AsynchronousSocketChannel.java

/*
 * Copyright 2007-2009 Sun Microsystems, Inc.  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.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

package java.nio.channels;

import java.nio.channels.spi.*;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.Future;
import java.io.IOException;
import java.net.SocketOption;
import java.net.SocketAddress;
import java.nio.ByteBuffer;

/**
 * An asynchronous channel for stream-oriented connecting sockets.
 *
 * <p> Asynchronous socket channels are created in one of two ways. A newly-created
 * {@code AsynchronousSocketChannel} is created by invoking one of the {@link
 * #open open} methods defined by this class. A newly-created channel is open but
 * not yet connected. A connected {@code AsynchronousSocketChannel} is created
 * when a connection is made to the socket of an {@link AsynchronousServerSocketChannel}.
 * It is not possible to create an asynchronous socket channel for an arbitrary,
 * pre-existing {@link java.net.Socket socket}.
 *
 * <p> A newly-created channel is connected by invoking its {@link #connect connect}
 * method; once connected, a channel remains connected until it is closed.  Whether
 * or not a socket channel is connected may be determined by invoking its {@link
 * #getRemoteAddress getRemoteAddress} method. An attempt to invoke an I/O
 * operation upon an unconnected channel will cause a {@link NotYetConnectedException}
 * to be thrown.
 *
 * <p> Channels of this type are safe for use by multiple concurrent threads.
 * They support concurrent reading and writing, though at most one read operation
 * and one write operation can be outstanding at any time.
 * If a thread initiates a read operation before a previous read operation has
 * completed then a {@link ReadPendingException} will be thrown. Similarly, an
 * attempt to initiate a write operation before a previous write has completed
 * will throw a {@link WritePendingException}.
 *
 * <p> Socket options are configured using the {@link #setOption(SocketOption,Object)
 * setOption} method. Asynchronous socket channels support the following options:
 * <blockquote>
 * <table border>
 *   <tr>
 *     <th>Option Name</th>
 *     <th>Description</th>
 *   </tr>
 *   <tr>
 *     <td> {@link java.net.StandardSocketOption#SO_SNDBUF SO_SNDBUF} </td>
 *     <td> The size of the socket send buffer </td>
 *   </tr>
 *   <tr>
 *     <td> {@link java.net.StandardSocketOption#SO_RCVBUF SO_RCVBUF} </td>
 *     <td> The size of the socket receive buffer </td>
 *   </tr>
 *   <tr>
 *     <td> {@link java.net.StandardSocketOption#SO_KEEPALIVE SO_KEEPALIVE} </td>
 *     <td> Keep connection alive </td>
 *   </tr>
 *   <tr>
 *     <td> {@link java.net.StandardSocketOption#SO_REUSEADDR SO_REUSEADDR} </td>
 *     <td> Re-use address </td>
 *   </tr>
 *   <tr>
 *     <td> {@link java.net.StandardSocketOption#TCP_NODELAY TCP_NODELAY} </td>
 *     <td> Disable the Nagle algorithm </td>
 *   </tr>
 * </table>
 * </blockquote>
 * Additional (implementation specific) options may also be supported.
 *
 * <h4>Timeouts</h4>
 *
 * <p> The {@link #read(ByteBuffer,long,TimeUnit,Object,CompletionHandler) read}
 * and {@link #write(ByteBuffer,long,TimeUnit,Object,CompletionHandler) write}
 * methods defined by this class allow a timeout to be specified when initiating
 * a read or write operation. If the timeout elapses before an operation completes
 * then the operation completes with the exception {@link
 * InterruptedByTimeoutException}. A timeout may leave the channel, or the
 * underlying connection, in an inconsistent state. Where the implementation
 * cannot guarantee that bytes have not been read from the channel then it puts
 * the channel into an implementation specific <em>error state</em>. A subsequent
 * attempt to initiate a {@code read} operation causes an unspecified runtime
 * exception to be thrown. Similarly if a {@code write} operation times out and
 * the implementation cannot guarantee bytes have not been written to the
 * channel then further attempts to {@code write} to the channel cause an
 * unspecified runtime exception to be thrown. When a timeout elapses then the
 * state of the {@link ByteBuffer}, or the sequence of buffers, for the I/O
 * operation is not defined. Buffers should be discarded or at least care must
 * be taken to ensure that the buffers are not accessed while the channel remains
 * open.
 *
 * @since 1.7
 */

