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   * Copyright (c) 2000, 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
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   * questions.
   */
  
  package java.nio.channels.spi;
  
  import java.io.IOException;
  import java.net.ProtocolFamily;
  import java.nio.channels.*;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.Iterator;
  import java.util.ServiceLoader;
  import java.util.ServiceConfigurationError;
  import sun.security.action.GetPropertyAction;
  
  
  /**
   * Service-provider class for selectors and selectable channels.
   *
   * <p> A selector provider is a concrete subclass of this class that has a
   * zero-argument constructor and implements the abstract methods specified
   * below.  A given invocation of the Java virtual machine maintains a single
   * system-wide default provider instance, which is returned by the {@link
   * #provider() provider} method.  The first invocation of that method will locate
   * the default provider as specified below.
   *
   * <p> The system-wide default provider is used by the static <tt>open</tt>
   * methods of the {@link DatagramChannel#open
   * DatagramChannel}, {@link Pipe#open Pipe}, {@link
   * Selector#open Selector}, {@link
   * ServerSocketChannel#open ServerSocketChannel}, and {@link
   * SocketChannel#open SocketChannel} classes.  It is also
   * used by the {@link System#inheritedChannel System.inheritedChannel()}
   * method. A program may make use of a provider other than the default provider
   * by instantiating that provider and then directly invoking the <tt>open</tt>
   * methods defined in this class.
   *
   * <p> All of the methods in this class are safe for use by multiple concurrent
   * threads.  </p>
   *
   *
   * @author Mark Reinhold
   * @author JSR-51 Expert Group
   * @since 1.4
   */
  
  public abstract class SelectorProvider {
  
      private static final Object lock = new Object();
      private static SelectorProvider provider = null;
  
      /**
       * Initializes a new instance of this class.
       *
       * @throws  SecurityException
       *          If a security manager has been installed and it denies
       *          {@link RuntimePermission}<tt>("selectorProvider")</tt>
       */
      protected SelectorProvider() {
          SecurityManager sm = System.getSecurityManager();
          if (sm != null)
              sm.checkPermission(new RuntimePermission("selectorProvider"));
      }
  
      private static boolean loadProviderFromProperty() {
          String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");
          if (cn == null)
              return false;
          try {
              Class<?> c = Class.forName(cn, true,
                                         ClassLoader.getSystemClassLoader());
              provider = (SelectorProvider)c.newInstance();
              return true;
          } catch (ClassNotFoundException x) {
              throw new ServiceConfigurationError(null, x);
          } catch (IllegalAccessException x) {
              throw new ServiceConfigurationError(null, x);
          } catch (InstantiationException x) {
              throw new ServiceConfigurationError(null, x);
          } catch (SecurityException x) {
              throw new ServiceConfigurationError(null, x);
          }
      }
  
      private static boolean loadProviderAsService() {
  
          ServiceLoader<SelectorProvider> sl =
              ServiceLoader.load(SelectorProvider.class,
                                 ClassLoader.getSystemClassLoader());
          Iterator<SelectorProvider> i = sl.iterator();
          for (;;) {
              try {
                  if (!i.hasNext())
                      return false;
                  provider = i.next();
                  return true;
              } catch (ServiceConfigurationError sce) {
                  if (sce.getCause() instanceof SecurityException) {
                      // Ignore the security exception, try the next provider
                      continue;
                  }
                  throw sce;
              }
          }
      }
  
