/* * @(#)Service.java	1.16 06/10/10 * * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER *  * This program 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.  *  * This program 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 at /legal/license.txt).  *  * 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 Sun Microsystems, Inc., 4150 Network Circle, Santa * Clara, CA 95054 or visit www.sun.com if you need additional * information or have any questions.  */package sun.misc;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.net.URL;import java.util.ArrayList;import java.util.Enumeration;import java.util.Iterator;import java.util.List;import java.util.NoSuchElementException;import java.util.Set;import java.util.TreeSet;/** * A simple service-provider lookup mechanism.  A <i>service</i> is a * well-known set of interfaces and (usually abstract) classes.  A <i>service * provider</i> is a specific implementation of a service.  The classes in a * provider typically implement the interfaces and subclass the classes defined * in the service itself.  Service providers may be installed in an * implementation of the Java platform in the form of extensions, that is, jar * files placed into any of the usual extension directories.  Providers may * also be made available by adding them to the applet or application class * path or by some other platform-specific means. * * <p> In this lookup mechanism a service is represented by an interface or an * abstract class.  (A concrete class may be used, but this is not * recommended.)  A provider of a given service contains one or more concrete * classes that extend this <i>service class</i> with data and code specific to * the provider.  This <i>provider class</i> will typically not be the entire * provider itself but rather a proxy that contains enough information to * decide whether the provider is able to satisfy a particular request together * with code that can create the actual provider on demand.  The details of * provider classes tend to be highly service-specific; no single class or * interface could possibly unify them, so no such class has been defined.  The * only requirement enforced here is that provider classes must have a * zero-argument constructor so that they may be instantiated during lookup. * * <p> A service provider identifies itself by placing a provider-configuration * file in the resource directory <tt>META-INF/services</tt>.  The file's name * should consist of the fully-qualified name of the abstract service class. * The file should contain a list of fully-qualified concrete provider-class * names, one per line.  Space and tab characters surrounding each name, as * well as blank lines, are ignored.  The comment character is <tt>'#'</tt> * (<tt>0x23</tt>); on each line all characters following the first comment * character are ignored.  The file must be encoded in UTF-8. * * <p> If a particular concrete provider class is named in more than one * configuration file, or is named in the same configuration file more than * once, then the duplicates will be ignored.  The configuration file naming a * particular provider need not be in the same jar file or other distribution * unit as the provider itself.  The provider must be accessible from the same * class loader that was initially queried to locate the configuration file; * note that this is not necessarily the class loader that found the file. * * <p> <b>Example:</b> Suppose we have a service class named * <tt>java.io.spi.CharCodec</tt>.  It has two abstract methods: * * <pre> *   public abstract CharEncoder getEncoder(String encodingName); *   public abstract CharDecoder getDecoder(String encodingName); * </pre> * * Each method returns an appropriate object or <tt>null</tt> if it cannot * translate the given encoding.  Typical <tt>CharCodec</tt> providers will * support more than one encoding. * * <p> If <tt>sun.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt> * service then its jar file would contain the file * <tt>META-INF/services/java.io.spi.CharCodec</tt>.  This file would contain * the single line: * * <pre> *   sun.io.StandardCodec    # Standard codecs for the platform * </pre> * * To locate an encoder for a given encoding name, the internal I/O code would * do something like this: * * <pre> *   CharEncoder getEncoder(String encodingName) { *       Iterator ps = Service.providers(CharCodec.class); *       while (ps.hasNext()) { *           CharCodec cc = (CharCodec)ps.next(); *           CharEncoder ce = cc.getEncoder(encodingName); *           if (ce != null) *               return ce; *       } *       return null; *   } * </pre> * * The provider-lookup mechanism always executes in the security context of the * caller.  Trusted system code should typically invoke the methods in this * class from within a privileged security context. * * @author Mark Reinhold * @version 1.10, 02/08/19 * @since 1.3 */public final class Service {    private static final String prefix = "META-INF/services/";    private Service() {}    private static void fail(Class service, String msg, Throwable cause)	throws ServiceConfigurationError    {	ServiceConfigurationError sce	    = new ServiceConfigurationError(service.getName() + ": " + msg);	sce.initCause(cause);	throw sce;    }    private static void fail(Class service, String msg)        throws ServiceConfigurationError {        throw new ServiceConfigurationError(service.getName() + ": " + msg);    }    private static void fail(Class service, URL u, int line, String msg)        throws ServiceConfigurationError {        fail(service, u + ":" + line + ": " + msg);    }    /**     * Parse a single line from the given configuration file, adding the name     * on the line to both the names list and the returned set iff the name is     * not already a member of the returned set.     */    private static int parseLine(Class service, URL u, BufferedReader r, int lc,        List names, Set returned)        throws IOException, ServiceConfigurationError {        String ln = r.readLine();        if (ln == null) {            return -1;        }        int ci = ln.indexOf('#');        if (ci >= 0) ln = ln.substring(0, ci);        ln = ln.trim();        int n = ln.length();        if (n != 0) {            if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))                fail(service, u, lc, "Illegal configuration-file syntax");            if (!Character.isJavaIdentifierStart(ln.charAt(0)))                fail(service, u, lc, "Illegal provider-class name: " + ln);            for (int i = 1; i < n; i++) {                char c = ln.charAt(i);                if (!Character.isJavaIdentifierPart(c) && (c != '.'))                    fail(service, u, lc, "Illegal provider-class name: " + ln);            }            if (!returned.contains(ln)) {                names.add(ln);                returned.add(ln);            }        }        return lc + 1;    }    /**     * Parse the content of the given URL as a provider-configuration file.     *     * @param  service     *         The service class for which providers are being sought;     *         used to construct error detail strings     *     * @param  url     *         The URL naming the configuration file to be parsed     *     * @param  returned     *         A Set containing the names of provider classes that have already     *         been returned.  This set will be updated to contain the names     *         that will be yielded from the returned <tt>Iterator</tt>.     *     * @return A (possibly empty) <tt>Iterator</tt> that will yield the     *         provider-class names in the given configuration file that are     *         not yet members of the returned set     *     * @throws ServiceConfigurationError