Java tutorial
/* * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * 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 * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.net; import java.io.IOException; import java.util.Map; import java.util.List; import sun.security.util.SecurityConstants; /** * Represents implementations of URLConnection caches. An instance of * such a class can be registered with the system by doing * ResponseCache.setDefault(ResponseCache), and the system will call * this object in order to: * * <ul><li>store resource data which has been retrieved from an * external source into the cache</li> * <li>try to fetch a requested resource that may have been * stored in the cache</li> * </ul> * * The ResponseCache implementation decides which resources * should be cached, and for how long they should be cached. If a * request resource cannot be retrieved from the cache, then the * protocol handlers will fetch the resource from its original * location. * * The settings for URLConnection#useCaches controls whether the * protocol is allowed to use a cached response. * * For more information on HTTP caching, see <a * href="http://www.ietf.org/rfc/rfc2616.txt"><i>RFC 2616: Hypertext * Transfer Protocol -- HTTP/1.1</i></a> * * @author Yingxian Wang * @since 1.5 */ public abstract class ResponseCache { /** * The system wide cache that provides access to a url * caching mechanism. * * @see #setDefault(ResponseCache) * @see #getDefault() */ private static ResponseCache theResponseCache; /** * Gets the system-wide response cache. * * @throws SecurityException * If a security manager has been installed and it denies * {@link NetPermission}{@code ("getResponseCache")} * * @see #setDefault(ResponseCache) * @return the system-wide {@code ResponseCache} * @since 1.5 */ public static synchronized ResponseCache getDefault() { SecurityManager sm = System.getSecurityManager(); if (sm != null) { sm.checkPermission(SecurityConstants.GET_RESPONSECACHE_PERMISSION); } return theResponseCache; } /** * Sets (or unsets) the system-wide cache. * * Note: non-standard protocol handlers may ignore this setting. * * @param responseCache The response cache, or * {@code null} to unset the cache. * * @throws SecurityException * If a security manager has been installed and it denies * {@link NetPermission}{@code ("setResponseCache")} * * @see #getDefault() * @since 1.5 */ public static synchronized void setDefault(ResponseCache responseCache) { SecurityManager sm = System.getSecurityManager(); if (sm != null) { sm.checkPermission(SecurityConstants.SET_RESPONSECACHE_PERMISSION); } theResponseCache = responseCache; } /** * Retrieve the cached response based on the requesting uri, * request method and request headers. Typically this method is * called by the protocol handler before it sends out the request * to get the network resource. If a cached response is returned, * that resource is used instead. * * @param uri a {@code URI} used to reference the requested * network resource * @param rqstMethod a {@code String} representing the request * method * @param rqstHeaders - a Map from request header * field names to lists of field values representing * the current request headers * @return a {@code CacheResponse} instance if available * from cache, or null otherwise * @throws IOException if an I/O error occurs * @throws IllegalArgumentException if any one of the arguments is null * * @see java.net.URLConnection#setUseCaches(boolean) * @see java.net.URLConnection#getUseCaches() * @see java.net.URLConnection#setDefaultUseCaches(boolean) * @see java.net.URLConnection#getDefaultUseCaches() */ public abstract CacheResponse get(URI uri, String rqstMethod, Map<String, List<String>> rqstHeaders) throws IOException; /** * The protocol handler calls this method after a resource has * been retrieved, and the ResponseCache must decide whether or * not to store the resource in its cache. If the resource is to * be cached, then put() must return a CacheRequest object which * contains an OutputStream that the protocol handler will * use to write the resource into the cache. If the resource is * not to be cached, then put must return null. * * @param uri a {@code URI} used to reference the requested * network resource * @param conn - a URLConnection instance that is used to fetch * the response to be cached * @return a {@code CacheRequest} for recording the * response to be cached. Null return indicates that * the caller does not intend to cache the response. * @throws IOException if an I/O error occurs * @throws IllegalArgumentException if any one of the arguments is * null */ public abstract CacheRequest put(URI uri, URLConnection conn) throws IOException; }