/*

 * Copyright (c) 2005, 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 java.util;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.net.URL;

import java.security.AccessController;

import java.security.AccessControlContext;

import java.security.PrivilegedAction;

import java.util.ArrayList;

import java.util.Enumeration;

import java.util.Iterator;

import java.util.List;

import java.util.NoSuchElementException;

/**

 * A simple service-provider loading facility.

 *

 * <p> 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

 * can 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 can also be made available by adding them to the

 * application's class path or by some other platform-specific means.

 *

 * <p> For the purpose of loading, a service is represented by a single type,

 * that is, a single interface or abstract class.  (A concrete class can be

 * used, but this is not recommended.)  A provider of a given service contains

 * one or more concrete classes that extend this <i>service type</i> with data

 * and code specific to the provider.  The <i>provider class</i> is typically

 * not the entire provider itself but rather a proxy which 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 type is

 * defined here.  The only requirement enforced by this facility is that

 * provider classes must have a zero-argument constructor so that they can be

 * instantiated during loading.

 *

 * <p><a name="format"> A service provider is identified by placing a

 * <i>provider-configuration file</i> in the resource directory

 * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.

 * The file contains a list of fully-qualified binary names of concrete

 * provider classes, one per line.  Space and tab characters surrounding each

 * name, as well as blank lines, are ignored.  The comment character is

 * 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 are 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 from which the file was

 * actually loaded.

 *

 * <p> Providers are located and instantiated lazily, that is, on demand.  A

 * service loader maintains a cache of the providers that have been loaded so

 * far.  Each invocation of the {@link #iterator iterator} method returns an

 * iterator that first yields all of the elements of the cache, in

 * instantiation order, and then lazily locates and instantiates any remaining

 * providers, adding each one to the cache in turn.  The cache can be cleared

 * via the {@link #reload reload} method.

 *

 * <p> Service loaders always execute in the security context of the caller.

 * Trusted system code should typically invoke the methods in this class, and

 * the methods of the iterators which they return, from within a privileged

 * security context.

 *

 * <p> Instances of this class are not safe for use by multiple concurrent

 * threads.

 *

 * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any

 * method in this class will cause a {@link NullPointerException} to be thrown.

 *

 *

 * <p><span style="font-weight: bold; padding-right: 1em">Example</span>

 * Suppose we have a service type <tt>com.example.CodecSet</tt> which is

 * intended to represent sets of encoder/decoder pairs for some protocol.  In

 * this case it is an abstract class with two abstract methods:

 *

 * <blockquote><pre>

 * public abstract Encoder getEncoder(String encodingName);

 * public abstract Decoder getDecoder(String encodingName);</pre></blockquote>

 *

 * Each method returns an appropriate object or <tt>null</tt> if the provider

 * does not support the given encoding.  Typical providers support more than

 * one encoding.

 *

 * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the

 * <tt>CodecSet</tt> service then its jar file also contains a file named

 *

 * <blockquote><pre>

 * META-INF/services/com.example.CodecSet</pre></blockquote>

 *

 * <p> This file contains the single line:

 *

 * <blockquote><pre>

 * com.example.impl.StandardCodecs    # Standard codecs</pre></blockquote>

 *

 * <p> The <tt>CodecSet</tt> class creates and saves a single service instance

 * at initialization:

 *

 * <blockquote><pre>

 * private static ServiceLoader<CodecSet> codecSetLoader

 *     = ServiceLoader.load(CodecSet.class);</pre></blockquote>

 *

 * <p> To locate an encoder for a given encoding name it defines a static

 * factory method which iterates through the known and available providers,

 * returning only when it has located a suitable encoder or has run out of

 * providers.

 *

 * <blockquote><pre>

 * public static Encoder getEncoder(String encodingName) {

 *     for (CodecSet cp : codecSetLoader) {

 *         Encoder enc = cp.getEncoder(encodingName);

 *         if (enc != null)

 *             return enc;

 *     }

 *     return null;

 * }</pre></blockquote>

 *

 * <p> A <tt>getDecoder</tt> method is defined similarly.

