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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  package java.nio.channels;
  
  import java.io.IOException;
  import java.nio.channels.spi.AbstractInterruptibleChannel;
  import java.nio.channels.spi.SelectorProvider;
  
  
  /**
   * A channel that can be multiplexed via a {@link Selector}.
   *
   * <p> In order to be used with a selector, an instance of this class must
   * first be <i>registered</i> via the {@link #register(Selector,int,Object)
   * register} method.  This method returns a new {@link SelectionKey} object
   * that represents the channel's registration with the selector.
   *
   * <p> Once registered with a selector, a channel remains registered until it
   * is <i>deregistered</i>.  This involves deallocating whatever resources were
   * allocated to the channel by the selector.
   *
   * <p> A channel cannot be deregistered directly; instead, the key representing
   * its registration must be <i>cancelled</i>.  Cancelling a key requests that
   * the channel be deregistered during the selector's next selection operation.
   * A key may be cancelled explicitly by invoking its {@link
   * SelectionKey#cancel() cancel} method.  All of a channel's keys are cancelled
   * implicitly when the channel is closed, whether by invoking its {@link
   * Channel#close close} method or by interrupting a thread blocked in an I/O
   * operation upon the channel.
   *
   * <p> If the selector itself is closed then the channel will be deregistered,
   * and the key representing its registration will be invalidated, without
   * further delay.
   *
   * <p> A channel may be registered at most once with any particular selector.
   *
   * <p> Whether or not a channel is registered with one or more selectors may be
   * determined by invoking the {@link #isRegistered isRegistered} method.
   *
   * <p> Selectable channels are safe for use by multiple concurrent
   * threads. </p>
   *
   *
   * <a name="bm"></a>
   * <h2>Blocking mode</h2>
   *
   * A selectable channel is either in <i>blocking</i> mode or in
   * <i>non-blocking</i> mode.  In blocking mode, every I/O operation invoked
   * upon the channel will block until it completes.  In non-blocking mode an I/O
   * operation will never block and may transfer fewer bytes than were requested
   * or possibly no bytes at all.  The blocking mode of a selectable channel may
   * be determined by invoking its {@link #isBlocking isBlocking} method.
   *
   * <p> Newly-created selectable channels are always in blocking mode.
   * Non-blocking mode is most useful in conjunction with selector-based
   * multiplexing.  A channel must be placed into non-blocking mode before being
   * registered with a selector, and may not be returned to blocking mode until
   * it has been deregistered.
   *
   *
   * @author Mark Reinhold
   * @author JSR-51 Expert Group
   * @since 1.4
   *
   * @see SelectionKey
   * @see Selector
   */
  
  public abstract class SelectableChannel
      extends AbstractInterruptibleChannel
      implements Channel
  {
  
      /**
       * Initializes a new instance of this class.
       */
      protected SelectableChannel() { }
  
      /**
       * Returns the provider that created this channel.
       *
       * @return  The provider that created this channel
       */
      public abstract SelectorProvider provider();
  
      /**
       * Returns an <a href="SelectionKey.html#opsets">operation set</a>
       * identifying this channel's supported operations.  The bits that are set
       * in this integer value denote exactly the operations that are valid for
       * this channel.  This method always returns the same value for a given
       * concrete channel class.
       *
       * @return  The valid-operation set
       */
      public abstract int validOps();
  
      // Internal state:
      //   keySet, may be empty but is never null, typ. a tiny array
      //   boolean isRegistered, protected by key set
      //   regLock, lock object to prevent duplicate registrations
      //   boolean isBlocking, protected by regLock
  
      /**
       * Tells whether or not this channel is currently registered with any
       * selectors.  A newly-created channel is not registered.
       *
       * <p> Due to the inherent delay between key cancellation and channel
       * deregistration, a channel may remain registered for some time after all
       * of its keys have been cancelled.  A channel may also remain registered
       * for some time after it is closed.  </p>
       *
       * @return <tt>true</tt> if, and only if, this channel is registered
       */
      public abstract boolean isRegistered();
      //
      // sync(keySet) { return isRegistered; }
  
      /**
       * Retrieves the key representing the channel's registration with the given
       * selector.
       *
       * @param   sel
       *          The selector
       *
       * @return  The key returned when this channel was last registered with the
       *          given selector, or <tt>null</tt> if this channel is not
       *          currently registered with that selector
       */
      public abstract SelectionKey keyFor(Selector sel);
      //
      // sync(keySet) { return findKey(sel); }
  