public abstract class AsynchronousSocketChannel
    implements AsynchronousByteChannel, NetworkChannel
{
    private final AsynchronousChannelProvider provider;

    /**
     * Initializes a new instance of this class.
     */
    protected AsynchronousSocketChannel(AsynchronousChannelProvider provider) {
        this.provider = provider;
    }

    /**
     * Returns the provider that created this channel.
     */
    public final AsynchronousChannelProvider provider() {
        return provider;
    }

    /**
     * Opens an asynchronous socket channel.
     *
     * <p> The new channel is created by invoking the {@link
     * AsynchronousChannelProvider#openAsynchronousSocketChannel
     * openAsynchronousSocketChannel} method on the {@link
     * AsynchronousChannelProvider} that created the group. If the group parameter
     * is {@code null} then the resulting channel is created by the system-wide
     * default provider, and bound to the <em>default group</em>.
     *
     * @param   group
     *          The group to which the newly constructed channel should be bound,
     *          or {@code null} for the default group
     *
     * @return  A new asynchronous socket channel
     *
     * @throws  ShutdownChannelGroupException
     *          If the channel group is shutdown
     * @throws  IOException
     *          If an I/O error occurs
     */
    public static AsynchronousSocketChannel open(AsynchronousChannelGroup group)
        throws IOException
    {
        AsynchronousChannelProvider provider = (group == null) ?
            AsynchronousChannelProvider.provider() : group.provider();
        return provider.openAsynchronousSocketChannel(group);
    }

    /**
     * Opens an asynchronous socket channel.
     *
     * <p> This method returns an asynchronous socket channel that is bound to
     * the <em>default group</em>.This method is equivalent to evaluating the
     * expression:
     * <blockquote><pre>
     * open((AsynchronousChannelGroup)null);
     * </pre></blockquote>
     *
     * @return  A new asynchronous socket channel
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    public static AsynchronousSocketChannel open()
        throws IOException
    {
        return open(null);
    }


    // -- socket options and related --

    /**
     * @throws  ConnectionPendingException
     *          If a connection operation is already in progress on this channel
     * @throws  AlreadyBoundException               {@inheritDoc}
     * @throws  UnsupportedAddressTypeException     {@inheritDoc}
     * @throws  ClosedChannelException              {@inheritDoc}
     * @throws  IOException                         {@inheritDoc}
     */
    @Override
    public abstract AsynchronousSocketChannel bind(SocketAddress local)
        throws IOException;

    /**
     * @throws  IllegalArgumentException                {@inheritDoc}
     * @throws  ClosedChannelException                  {@inheritDoc}
     * @throws  IOException                             {@inheritDoc}
     */
    @Override
    public abstract <T> AsynchronousSocketChannel setOption(SocketOption<T> name, T value)
        throws IOException;

    /**
     * Shutdown the connection for reading without closing the channel.
     *
     * <p> Once shutdown for reading then further reads on the channel will
     * return {@code -1}, the end-of-stream indication. If the input side of the
     * connection is already shutdown then invoking this method has no effect.
     * The effect on an outstanding read operation is system dependent and
     * therefore not specified. The effect, if any, when there is data in the
     * socket receive buffer that has not been read, or data arrives subsequently,
     * is also system dependent.
     *
     * @return  The channel
     *
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ClosedChannelException
     *          If this channel is closed
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract AsynchronousSocketChannel shutdownInput() throws IOException;

