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  package java.nio.channels;
  
  import java.nio.channels.spi.AsynchronousChannelProvider;
  import java.io.IOException;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.TimeUnit;
  
  /**
   * A grouping of asynchronous channels for the purpose of resource sharing.
   *
   * <p> An asynchronous channel group encapsulates the mechanics required to
   * handle the completion of I/O operations initiated by {@link AsynchronousChannel
   * asynchronous channels} that are bound to the group. A group has an associated
   * thread pool to which tasks are submitted to handle I/O events and dispatch to
   * {@link CompletionHandler completion-handlers} that consume the result of
   * asynchronous operations performed on channels in the group. In addition to
   * handling I/O events, the pooled threads may also execute other tasks required
   * to support the execution of asynchronous I/O operations.
   *
   * <p> An asynchronous channel group is created by invoking the {@link
   * #withFixedThreadPool withFixedThreadPool} or {@link #withCachedThreadPool
   * withCachedThreadPool} methods defined here. Channels are bound to a group by
   * specifying the group when constructing the channel. The associated thread
   * pool is <em>owned</em> by the group; termination of the group results in the
   * shutdown of the associated thread pool.
   *
   * <p> In addition to groups created explicitly, the Java virtual machine
   * maintains a system-wide <em>default group</em> that is constructed
   * automatically. Asynchronous channels that do not specify a group at
   * construction time are bound to the default group. The default group has an
   * associated thread pool that creates new threads as needed. The default group
   * may be configured by means of system properties defined in the table below.
   * Where the {@link ThreadFactory ThreadFactory} for the
   * default group is not configured then the pooled threads of the default group
   * are {@link Thread#isDaemon daemon} threads.
   *
   * <table border summary="System properties">
   *   <tr>
   *     <th>System property</th>
   *     <th>Description</th>
   *   </tr>
   *   <tr>
   *     <td> {@code java.nio.channels.DefaultThreadPool.threadFactory} </td>
   *     <td> The value of this property is taken to be the fully-qualified name
   *     of a concrete {@link ThreadFactory ThreadFactory}
   *     class. The class is loaded using the system class loader and instantiated.
   *     The factory's {@link ThreadFactory#newThread
   *     newThread} method is invoked to create each thread for the default
   *     group's thread pool. If the process to load and instantiate the value
   *     of the property fails then an unspecified error is thrown during the
   *     construction of the default group. </td>
   *   </tr>
   *   <tr>
   *     <td> {@code java.nio.channels.DefaultThreadPool.initialSize} </td>
   *     <td> The value of the {@code initialSize} parameter for the default
   *     group (see {@link #withCachedThreadPool withCachedThreadPool}).
   *     The value of the property is taken to be the {@code String}
   *     representation of an {@code Integer} that is the initial size parameter.
   *     If the value cannot be parsed as an {@code Integer} it causes an
   *     unspecified error to be thrown during the construction of the default
   *     group. </td>
   *   </tr>
   * </table>
   *
   * <a name="threading"></a><h2>Threading</h2>
   *
   * <p> The completion handler for an I/O operation initiated on a channel bound
   * to a group is guaranteed to be invoked by one of the pooled threads in the
   * group. This ensures that the completion handler is run by a thread with the
   * expected <em>identity</em>.
   *
   * <p> Where an I/O operation completes immediately, and the initiating thread
   * is one of the pooled threads in the group then the completion handler may
   * be invoked directly by the initiating thread. To avoid stack overflow, an
   * implementation may impose a limit as to the number of activations on the
   * thread stack. Some I/O operations may prohibit invoking the completion
   * handler directly by the initiating thread (see {@link
   * AsynchronousServerSocketChannel#accept(Object,CompletionHandler) accept}).
   *
   * <a name="shutdown"></a><h2>Shutdown and Termination</h2>
   *
   * <p> The {@link #shutdown() shutdown} method is used to initiate an <em>orderly
   * shutdown</em> of a group. An orderly shutdown marks the group as shutdown;
   * further attempts to construct a channel that binds to the group will throw
   * {@link ShutdownChannelGroupException}. Whether or not a group is shutdown can
   * be tested using the {@link #isShutdown() isShutdown} method. Once shutdown,
   * the group <em>terminates</em> when all asynchronous channels that are bound to
   * the group are closed, all actively executing completion handlers have run to
   * completion, and resources used by the group are released. No attempt is made
   * to stop or interrupt threads that are executing completion handlers. The
   * {@link #isTerminated() isTerminated} method is used to test if the group has
   * terminated, and the {@link #awaitTermination awaitTermination} method can be
   * used to block until the group has terminated.
   *
   * <p> The {@link #shutdownNow() shutdownNow} method can be used to initiate a
   * <em>forceful shutdown</em> of the group. In addition to the actions performed
   * by an orderly shutdown, the {@code shutdownNow} method closes all open channels
   * in the group as if by invoking the {@link AsynchronousChannel#close close}
   * method.
   *
   * @since 1.7
   *
   * @see AsynchronousSocketChannel#open(AsynchronousChannelGroup)
   * @see AsynchronousServerSocketChannel#open(AsynchronousChannelGroup)
   */
  
