content: /*
   * Copyright (C) 2014 The Android Open Source Project
   * Copyright (c) 2005, 2013, 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 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>.</a>  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 style="font-size:smaller;">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&lt;CodecSet&gt; 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
      // Android-changed: do not use legacy security code.
      // 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;
          // Android-changed: Do not use legacy security code.
          // On Android, System.getSecurityManager() is always null.
          // 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,
                       // Android-changed: Let the ServiceConfigurationError have a cause.
                       "Provider " + cn + " not found", x);
                       // "Provider " + cn + " not found");
              }
              if (!service.isAssignableFrom(c)) {
                  // Android-changed: Let the ServiceConfigurationError have a cause.
                  ClassCastException cce = new ClassCastException(
                          service.getCanonicalName() + " is not assignable from " + c.getCanonicalName());
                  fail(service,
                       "Provider " + cn  + " not a subtype", cce);
                  // 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);
              }
              throw new Error();          // This cannot happen
          }
  
          public boolean hasNext() {
              // Android-changed: do not use legacy security code
              /* 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() {
              // Android-changed: do not use legacy security code
              /* 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 Iterator#hasNext hasNext} and
       * {@link 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 Iterator#remove() remove} method will
       * cause an {@link UnsupportedOperationException} to be thrown.
       *
       * @implNote When adding providers to the cache, the {@link #iterator
       * Iterator} processes resources in the order that the {@link
       * ClassLoader#getResources(String)
       * ClassLoader.getResources(String)} method finds the service configuration
       * files.
       *
       * @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  <S> the class of the service type
       *
       * @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 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  <S> the class of the service type
       *
       * @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 = 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  <S> the class of the service type
       *
       * @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);
      }
  
      // BEGIN Android-added: loadFromSystemProperty(), for internal use.
      // Instantiates a class from a system property (used elsewhere in libcore).
      /**
       * Internal API to support built-in SPIs that check a system property first.
       * Returns an instance specified by a property with the class' binary name, or null if
       * no such property is set.
       * @hide
       */
      public static <S> S loadFromSystemProperty(final Class<S> service) {
          try {
              final String className = System.getProperty(service.getName());
              if (className != null) {
                  Class<?> c = ClassLoader.getSystemClassLoader().loadClass(className);
                  return (S) c.newInstance();
              }
              return null;
          } catch (Exception e) {
              throw new Error(e);
          }
      }
      // END Android-added: loadFromSystemProperty(), for internal use.
  
      /**
       * Returns a string describing this service.
       *
       * @return  A descriptive string
       */
      public String toString() {
          return "java.util.ServiceLoader[" + service.getName() + "]";
      }
  
  }
  