src/java.base/share/classes/java/net/ResponseCache.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
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package java.net;

import java.io.IOException;
import java.util.Map;
import java.util.List;

/**
 * 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&nbsp;2616: Hypertext
 * Transfer Protocol -- HTTP/1.1</i></a>
 *
 * @spec https://www.rfc-editor.org/info/rfc2616
 *      RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1
 * @author Yingxian Wang
 * @since 1.5
 */
public abstract class ResponseCache {

    /**
     * Constructor for subclasses to call.
     */
    public 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.
     *
     * @see #setDefault(ResponseCache)
     * @return the system-wide {@code ResponseCache}
     * @since 1.5
     */
    public static synchronized ResponseCache getDefault() {
        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.
     *
     * @see #getDefault()
     * @since 1.5
     */
    public static synchronized void setDefault(ResponseCache responseCache) {
        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;
}