  public abstract class AsynchronousChannelGroup {
      private final AsynchronousChannelProvider provider;
  
      /**
       * Initialize a new instance of this class.
       *
       * @param   provider
       *          The asynchronous channel provider for this group
       */
      protected AsynchronousChannelGroup(AsynchronousChannelProvider provider) {
          this.provider = provider;
      }
  
      /**
       * Returns the provider that created this channel group.
       *
       * @return  The provider that created this channel group
       */
      public final AsynchronousChannelProvider provider() {
          return provider;
      }
  
      /**
       * Creates an asynchronous channel group with a fixed thread pool.
       *
       * <p> The resulting asynchronous channel group reuses a fixed number of
       * threads. At any point, at most {@code nThreads} threads will be active
       * processing tasks that are submitted to handle I/O events and dispatch
       * completion results for operations initiated on asynchronous channels in
       * the group.
       *
       * <p> The group is created by invoking the {@link
       * AsynchronousChannelProvider#openAsynchronousChannelGroup(int,ThreadFactory)
       * openAsynchronousChannelGroup(int,ThreadFactory)} method of the system-wide
       * default {@link AsynchronousChannelProvider} object.
       *
       * @param   nThreads
       *          The number of threads in the pool
       * @param   threadFactory
       *          The factory to use when creating new threads
       *
       * @return  A new asynchronous channel group
       *
       * @throws  IllegalArgumentException
       *          If {@code nThreads <= 0}
       * @throws  IOException
       *          If an I/O error occurs
       */
      public static AsynchronousChannelGroup withFixedThreadPool(int nThreads,
                                                                 ThreadFactory threadFactory)
          throws IOException
      {
          return AsynchronousChannelProvider.provider()
              .openAsynchronousChannelGroup(nThreads, threadFactory);
      }
  
      /**
       * Creates an asynchronous channel group with a given thread pool that
       * creates new threads as needed.
       *
       * <p> The {@code executor} parameter is an {@code ExecutorService} that
       * creates new threads as needed to execute tasks that are submitted to
       * handle I/O events and dispatch completion results for operations initiated
       * on asynchronous channels in the group. It may reuse previously constructed
       * threads when they are available.
       *
       * <p> The {@code initialSize} parameter may be used by the implementation
       * as a <em>hint</em> as to the initial number of tasks it may submit. For
       * example, it may be used to indicate the initial number of threads that
       * wait on I/O events.
       *
       * <p> The executor is intended to be used exclusively by the resulting
       * asynchronous channel group. Termination of the group results in the
       * orderly  {@link ExecutorService#shutdown shutdown} of the executor
       * service. Shutting down the executor service by other means results in
       * unspecified behavior.
       *
       * <p> The group is created by invoking the {@link
       * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
       * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
       * default {@link AsynchronousChannelProvider} object.
       *
       * @param   executor
       *          The thread pool for the resulting group
       * @param   initialSize
       *          A value {@code >=0} or a negative value for implementation
       *          specific default
       *
       * @return  A new asynchronous channel group
       *
       * @throws  IOException
       *          If an I/O error occurs
       *
       * @see java.util.concurrent.Executors#newCachedThreadPool
       */
      public static AsynchronousChannelGroup withCachedThreadPool(ExecutorService executor,
                                                                  int initialSize)
          throws IOException
      {
          return AsynchronousChannelProvider.provider()
              .openAsynchronousChannelGroup(executor, initialSize);
      }
  