    /**
     * Shutdown the connection for writing without closing the channel.
     *
     * <p> Once shutdown for writing then further attempts to write to the
     * channel will throw {@link ClosedChannelException}. If the output side of
     * the connection is already shutdown then invoking this method has no
     * effect. The effect on an outstanding write operation is system dependent
     * and therefore not specified.
     *
     * @return  The channel
     *
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ClosedChannelException
     *          If this channel is closed
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract AsynchronousSocketChannel shutdownOutput() throws IOException;

    // -- state --

    /**
     * Returns the remote address to which this channel's socket is connected.
     *
     * <p> Where the channel is bound and connected to an Internet Protocol
     * socket address then the return value from this method is of type {@link
     * java.net.InetSocketAddress}.
     *
     * @return  The remote address; {@code null} if the channel's socket is not
     *          connected
     *
     * @throws  ClosedChannelException
     *          If the channel is closed
     * @throws  IOException
     *          If an I/O error occurs
     */
    public abstract SocketAddress getRemoteAddress() throws IOException;

    // -- asynchronous operations --

    /**
     * Connects this channel.
     *
     * <p> This method initiates an operation to connect this channel, returning
     * a {@code Future} representing the pending result of the operation. If
     * the connection is successfully established then the {@code Future}'s
     * {@link Future#get() get} method will return {@code null}. If the
     * connection cannot be established then the channel is closed. In that case,
     * invoking the {@code get} method throws {@link
     * java.util.concurrent.ExecutionException} with an {@code IOException} as
     * the cause.
     *
     * <p> This method performs exactly the same security checks as the {@link
     * java.net.Socket} class.  That is, if a security manager has been
     * installed then this method verifies that its {@link
     * java.lang.SecurityManager#checkConnect checkConnect} method permits
     * connecting to the address and port number of the given remote endpoint.
     *
     * @param   remote
     *          The remote address to which this channel is to be connected
     * @param   attachment
     *          The object to attach to the I/O operation; can be {@code null}
     * @param   handler
     *          The handler for consuming the result; can be {@code null}
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  UnresolvedAddressException
     *          If the given remote address is not fully resolved
     * @throws  UnsupportedAddressTypeException
     *          If the type of the given remote address is not supported
     * @throws  AlreadyConnectedException
     *          If this channel is already connected
     * @throws  ConnectionPendingException
     *          If a connection operation is already in progress on this channel
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     * @throws  SecurityException
     *          If a security manager has been installed
     *          and it does not permit access to the given remote endpoint
     *
     * @see #getRemoteAddress
     */
    public abstract <A> Future<Void> connect(SocketAddress remote,
                                             A attachment,
                                             CompletionHandler<Void,? super A> handler);

    /**
     * Connects this channel.
     *
     * <p> This method is equivalent to invoking {@link
     * #connect(SocketAddress,Object,CompletionHandler)} with the {@code attachment}
     * and handler parameters set to {@code null}.
     *
     * @param   remote
     *          The remote address to which this channel is to be connected
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  UnresolvedAddressException
     *          If the given remote address is not fully resolved
     * @throws  UnsupportedAddressTypeException
     *          If the type of the given remote address is not supported
     * @throws  AlreadyConnectedException
     *          If this channel is already connected
     * @throws  ConnectionPendingException
     *          If a connection operation is already in progress on this channel
     * @throws  SecurityException
     *          If a security manager has been installed
     *          and it does not permit access to the given remote endpoint
     */
    public final Future<Void> connect(SocketAddress remote) {
        return connect(remote, null, null);
    }