      /**
       * Registers this channel with the given selector, returning a selection
       * key.
       *
       * <p> If this channel is currently registered with the given selector then
       * the selection key representing that registration is returned.  The key's
       * interest set will have been changed to <tt>ops</tt>, as if by invoking
       * the {@link SelectionKey#interestOps(int) interestOps(int)} method.  If
       * the <tt>att</tt> argument is not <tt>null</tt> then the key's attachment
       * will have been set to that value.  A {@link CancelledKeyException} will
       * be thrown if the key has already been cancelled.
       *
       * <p> Otherwise this channel has not yet been registered with the given
       * selector, so it is registered and the resulting new key is returned.
       * The key's initial interest set will be <tt>ops</tt> and its attachment
       * will be <tt>att</tt>.
       *
       * <p> This method may be invoked at any time.  If this method is invoked
       * while another invocation of this method or of the {@link
       * #configureBlocking(boolean) configureBlocking} method is in progress
       * then it will first block until the other operation is complete.  This
       * method will then synchronize on the selector's key set and therefore may
       * block if invoked concurrently with another registration or selection
       * operation involving the same selector. </p>
       *
       * <p> If this channel is closed while this operation is in progress then
       * the key returned by this method will have been cancelled and will
       * therefore be invalid. </p>
       *
       * @param  sel
       *         The selector with which this channel is to be registered
       *
       * @param  ops
       *         The interest set for the resulting key
       *
       * @param  att
       *         The attachment for the resulting key; may be <tt>null</tt>
       *
       * @throws  ClosedChannelException
       *          If this channel is closed
       *
       * @throws  ClosedSelectorException
       *          If the selector is closed
       *
       * @throws  IllegalBlockingModeException
       *          If this channel is in blocking mode
       *
       * @throws  IllegalSelectorException
       *          If this channel was not created by the same provider
       *          as the given selector
       *
       * @throws  CancelledKeyException
       *          If this channel is currently registered with the given selector
       *          but the corresponding key has already been cancelled
       *
       * @throws  IllegalArgumentException
       *          If a bit in the <tt>ops</tt> set does not correspond to an
       *          operation that is supported by this channel, that is, if
       *          {@code set & ~validOps() != 0}
       *
       * @return  A key representing the registration of this channel with
       *          the given selector
       */
      public abstract SelectionKey register(Selector sel, int ops, Object att)
          throws ClosedChannelException;
      //
      // sync(regLock) {
      //   sync(keySet) { look for selector }
      //   if (channel found) { set interest ops -- may block in selector;
      //                        return key; }
      //   create new key -- may block somewhere in selector;
      //   sync(keySet) { add key; }
      //   attach(attachment);
      //   return key;
      // }
  
      /**
       * Registers this channel with the given selector, returning a selection
       * key.
       *
       * <p> An invocation of this convenience method of the form
       *
       * <blockquote><tt>sc.register(sel, ops)</tt></blockquote>
       *
       * behaves in exactly the same way as the invocation
       *
       * <blockquote><tt>sc.{@link
       * #register(Selector,int, Object)
       * register}(sel, ops, null)</tt></blockquote>
       *
       * @param  sel
       *         The selector with which this channel is to be registered
       *
       * @param  ops
       *         The interest set for the resulting key
       *
       * @throws  ClosedChannelException
       *          If this channel is closed
       *
       * @throws  ClosedSelectorException
       *          If the selector is closed
       *
       * @throws  IllegalBlockingModeException
       *          If this channel is in blocking mode
       *
       * @throws  IllegalSelectorException
       *          If this channel was not created by the same provider
       *          as the given selector
       *
       * @throws  CancelledKeyException
       *          If this channel is currently registered with the given selector
       *          but the corresponding key has already been cancelled
       *
       * @throws  IllegalArgumentException
       *          If a bit in <tt>ops</tt> does not correspond to an operation
       *          that is supported by this channel, that is, if {@code set &
       *          ~validOps() != 0}
       *
       * @return  A key representing the registration of this channel with
       *          the given selector
       */
      public final SelectionKey register(Selector sel, int ops)
          throws ClosedChannelException
      {
          return register(sel, ops, null);
      }
  
      /**
       * Adjusts this channel's blocking mode.
       *
       * <p> If this channel is registered with one or more selectors then an
       * attempt to place it into blocking mode will cause an {@link
       * IllegalBlockingModeException} to be thrown.
       *
       * <p> This method may be invoked at any time.  The new blocking mode will
       * only affect I/O operations that are initiated after this method returns.
       * For some implementations this may require blocking until all pending I/O
       * operations are complete.
       *
       * <p> If this method is invoked while another invocation of this method or
       * of the {@link #register(Selector, int) register} method is in progress
       * then it will first block until the other operation is complete. </p>
       *
       * @param  block  If <tt>true</tt> then this channel will be placed in
       *                blocking mode; if <tt>false</tt> then it will be placed
       *                non-blocking mode
       *
       * @return  This selectable channel
       *
       * @throws  ClosedChannelException
       *          If this channel is closed
       *
       * @throws  IllegalBlockingModeException
       *          If <tt>block</tt> is <tt>true</tt> and this channel is
       *          registered with one or more selectors
       *
       * @throws IOException
       *         If an I/O error occurs
       */
      public abstract SelectableChannel configureBlocking(boolean block)
          throws IOException;
      //
      // sync(regLock) {
      //   sync(keySet) { throw IBME if block && isRegistered; }
      //   change mode;
      // }
  
      /**
       * Tells whether or not every I/O operation on this channel will block
       * until it completes.  A newly-created channel is always in blocking mode.
       *
       * <p> If this channel is closed then the value returned by this method is
       * not specified. </p>
       *
       * @return <tt>true</tt> if, and only if, this channel is in blocking mode
       */
      public abstract boolean isBlocking();
  
      /**
       * Retrieves the object upon which the {@link #configureBlocking
       * configureBlocking} and {@link #register register} methods synchronize.
       * This is often useful in the implementation of adaptors that require a
       * specific blocking mode to be maintained for a short period of time.
       *
       * @return  The blocking-mode lock object
       */
      public abstract Object blockingLock();
  
  }
  