/*

  * Copyright (c) 2005, 2010, 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.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

  * <tt>META-INF/services</tt>.  The file's name is the fully-qualified <a

  * 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

  * <tt>'#'</tt> (<tt>'&#92;u0023'</tt>, <font size="-1">NUMBER SIGN</font>); 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 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 Class<S> service;

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

     private ClassLoader loader;

     // 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 = svc;

         loader = cl;

         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;

         }

         public boolean hasNext() {

             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;

         }

         public S next() {

             if (!hasNext()) {

                 throw new NoSuchElementException();

             }

             String cn = nextName;

             nextName = null;

             try {

                 S p = service.cast(Class.forName(cn, true, loader)

                                    .newInstance());

                 providers.put(cn, p);

                 return p;

             } catch (ClassNotFoundException x) {

                 fail(service,

                      "Provider " + cn + " not found");

             } catch (Throwable x) {

                 fail(service,

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

                      x);

             }

             throw new Error();          // This cannot happen

         }

         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>

      *

      * @param  service

      *         The interface or abstract class representing the service

      *

      * @return A new service loader

      */

     public static <S> ServiceLoader<S> load(Class<S> service) {

         ClassLoader cl = null; // XXX: Thread.currentThread().getContextClassLoader();

         return ServiceLoader.load(service, cl);

     }

     /**

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

      * extension class loader.

      *

      * <p> This convenience method simply locates the extension class loader,

      * call it <tt><i>extClassLoader</i></tt>, and then returns

      *

      * <blockquote><pre>

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

      *

      * <p> 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.

      *

      * <p> This method is intended for use when only installed providers are

      * desired.  The resulting service will only find and load providers that

      * have been installed into the current Java virtual machine; providers on

      * the application's class path will be ignored.

      *

      * @param  service

      *         The interface or abstract class representing the service

      *

      * @return A new service loader

      */

     public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {

         ClassLoader cl = ClassLoader.getSystemClassLoader();

         ClassLoader prev = null;

         while (cl != null) {

             prev = cl;

             cl = cl.getParent();

         }

         return ServiceLoader.load(service, prev);

     }

     /**

      * Returns a string describing this service.

      *

      * @return  A descriptive string

      */

     public String toString() {

         return "java.util.ServiceLoader[" + service.getName() + "]";

     }

 }