      /**
       * Creates an asynchronous channel group with a given thread pool.
       *
       * <p> The {@code executor} parameter is an {@code ExecutorService} that
       * executes tasks submitted to dispatch completion results for operations
       * initiated on asynchronous channels in the group.
       *
       * <p> Care should be taken when configuring the executor service. It
       * should support <em>direct handoff</em> or <em>unbounded queuing</em> of
       * submitted tasks, and the thread that invokes the {@link
       * ExecutorService#execute execute} method should never invoke the task
       * directly. An implementation may mandate additional constraints.
       *
       * <p> The executor is intended to be used exclusively by the resulting
       * asynchronous channel group. Termination of the group results in the
       * orderly  {@link ExecutorService#shutdown shutdown} of the executor
       * service. Shutting down the executor service by other means results in
       * unspecified behavior.
       *
       * <p> The group is created by invoking the {@link
       * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
       * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
       * default {@link AsynchronousChannelProvider} object with an {@code
       * initialSize} of {@code 0}.
       *
       * @param   executor
       *          The thread pool for the resulting group
       *
       * @return  A new asynchronous channel group
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public static AsynchronousChannelGroup withThreadPool(ExecutorService executor)
          throws IOException
      {
          return AsynchronousChannelProvider.provider()
              .openAsynchronousChannelGroup(executor, 0);
      }
  
      /**
       * Tells whether or not this asynchronous channel group is shutdown.
       *
       * @return  {@code true} if this asynchronous channel group is shutdown or
       *          has been marked for shutdown.
       */
      public abstract boolean isShutdown();
  
      /**
       * Tells whether or not this group has terminated.
       *
       * <p> Where this method returns {@code true}, then the associated thread
       * pool has also {@link ExecutorService#isTerminated terminated}.
       *
       * @return  {@code true} if this group has terminated
       */
      public abstract boolean isTerminated();
  
      /**
       * Initiates an orderly shutdown of the group.
       *
       * <p> This method marks the group as shutdown. Further attempts to construct
       * channel that binds to this group will throw {@link ShutdownChannelGroupException}.
       * The group terminates when all asynchronous channels in the group are
       * closed, all actively executing completion handlers have run to completion,
       * and all resources have been released. This method has no effect if the
       * group is already shutdown.
       */
      public abstract void shutdown();
  
      /**
       * Shuts down the group and closes all open channels in the group.
       *
       * <p> In addition to the actions performed by the {@link #shutdown() shutdown}
       * method, this method invokes the {@link AsynchronousChannel#close close}
       * method on all open channels in the group. This method does not attempt to
       * stop or interrupt threads that are executing completion handlers. The
       * group terminates when all actively executing completion handlers have run
       * to completion and all resources have been released. This method may be
       * invoked at any time. If some other thread has already invoked it, then
       * another invocation will block until the first invocation is complete,
       * after which it will return without effect.
       *
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract void shutdownNow() throws IOException;
  
      /**
       * Awaits termination of the group.
  
       * <p> This method blocks until the group has terminated, or the timeout
       * occurs, or the current thread is interrupted, whichever happens first.
       *
       * @param   timeout
       *          The maximum time to wait, or zero or less to not wait
       * @param   unit
       *          The time unit of the timeout argument
       *
       * @return  {@code true} if the group has terminated; {@code false} if the
       *          timeout elapsed before termination
       *
       * @throws  InterruptedException
       *          If interrupted while waiting
       */
      public abstract boolean awaitTermination(long timeout, TimeUnit unit)
          throws InterruptedException;
  }
  