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	/*
	* Copyright (c) 1999, 2012, 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 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
	* @since 1.3
	*/

	public final class Service<S> {

	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<String> names, Set<String> 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");
	int cp = ln.codePointAt(0);
	if (!Character.isJavaIdentifierStart(cp))
	fail(service, u, lc, "Illegal provider-class name: " + ln);
	for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
	cp = ln.codePointAt(i);
	if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
	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
	* If an I/O error occurs while reading from the given URL, or
	* if a configuration-file format error is detected
	*/
	private static Iterator<String> parse(Class<?> service, URL u, Set<String> returned)
	throws ServiceConfigurationError
	{
	InputStream in = null;
	BufferedReader r = null;
	ArrayList<String> 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<S> implements Iterator<S> {

	Class<S> service;
	ClassLoader loader;
	Enumeration<URL> configs = null;
	Iterator<String> pending = null;
	Set<String> returned = new TreeSet<>();
	String nextName = null;

	private LazyIterator(Class<S> service, ClassLoader loader) {
	this.service = service;
	this.loader = loader;
	}

	public boolean hasNext() throws ServiceConfigurationError {
	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, configs.nextElement(), returned);
	}
	nextName = pending.next();
	return true;
	}

	public S next() throws ServiceConfigurationError {
	if (!hasNext()) {
	throw new NoSuchElementException();
	}
	String cn = nextName;
	nextName = null;
	Class<?> c = null;
	try {
	c = Class.forName(cn, false, loader);
	} catch (ClassNotFoundException x) {
	fail(service,
	"Provider " + cn + " not found");
	}
	if (!service.isAssignableFrom(c)) {
	fail(service,
	"Provider " + cn + " not a subtype");
	}
	try {
	return service.cast(c.newInstance());
	} catch (Throwable x) {
	fail(service,
	"Provider " + cn + " could not be instantiated",
	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>ServiceConfigurationError</tt> if a provider-configuration
	* file violates the specified format or if a provider class cannot
	* be found and instantiated.
	*
	* @throws ServiceConfigurationError
	* If a provider-configuration file violates the specified format
	* or names a provider class that cannot be found and instantiated
	*
	* @see #providers(java.lang.Class)
	* @see #installedProviders(java.lang.Class)
	*/
	public static <S> Iterator<S> providers(Class<S> service, ClassLoader loader)
	throws ServiceConfigurationError
	{
	return new LazyIterator<S>(service, loader);
	}


	/**
	* Locates and incrementally instantiates the available providers of a
	* given service using the context class loader. This convenience method
	* is equivalent to
	*
	* <pre>
	* ClassLoader cl = Thread.currentThread().getContextClassLoader();
	* return Service.providers(service, cl);
	* </pre>
	*
	* @param service
	* The service's abstract service class
	*
	* @return An <tt>Iterator</tt> that yields provider objects for the given
	* service, in some arbitrary order. The iterator will throw a
	* <tt>ServiceConfigurationError</tt> if a provider-configuration
	* file violates the specified format or if a provider class cannot
	* be found and instantiated.
	*
	* @throws ServiceConfigurationError
	* If a provider-configuration file violates the specified format
	* or names a provider class that cannot be found and instantiated
	*
	* @see #providers(java.lang.Class, java.lang.ClassLoader)
	*/
	public static <S> Iterator<S> providers(Class<S> service)
	throws ServiceConfigurationError
	{
	ClassLoader cl = Thread.currentThread().getContextClassLoader();
	return Service.providers(service, cl);
	}


	/**
	* Locates and incrementally instantiates the available providers of a
	* given service using the extension class loader. This convenience method
	* simply locates the extension class loader, call it
	* <tt>extClassLoader</tt>, and then does
	*
	* <pre>
	* return Service.providers(service, extClassLoader);
	* </pre>
	*
	* If the extension class loader cannot be found then the system class
	* loader is used; if there is no system class loader then the bootstrap
	* class loader is used.
	*
	* @param service
	* The service's abstract service class
	*
	* @return An <tt>Iterator</tt> that yields provider objects for the given
	* service, in some arbitrary order. The iterator will throw a
	* <tt>ServiceConfigurationError</tt> if a provider-configuration
	* file violates the specified format or if a provider class cannot
	* be found and instantiated.
	*
	* @throws ServiceConfigurationError
	* If a provider-configuration file violates the specified format
	* or names a provider class that cannot be found and instantiated
	*
	* @see #providers(java.lang.Class, java.lang.ClassLoader)
	*/
	public static <S> Iterator<S> installedProviders(Class<S> service)
	throws ServiceConfigurationError
	{
	ClassLoader cl = ClassLoader.getSystemClassLoader();
	ClassLoader prev = null;
	while (cl != null) {
	prev = cl;
	cl = cl.getParent();
	}
	return Service.providers(service, prev);
	}

	}

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