/* * @(#)Service.java	1.7 05/12/30 *  * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package com.sun.jmx.remote.util;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;import java.security.AccessController;import java.security.PrivilegedAction;/** * EXTRACTED FROM sun.misc.Service * A simple service-provider lookup mechanism.  A <i>service</i> is a * well-known set of intjavax.management.remoteerfaces 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. * */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 IllegalArgumentException    {	IllegalArgumentException sce	    = new IllegalArgumentException(service.getName() + ": " + msg);		throw (IllegalArgumentException) EnvHelp.initCause(sce, cause);    }    private static void fail(Class service, String msg)	throws IllegalArgumentException    {	throw new IllegalArgumentException(service.getName() + ": " + msg);    }    private static void fail(Class service, URL u, int line, String msg)	throws IllegalArgumentException    {	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 if and only if 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, IllegalArgumentException    {	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 IllegalArgumentException     *         If an I/O error occurs while reading from the given URL, or     *         if a configuration-file format error is detected     */    private static Iterator parse(Class service, URL u, Set returned)	throws IllegalArgumentException    {	InputStream in = null;	BufferedReader r = null;	ArrayList names = new ArrayList();	try {	    in = u.openStream();	    r = new BufferedReader(new InputStreamReader(in, "utf-8"));	    int lc = 1;	    while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0);	} catch (IOException x) {	    fail(service, ": " + x);	} finally {	    try {		if (r != null) r.close();		if (in != null) in.close();	    } catch (IOException y) {		fail(service, ": " + y);	    }	}	return names.iterator();    }    /**     * Private inner class implementing fully-lazy provider lookup     */    private static class LazyIterator implements Iterator {	Class service;	ClassLoader loader;	Enumeration configs = null;	Iterator pending = null;	Set returned = new TreeSet();	String nextName = null;	private LazyIterator(Class service, ClassLoader loader) {	    this.service = service;	    this.loader = loader;	}	public boolean hasNext() throws IllegalArgumentException {	    if (nextName != null) {		return true;	    }	    if (configs == null) {		try {		    String fullName = prefix + service.getName();		    if (loader == null)			configs = ClassLoader.getSystemResources(fullName);		    else			configs = loader.getResources(fullName);		} catch (IOException x) {		    fail(service, ": " + x);		}	    }	    while ((pending == null) || !pending.hasNext()) {		if (!configs.hasMoreElements()) {		    return false;		}		pending = parse(service, (URL)configs.nextElement(), returned);	    }	    nextName = (String)pending.next();	    return true;	}	public Object next() throws IllegalArgumentException {	    if (!hasNext()) {		throw new NoSuchElementException();	    }	    String cn = nextName;	    nextName = null;	    try {		return Class.forName(cn, true, loader).newInstance();	    } catch (ClassNotFoundException x) {		fail(service,		     "Provider " + cn + " not found");	    } catch (Exception x) {		fail(service,		     "Provider " + cn + " could not be instantiated: " + x,		     x);	    }	    return null;	/* This cannot happen */	}	public void remove() {	    throw new UnsupportedOperationException();	}    }    /**     * Locates and incrementally instantiates the available providers of a     * given service using the given class loader.     *     * <p> This method transforms the name of the given service class into a     * provider-configuration filename as described above and then uses the     * <tt>getResources</tt> method of the given class loader to find all     * available files with that name.  These files are then read and parsed to     * produce a list of provider-class names.  The iterator that is returned     * uses the given class loader to lookup and then instantiate each element     * of the list.     *     * <p> Because it is possible for extensions to be installed into a running     * Java virtual machine, this method may return different results each time     * it is invoked. <p>     *     * @param  service     *         The service's abstract service class     *     * @param  loader     *         The class loader to be used to load provider-configuration files     *         and instantiate provider classes, or <tt>null</tt> if the system     *         class loader (or, failing that the bootstrap class loader) is to     *         be used     *      * @return An <tt>Iterator</tt> that yields provider objects for the given     *         service, in some arbitrary order.  The iterator will throw a     *         <tt>IllegalArgumentException</tt> if a provider-configuration     *         file violates the specified format or if a provider class cannot     *         be found and instantiated.     *     * @throws IllegalArgumentException     *         If a provider-configuration file violates the specified format     *         or names a provider class that cannot be found and instantiated     *     */    public static Iterator providers(Class service, ClassLoader loader)	throws IllegalArgumentException    {	return new LazyIterator(service, loader);    }}