      /**
       * Returns the system-wide default selector provider for this invocation of
       * the Java virtual machine.
       *
       * <p> The first invocation of this method locates the default provider
       * object as follows: </p>
       *
       * <ol>
       *
       *   <li><p> If the system property
       *   <tt>java.nio.channels.spi.SelectorProvider</tt> is defined then it is
       *   taken to be the fully-qualified name of a concrete provider class.
       *   The class is loaded and instantiated; if this process fails then an
       *   unspecified error is thrown.  </p></li>
       *
       *   <li><p> If a provider class has been installed in a jar file that is
       *   visible to the system class loader, and that jar file contains a
       *   provider-configuration file named
       *   <tt>java.nio.channels.spi.SelectorProvider</tt> in the resource
       *   directory <tt>META-INF/services</tt>, then the first class name
       *   specified in that file is taken.  The class is loaded and
       *   instantiated; if this process fails then an unspecified error is
       *   thrown.  </p></li>
       *
       *   <li><p> Finally, if no provider has been specified by any of the above
       *   means then the system-default provider class is instantiated and the
       *   result is returned.  </p></li>
       *
       * </ol>
       *
       * <p> Subsequent invocations of this method return the provider that was
       * returned by the first invocation.  </p>
       *
       * @return  The system-wide default selector provider
       */
      public static SelectorProvider provider() {
          synchronized (lock) {
              if (provider != null)
                  return provider;
              return AccessController.doPrivileged(
                  new PrivilegedAction<SelectorProvider>() {
                      public SelectorProvider run() {
                              if (loadProviderFromProperty())
                                  return provider;
                              if (loadProviderAsService())
                                  return provider;
                              provider = sun.nio.ch.DefaultSelectorProvider.create();
                              return provider;
                          }
                      });
          }
      }
  
      /**
       * Opens a datagram channel.
       *
       * @return  The new channel
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract DatagramChannel openDatagramChannel()
          throws IOException;
  
      /**
       * Opens a datagram channel.
       *
       * @param   family
       *          The protocol family
       *
       * @return  A new datagram channel
       *
       * @throws  UnsupportedOperationException
       *          If the specified protocol family is not supported
       * @throws  IOException
       *          If an I/O error occurs
       *
       * @since 1.7
       */
      public abstract DatagramChannel openDatagramChannel(ProtocolFamily family)
          throws IOException;
  
      /**
       * Opens a pipe.
       *
       * @return  The new pipe
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract Pipe openPipe()
          throws IOException;
  
      /**
       * Opens a selector.
       *
       * @return  The new selector
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract AbstractSelector openSelector()
          throws IOException;
  
      /**
       * Opens a server-socket channel.
       *
       * @return  The new channel
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract ServerSocketChannel openServerSocketChannel()
          throws IOException;
  
      /**
       * Opens a socket channel.
       *
       * @return  The new channel
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract SocketChannel openSocketChannel()
          throws IOException;
  
      /**
       * Returns the channel inherited from the entity that created this
       * Java virtual machine.
       *
       * <p> On many operating systems a process, such as a Java virtual
       * machine, can be started in a manner that allows the process to
       * inherit a channel from the entity that created the process. The
       * manner in which this is done is system dependent, as are the
       * possible entities to which the channel may be connected. For example,
       * on UNIX systems, the Internet services daemon (<i>inetd</i>) is used to
       * start programs to service requests when a request arrives on an
       * associated network port. In this example, the process that is started,
       * inherits a channel representing a network socket.
       *
       * <p> In cases where the inherited channel represents a network socket
       * then the {@link Channel Channel} type returned
       * by this method is determined as follows:
       *
       * <ul>
       *
       *  <li><p> If the inherited channel represents a stream-oriented connected
       *  socket then a {@link SocketChannel SocketChannel} is
       *  returned. The socket channel is, at least initially, in blocking
       *  mode, bound to a socket address, and connected to a peer.
       *  </p></li>
       *
       *  <li><p> If the inherited channel represents a stream-oriented listening
       *  socket then a {@link ServerSocketChannel
       *  ServerSocketChannel} is returned. The server-socket channel is, at
       *  least initially, in blocking mode, and bound to a socket address.
       *  </p></li>
       *
       *  <li><p> If the inherited channel is a datagram-oriented socket
       *  then a {@link DatagramChannel DatagramChannel} is
       *  returned. The datagram channel is, at least initially, in blocking
       *  mode, and bound to a socket address.
       *  </p></li>
       *
       * </ul>
       *
       * <p> In addition to the network-oriented channels described, this method
       * may return other kinds of channels in the future.
       *
       * <p> The first invocation of this method creates the channel that is
       * returned. Subsequent invocations of this method return the same
       * channel. </p>
       *
       * @return  The inherited channel, if any, otherwise <tt>null</tt>.
       *
       * @throws  IOException
       *          If an I/O error occurs
       *
       * @throws  SecurityException
       *          If a security manager has been installed and it denies
       *          {@link RuntimePermission}<tt>("inheritedChannel")</tt>
       *
       * @since 1.5
       */
     public Channel inheritedChannel() throws IOException {
          return null;
     }
  
  }
  