    /**
     * Reads a sequence of bytes from this channel into the given buffer.
     *
     * <p> This method initiates the reading of a sequence of bytes from this
     * channel into the given buffer, returning a {@code Future} representing
     * the pending result of the operation. The {@code Future}'s {@link
     * Future#get() get} method returns the number of bytes read or {@code -1}
     * if all bytes have been read and channel has reached end-of-stream.
     *
     * <p> If a timeout is specified and the timeout elapses before the operation
     * completes then the operation completes with the exception {@link
     * InterruptedByTimeoutException}. Where a timeout occurs, and the
     * implementation cannot guarantee that bytes have not been read, or will not
     * be read from the channel into the given buffer, then further attempts to
     * read from the channel will cause an unspecific runtime exception to be
     * thrown.
     *
     * <p> Otherwise this method works in the same manner as the {@link
     * AsynchronousByteChannel#read(ByteBuffer,Object,CompletionHandler)}
     * method.
     *
     * @param   dst
     *          The buffer into which bytes are to be transferred
     * @param   timeout
     *          The timeout, or {@code 0L} for no timeout
     * @param   unit
     *          The time unit of the {@code timeout} argument
     * @param   attachment
     *          The object to attach to the I/O operation; can be {@code null}
     * @param   handler
     *          The handler for consuming the result; can be {@code null}
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  IllegalArgumentException
     *          If the {@code timeout} parameter is negative or the buffer is
     *          read-only
     * @throws  ReadPendingException
     *          If a read operation is already in progress on this channel
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    public abstract <A> Future<Integer> read(ByteBuffer dst,
                                             long timeout,
                                             TimeUnit unit,
                                             A attachment,
                                             CompletionHandler<Integer,? super A> handler);

    /**
     * @throws  IllegalArgumentException        {@inheritDoc}
     * @throws  ReadPendingException            {@inheritDoc}
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    @Override
    public final <A> Future<Integer> read(ByteBuffer dst,
                                          A attachment,
                                          CompletionHandler<Integer,? super A> handler)
    {
        return read(dst, 0L, TimeUnit.MILLISECONDS, attachment, handler);
    }

    /**
     * @throws  IllegalArgumentException        {@inheritDoc}
     * @throws  ReadPendingException            {@inheritDoc}
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     */
    @Override
    public final Future<Integer> read(ByteBuffer dst) {
        return read(dst, 0L, TimeUnit.MILLISECONDS, null, null);
    }

    /**
     * Reads a sequence of bytes from this channel into a subsequence of the
     * given buffers. This operation, sometimes called a <em>scattering read</em>,
     * is often useful when implementing network protocols that group data into
     * segments consisting of one or more fixed-length headers followed by a
     * variable-length body.
     *
     * <p> This method initiates a read of up to <i>r</i> bytes from this channel,
     * where <i>r</i> is the total number of bytes remaining in the specified
     * subsequence of the given buffer array, that is,
     *
     * <blockquote><pre>
     * dsts[offset].remaining()
     *     + dsts[offset+1].remaining()
     *     + ... + dsts[offset+length-1].remaining()</pre></blockquote>
     *
     * at the moment that the read is attempted.
     *
     * <p> Suppose that a byte sequence of length <i>n</i> is read, where
     * <tt>0</tt>&nbsp;<tt>&lt;</tt>&nbsp;<i>n</i>&nbsp;<tt>&lt;=</tt>&nbsp;<i>r</i>.
     * Up to the first <tt>dsts[offset].remaining()</tt> bytes of this sequence
     * are transferred into buffer <tt>dsts[offset]</tt>, up to the next
     * <tt>dsts[offset+1].remaining()</tt> bytes are transferred into buffer
     * <tt>dsts[offset+1]</tt>, and so forth, until the entire byte sequence
     * is transferred into the given buffers.  As many bytes as possible are
     * transferred into each buffer, hence the final position of each updated
     * buffer, except the last updated buffer, is guaranteed to be equal to
     * that buffer's limit. The underlying operating system may impose a limit
     * on the number of buffers that may be used in an I/O operation. Where the
     * number of buffers (with bytes remaining), exceeds this limit, then the
     * I/O operation is performed with the maximum number of buffers allowed by
     * the operating system.
     *
     * <p> The return value from this method is a {@code Future} representing
     * the pending result of the operation. The {@code Future}'s {@link
     * Future#get() get} method returns the number of bytes read or {@code -1L}
     * if all bytes have been read and the channel has reached end-of-stream.
     *
     * <p> If a timeout is specified and the timeout elapses before the operation
     * completes then it completes with the exception {@link
     * InterruptedByTimeoutException}. Where a timeout occurs, and the
     * implementation cannot guarantee that bytes have not been read, or will not
     * be read from the channel into the given buffers, then further attempts to
     * read from the channel will cause an unspecific runtime exception to be
     * thrown.
     *
     * @param   dsts
     *          The buffers into which bytes are to be transferred
     * @param   offset
     *          The offset within the buffer array of the first buffer into which
     *          bytes are to be transferred; must be non-negative and no larger than
     *          {@code dsts.length}
     * @param   length
     *          The maximum number of buffers to be accessed; must be non-negative
     *          and no larger than {@code dsts.length - offset}
     * @param   timeout
     *          The timeout, or {@code 0L} for no timeout
     * @param   unit
     *          The time unit of the {@code timeout} argument
     * @param   attachment
     *          The object to attach to the I/O operation; can be {@code null}
     * @param   handler
     *          The handler for consuming the result; can be {@code null}
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  IndexOutOfBoundsException
     *          If the pre-conditions for the {@code offset}  and {@code length}
     *          parameter aren't met
     * @throws  IllegalArgumentException
     *          If the {@code timeout} parameter is negative, or a buffer is
     *          read-only
     * @throws  ReadPendingException
     *          If a read operation is already in progress on this channel
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    public abstract <A> Future<Long> read(ByteBuffer[] dsts,
                                          int offset,
                                          int length,
                                          long timeout,
                                          TimeUnit unit,
                                          A attachment,
                                          CompletionHandler<Long,? super A> handler);

