ContentHandler.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
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 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 * 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,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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package java.net;

import java.io.IOException;

/**
 * The abstract class {@code ContentHandler} is the superclass
 * of all classes that read an {@code Object} from a
 * {@code URLConnection}.
 * <p>
 * An application does not generally call the
 * {@code getContent} method in this class directly. Instead, an
 * application calls the {@code getContent} method in class
 * {@code URL} or in {@code URLConnection}.
 * The application's content handler factory (an instance of a class that
 * implements the interface {@code ContentHandlerFactory} set up by a call to
 * {@link URLConnection#setContentHandlerFactory(ContentHandlerFactory)
 * setContentHandlerFactory} is called with a {@code String} giving the
 * MIME type of the object being received on the socket. The factory returns an
 * instance of a subclass of {@code ContentHandler}, and its
 * {@code getContent} method is called to create the object.
 * <p>
 * If no content handler could be {@linkplain URLConnection#getContent() found},
 * URLConnection will look for a content handler in a user-definable set of places.
 * Users can define a vertical-bar delimited set of class prefixes
 * to search through by defining the <i>{@link java.net.URLConnection#contentPathProp}</i>
 * property. The class name must be of the form:
 * <blockquote>
 *     <i>{package-prefix}.{major}.{minor}</i>
 *     <p>
 *     where <i>{major}.{minor}</i> is formed by taking the
 *     content-type string, replacing all slash characters with a
 *     {@code period} ('.'), and all other non-alphanumeric characters
 *     with the underscore character '{@code _}'. The alphanumeric
 *     characters are specifically the 26 uppercase ASCII letters
 *     '{@code A}' through '{@code Z}', the 26 lowercase ASCII
 *     letters '{@code a}' through '{@code z}', and the 10 ASCII
 *     digits '{@code 0}' through '{@code 9}'.
 *     <p>
 *     e.g.
 *     YoyoDyne.experimental.text.plain
 * </blockquote>
 * If no user-defined content handler is found, then the system
 * tries to load a specific <i>content-type</i> handler from one
 * of the built-in handlers, if one exists.
 * <p>
 * If the loading of the content handler class would be performed by
 * a classloader that is outside of the delegation chain of the caller,
 * the JVM will need the RuntimePermission "getClassLoader".
 *
 * @author  James Gosling
 * @see     java.net.ContentHandler#getContent(java.net.URLConnection)
 * @see     java.net.ContentHandlerFactory
 * @see     java.net.URL#getContent()
 * @see     java.net.URLConnection
 * @see     java.net.URLConnection#getContent()
 * @see     java.net.URLConnection#setContentHandlerFactory(java.net.ContentHandlerFactory)
 * @since   1.0
 */
// 资源内容句柄
public abstract class ContentHandler {
    
    /**
     * Given a URL connect stream positioned at the beginning of the
     * representation of an object, this method reads that stream and
     * creates an object from it.
     *
     * @param urlc a URL connection.
     *
     * @return the object read by the {@code ContentHandler}.
     *
     * @throws IOException if an I/O error occurs while reading the object.
     */
    // 返回目标资源的内容，返回的形式取决于资源的类型(不一定总是输入流)
    public abstract Object getContent(URLConnection connection) throws IOException;
    
    /**
     * Given a URL connect stream positioned at the beginning of the
     * representation of an object, this method reads that stream and
     * creates an object that matches one of the types specified.
     *
     * The default implementation of this method should call
     * {@link #getContent(URLConnection)}
     * and screen the return type for a match of the suggested types.
     *
     * @param urlc    a URL connection.
     * @param classes an array of types requested
     *
     * @return the object read by the {@code ContentHandler} that is
     * the first match of the suggested types or
     * {@code null} if none of the requested  are supported.
     *
     * @throws IOException if an I/O error occurs while reading the object.
     * @since 1.3
     */
    // 返回目标资源的内容，且限定该"内容"只能是指定的类型；内容的返回形式取决于资源的类型(不一定总是输入流)
    @SuppressWarnings("rawtypes")
    public Object getContent(URLConnection connection, Class[] classes) throws IOException {
        // 返回目标资源的内容，返回的形式取决于资源的类型
        Object obj = getContent(connection);
        
        // 限定该"内容"只能是指定的类型
        for(Class<?> c : classes) {
            if(c.isInstance(obj)) {
                return obj;
            }
        }
        
        return null;
    }
}