the calling
     * filter is the last filter in the chain, causes the resource at the end of
     * the chain to be invoked.
     * 
     * @param request
     *            the request to pass along the chain.
     * @param response
     *            the response to pass along the chain.
     * 
     * @since 2.3
     */
    public void doFilter(ServletRequest request, ServletResponse response)
            throws IOException, ServletException;

}

ch.qos.logback.classic.selector.servlet.LoggerContextFilter

public class LoggerContextFilter implements Filter {

  public void destroy() {
    //do nothing
  }

  public void doFilter(ServletRequest request, ServletResponse response,
      FilterChain chain) throws IOException, ServletException {

    LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
    ContextSelector selector = ContextSelectorStaticBinder.getSingleton().getContextSelector();
    ContextJNDISelector sel = null;

    if (selector instanceof ContextJNDISelector) {
      sel = (ContextJNDISelector)selector;
      sel.setLocalContext(lc);
    }

    try {
      chain.doFilter(request, response);
    } finally {
      if (sel != null) {
        sel.removeLocalContext();
      }
    }
  }

  public void init(FilterConfig arg0) throws ServletException {
    //do nothing
  }
}