    /**
     * Writes a sequence of bytes to this channel from the given buffer.
     *
     * <p> This method initiates the writing of a sequence of bytes to this channel
     * from the given buffer, returning a {@code Future} representing the
     * pending result of the operation. The {@code Future}'s {@link Future#get()
     * get} method will return the number of bytes written.
     *
     * <p> If a timeout is specified and the timeout elapses before the operation
     * completes then it completes with the exception {@link
     * InterruptedByTimeoutException}. Where a timeout occurs, and the
     * implementation cannot guarantee that bytes have not been written, or will
     * not be written to the channel from the given buffer, then further attempts
     * to write to the channel will cause an unspecific runtime exception to be
     * thrown.
     *
     * <p> Otherwise this method works in the same manner as the {@link
     * AsynchronousByteChannel#write(ByteBuffer,Object,CompletionHandler)}
     * method.
     *
     * @param   src
     *          The buffer from which bytes are to be retrieved
     * @param   timeout
     *          The timeout, or {@code 0L} for no timeout
     * @param   unit
     *          The time unit of the {@code timeout} argument
     * @param   attachment
     *          The object to attach to the I/O operation; can be {@code null}
     * @param   handler
     *          The handler for consuming the result; can be {@code null}
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  IllegalArgumentException
     *          If the {@code timeout} parameter is negative
     * @throws  WritePendingException
     *          If a write operation is already in progress on this channel
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    public abstract <A> Future<Integer> write(ByteBuffer src,
                                              long timeout,
                                              TimeUnit unit,
                                              A attachment,
                                              CompletionHandler<Integer,? super A> handler);

    /**
     * @throws  WritePendingException          {@inheritDoc}
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    @Override
    public final <A> Future<Integer> write(ByteBuffer src,
                                           A attachment,
                                           CompletionHandler<Integer,? super A> handler)

    {
        return write(src, 0L, TimeUnit.MILLISECONDS, attachment, handler);
    }

    /**
     * @throws  WritePendingException       {@inheritDoc}
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     */
    @Override
    public final Future<Integer> write(ByteBuffer src) {
        return write(src, 0L, TimeUnit.MILLISECONDS, null, null);
    }

