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 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

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package com.sun.net.httpserver;

import java.io.IOException;

import java.util.*;

/**

 * A filter used to pre- and post-process incoming requests. Pre-processing occurs

 * before the application's exchange handler is invoked, and post-processing

 * occurs after the exchange handler returns.  Filters

 * are organised in chains, and are associated with HttpContext instances.

 * <p>

 * Each Filter in the chain, invokes the next filter within its own

 * doFilter() implementation. The final Filter in the chain invokes the applications

 * exchange handler.

 * @since 1.6

 */

public abstract class Filter {

    protected Filter () {}

    /**

     * a chain of filters associated with a HttpServer.

     * Each filter in the chain is given one of these

     * so it can invoke the next filter in the chain

     */

    public static class Chain {

        /* the last element in the chain must invoke the users

         * handler

         */

        private ListIterator<Filter> iter;

        private HttpHandler handler;

        public Chain (List<Filter> filters, HttpHandler handler) {

            iter = filters.listIterator();

            this.handler = handler;

        }

        /**

         * calls the next filter in the chain, or else

         * the users exchange handler, if this is the

         * final filter in the chain. The Filter may decide

         * to terminate the chain, by not calling this method.

         * In this case, the filter <b>must</b> send the

         * response to the request, because the application's

         * exchange handler will not be invoked.

         * @param exchange the HttpExchange

         * @throws IOException let exceptions pass up the stack

         * @throws NullPointerException if exchange is <code>null</code>

         */

        public void doFilter (HttpExchange exchange) throws IOException {

            if (!iter.hasNext()) {

                handler.handle (exchange);

            } else {

                Filter f = iter.next();

                f.doFilter (exchange, this);

            }

        }

    }

    /**

     * Asks this filter to pre/post-process the given exchange. The filter

     * can :-

     * <ul><li>examine or modify the request headers</li>

     * <li>filter the request body or the response body, by creating suitable

     * filter streams and calling

     * {@link HttpExchange#setStreams(InputStream,OutputStream)}</li>

     * <li>set attribute Objects in the exchange, which other filters or the

     * exchange handler can access.</li>

     * <li>decide to either :-<ol>

     * <li>invoke the next filter in the chain, by calling

     * {@link Filter.Chain#doFilter(HttpExchange)}</li>

     * <li>terminate the chain of invocation, by <b>not</b> calling

     * {@link Filter.Chain#doFilter(HttpExchange)}</li></ol>

     * <li>if option 1. above taken, then when doFilter() returns all subsequent

     * filters in the Chain have been called, and the response headers can be

     * examined or modified.</li>

     * <li>if option 2. above taken, then this Filter must use the HttpExchange

     * to send back an appropriate response</li></ul><p>

     * @param exchange the <code>HttpExchange</code> to be filtered.

     * @param chain the Chain which allows the next filter to be invoked.

     * @throws IOException may be thrown by any filter module, and if

     *          caught, must be rethrown again.

     * @throws NullPointerException if either exchange or chain are <code>null</code>

     */

    public abstract void doFilter (HttpExchange exchange, Chain chain)

        throws IOException;

    /**

     * returns a short description of this Filter

     * @return a string describing the Filter

     */

    public abstract String description ();

}