 *

 *

 * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If

 * the class path of a class loader that is used for provider loading includes

 * remote network URLs then those URLs will be dereferenced in the process of

 * searching for provider-configuration files.

 *

 * <p> This activity is normal, although it may cause puzzling entries to be

 * created in web-server logs.  If a web server is not configured correctly,

 * however, then this activity may cause the provider-loading algorithm to fail

 * spuriously.

 *

 * <p> A web server should return an HTTP 404 (Not Found) response when a

 * requested resource does not exist.  Sometimes, however, web servers are

 * erroneously configured to return an HTTP 200 (OK) response along with a

 * helpful HTML error page in such cases.  This will cause a {@link

 * ServiceConfigurationError} to be thrown when this class attempts to parse

 * the HTML page as a provider-configuration file.  The best solution to this

 * problem is to fix the misconfigured web server to return the correct

 * response code (HTTP 404) along with the HTML error page.

 *

 * @param  <S>

 *         The type of the service to be loaded by this loader

 *

 * @author Mark Reinhold

 * @since 1.6

 */

public final class ServiceLoader<S>

    implements Iterable<S>

{

    private static final String PREFIX = "META-INF/services/";

    // The class or interface representing the service being loaded

    private final Class<S> service;

    // The class loader used to locate, load, and instantiate providers

    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created

    private final AccessControlContext acc;

    // Cached providers, in instantiation order

    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    // The current lazy-lookup iterator

    private LazyIterator lookupIterator;

    /**

     * Clear this loader's provider cache so that all providers will be

     * reloaded.

     *

     * <p> After invoking this method, subsequent invocations of the {@link

     * #iterator() iterator} method will lazily look up and instantiate

     * providers from scratch, just as is done by a newly-created loader.

     *

     * <p> This method is intended for use in situations in which new providers

     * can be installed into a running Java virtual machine.

     */

    public void reload() {

        providers.clear();

        lookupIterator = new LazyIterator(service, loader);

    }

    private ServiceLoader(Class<S> svc, ClassLoader cl) {

        service = Objects.requireNonNull(svc, "Service interface cannot be null");

        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;

        acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;

        reload();

    }

    private static void fail(Class<?> service, String msg, Throwable cause)

        throws ServiceConfigurationError

    {

        throw new ServiceConfigurationError(service.getName() + ": " + msg,

                                            cause);

    }

    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 the names list.

    //

    private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,

                          List<String> names)

        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 (!providers.containsKey(ln) && !names.contains(ln))

                names.add(ln);

        }

        return lc + 1;

    }

    // Parse the content of the given URL as a provider-configuration file.

    //

    // @param  service

    //         The service type for which providers are being sought;

    //         used to construct error detail strings

    //

    // @param  u

    //         The URL naming the configuration file to be parsed

    //

    // @return A (possibly empty) iterator 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 Iterator<String> parse(Class<?> service, URL u)

        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)) >= 0);

        } catch (IOException x) {

            fail(service, "Error reading configuration file", x);

        } finally {

            try {

                if (r != null) r.close();

                if (in != null) in.close();

            } catch (IOException y) {

                fail(service, "Error closing configuration file", y);

            }

        }

        return names.iterator();

    }

    // Private inner class implementing fully-lazy provider lookup

    //

    private class LazyIterator

        implements Iterator<S>

    {

        Class<S> service;

        ClassLoader loader;

        Enumeration<URL> configs = null;

        Iterator<String> pending = null;

        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {

            this.service = service;

            this.loader = loader;

        }

        private boolean hasNextService() {

            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, "Error locating configuration files", x);

                }

            }

            while ((pending == null) || !pending.hasNext()) {

                if (!configs.hasMoreElements()) {

                    return false;

                }

                pending = parse(service, configs.nextElement());

            }

            nextName = pending.next();

            return true;

        }

        private S nextService() {

            if (!hasNextService())

                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 {

                S p = service.cast(c.newInstance());

                providers.put(cn, p);

                return p;

            } catch (Throwable x) {

                fail(service,

                     "Provider " + cn + " could not be instantiated: " + x,

                     x);