    /**
     * Writes a sequence of bytes to this channel from a subsequence of the given
     * buffers. This operation, sometimes called a <em>gathering write</em>, is
     * often useful when implementing network protocols that group data into
     * segments consisting of one or more fixed-length headers followed by a
     * variable-length body.
     *
     * <p> This method initiates a write of up to <i>r</i> bytes to this channel,
     * where <i>r</i> is the total number of bytes remaining in the specified
     * subsequence of the given buffer array, that is,
     *
     * <blockquote><pre>
     * srcs[offset].remaining()
     *     + srcs[offset+1].remaining()
     *     + ... + srcs[offset+length-1].remaining()</pre></blockquote>
     *
     * at the moment that the write is attempted.
     *
     * <p> Suppose that a byte sequence of length <i>n</i> is written, where
     * <tt>0</tt>&nbsp;<tt>&lt;</tt>&nbsp;<i>n</i>&nbsp;<tt>&lt;=</tt>&nbsp;<i>r</i>.
     * Up to the first <tt>srcs[offset].remaining()</tt> bytes of this sequence
     * are written from buffer <tt>srcs[offset]</tt>, up to the next
     * <tt>srcs[offset+1].remaining()</tt> bytes are written from buffer
     * <tt>srcs[offset+1]</tt>, and so forth, until the entire byte sequence is
     * written.  As many bytes as possible are written from each buffer, hence
     * the final position of each updated buffer, except the last updated
     * buffer, is guaranteed to be equal to that buffer's limit. The underlying
     * operating system may impose a limit on the number of buffers that may be
     * used in an I/O operation. Where the number of buffers (with bytes
     * remaining), exceeds this limit, then the I/O operation is performed with
     * the maximum number of buffers allowed by the operating system.
     *
     * <p> The return value from this method is a {@code Future} representing
     * the pending result of the operation. The {@code Future}'s {@link
     * Future#get() get} method will return the number of bytes written.
     *
     * <p> If a timeout is specified and the timeout elapses before the operation
     * completes then it completes with the exception {@link
     * InterruptedByTimeoutException}. Where a timeout occurs, and the
     * implementation cannot guarantee that bytes have not been written, or will
     * not be written to the channel from the given buffers, then further attempts
     * to write to the channel will cause an unspecific runtime exception to be
     * thrown.
     *
     * @param   srcs
     *          The buffers from which bytes are to be retrieved
     * @param   offset
     *          The offset within the buffer array of the first buffer from which
     *          bytes are to be retrieved; must be non-negative and no larger
     *          than {@code srcs.length}
     * @param   length
     *          The maximum number of buffers to be accessed; must be non-negative
     *          and no larger than {@code srcs.length - offset}
     * @param   timeout
     *          The timeout, or {@code 0L} for no timeout
     * @param   unit
     *          The time unit of the {@code timeout} argument
     * @param   attachment
     *          The object to attach to the I/O operation; can be {@code null}
     * @param   handler
     *          The handler for consuming the result; can be {@code null}
     *
     * @return  A {@code Future} object representing the pending result
     *
     * @throws  IndexOutOfBoundsException
     *          If the pre-conditions for the {@code offset}  and {@code length}
     *          parameter aren't met
     * @throws  IllegalArgumentException
     *          If the {@code timeout} parameter is negative
     * @throws  WritePendingException
     *          If a write operation is already in progress on this channel
     * @throws  NotYetConnectedException
     *          If this channel is not yet connected
     * @throws  ShutdownChannelGroupException
     *          If a handler is specified, and the channel group is shutdown
     */
    public abstract <A> Future<Long> write(ByteBuffer[] srcs,
                                           int offset,
                                           int length,
                                           long timeout,
                                           TimeUnit unit,
                                           A attachment,
                                           CompletionHandler<Long,? super A> handler);
}