/*

 * 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

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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,

                     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);

    }

}