            }

            throw new Error();          // This cannot happen

        }

        public boolean hasNext() {

            if (acc == null) {

                return hasNextService();

            } else {

                PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {

                    public Boolean run() { return hasNextService(); }

                };

                return AccessController.doPrivileged(action, acc);

            }

        }

        public S next() {

            if (acc == null) {

                return nextService();

            } else {

                PrivilegedAction<S> action = new PrivilegedAction<S>() {

                    public S run() { return nextService(); }

                };

                return AccessController.doPrivileged(action, acc);

            }

        }

        public void remove() {

            throw new UnsupportedOperationException();

        }

    }

    /**

     * Lazily loads the available providers of this loader's service.

     *

     * <p> The iterator returned by this method first yields all of the

     * elements of the provider cache, in instantiation order.  It then lazily

     * loads and instantiates any remaining providers, adding each one to the

     * cache in turn.

     *

     * <p> To achieve laziness the actual work of parsing the available

     * provider-configuration files and instantiating providers must be done by

     * the iterator itself.  Its {@link java.util.Iterator#hasNext hasNext} and

     * {@link java.util.Iterator#next next} methods can therefore throw a

     * {@link ServiceConfigurationError} if a provider-configuration file

     * violates the specified format, or if it names a provider class that

     * cannot be found and instantiated, or if the result of instantiating the

     * class is not assignable to the service type, or if any other kind of

     * exception or error is thrown as the next provider is located and

     * instantiated.  To write robust code it is only necessary to catch {@link

     * ServiceConfigurationError} when using a service iterator.

     *

     * <p> If such an error is thrown then subsequent invocations of the

     * iterator will make a best effort to locate and instantiate the next

     * available provider, but in general such recovery cannot be guaranteed.

     *

     * <blockquote style="font-size: smaller; line-height: 1.2"><span

     * style="padding-right: 1em; font-weight: bold">Design Note</span>

     * Throwing an error in these cases may seem extreme.  The rationale for

     * this behavior is that a malformed provider-configuration file, like a

     * malformed class file, indicates a serious problem with the way the Java

     * virtual machine is configured or is being used.  As such it is

     * preferable to throw an error rather than try to recover or, even worse,

     * fail silently.</blockquote>

     *

     * <p> The iterator returned by this method does not support removal.

     * Invoking its {@link java.util.Iterator#remove() remove} method will

     * cause an {@link UnsupportedOperationException} to be thrown.

     *

     * @return  An iterator that lazily loads providers for this loader's

     *          service

     */

    public Iterator<S> iterator() {

        return new Iterator<S>() {

            Iterator<Map.Entry<String,S>> knownProviders

                = providers.entrySet().iterator();

            public boolean hasNext() {

                if (knownProviders.hasNext())

                    return true;

                return lookupIterator.hasNext();

            }

            public S next() {

                if (knownProviders.hasNext())

                    return knownProviders.next().getValue();

                return lookupIterator.next();

            }

            public void remove() {

                throw new UnsupportedOperationException();

            }

        };

    }

    /**

     * Creates a new service loader for the given service type and class

     * loader.

     *

     * @param  service

     *         The interface or abstract class representing the service

     *

     * @param  loader

     *         The class loader to be used to load provider-configuration files

     *         and provider classes, or <tt>null</tt> if the system class

     *         loader (or, failing that, the bootstrap class loader) is to be

     *         used

     *

     * @return A new service loader

     */

    public static <S> ServiceLoader<S> load(Class<S> service,

                                            ClassLoader loader)

    {

        return new ServiceLoader<>(service, loader);

    }

    /**

     * Creates a new service loader for the given service type, using the

     * current thread's {@linkplain java.lang.Thread#getContextClassLoader

     * context class loader}.

     *

     * <p> An invocation of this convenience method of the form

     *

     * <blockquote><pre>

     * ServiceLoader.load(<i>service</i>)</pre></blockquote>

     *

     * is equivalent to

     *

     * <blockquote><pre>

     * ServiceLoader.load(<i>service</i>,

     *                    Thread.currentThread().getContextClassLoader())</pre></blockquote>