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+ Copyright 2001-2005 Sun Microsystems, Inc.  All Rights Reserved.
+ 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.
+<!doctype html public "-//IETF//DTD HTML//EN">
+<body bgcolor="white">
+Defines channels, which represent connections to entities that are capable of
+performing I/O operations, such as files and sockets; defines selectors, for
+multiplexed, non-blocking I/O operations.
+<a name="channels">
+<blockquote><table cellspacing=1 cellpadding=0 summary="Lists channels and their descriptions">
+  <tr><th><p align="left">Channels</p></th><th><p align="left">Description</p></th></tr>
+  <tr><td valign=top><tt><i>{@link java.nio.channels.Channel}</i></tt></td>
+      <td>A nexus for I/O operations</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;<i>{@link java.nio.channels.ReadableByteChannel}</i></tt></td>
+      <td>Can read into a buffer</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>{@link java.nio.channels.ScatteringByteChannel}&nbsp;&nbsp;</i></tt></td>
+      <td>Can read into a sequence of&nbsp;buffers</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;<i>{@link java.nio.channels.WritableByteChannel}</i></tt></td>
+      <td>Can write from a buffer</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>{@link java.nio.channels.GatheringByteChannel}</i></tt></td>
+      <td>Can write from a sequence of&nbsp;buffers</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;<i>{@link java.nio.channels.ByteChannel}</i></tt></td>
+      <td>Can read/write to/from a&nbsp;buffer</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.Channels}</tt></td>
+      <td>Utility methods for channel/stream interoperation</td></tr>
+<p> A <i>channel</i> represents an open connection to an entity such as a
+hardware device, a file, a network socket, or a program component that is
+capable of performing one or more distinct I/O operations, for example reading
+or writing.  As specified in the {@link java.nio.channels.Channel} interface,
+channels are either open or closed, and they are both <i>asynchronously
+closeable</i> and <i>interruptible</i>.
+<p> The {@link java.nio.channels.Channel} interface is extended by several
+other interfaces, each of which specifies a new I/O operation.
+<p> The {@link java.nio.channels.ReadableByteChannel} interface specifies a
+{@link java.nio.channels.ReadableByteChannel#read read} method that reads bytes
+from the channel into a buffer; similarly, the {@link
+java.nio.channels.WritableByteChannel} interface specifies a {@link
+java.nio.channels.WritableByteChannel#write write} method that writes bytes
+from a buffer to the channel.  The {@link java.nio.channels.ByteChannel}
+interface unifies these two interfaces for the common case of channels that can
+both read and write bytes.
+<p> The {@link java.nio.channels.ScatteringByteChannel} and {@link
+java.nio.channels.GatheringByteChannel} interfaces extend the {@link
+java.nio.channels.ReadableByteChannel} and {@link
+java.nio.channels.WritableByteChannel} interfaces, respectively, adding {@link
+java.nio.channels.ScatteringByteChannel#read read} and {@link
+java.nio.channels.GatheringByteChannel#write write} methods that take a
+sequence of buffers rather than a single buffer.
+<p> The {@link java.nio.channels.Channels} utility class defines static methods
+that support the interoperation of the stream classes of the <tt>{@link
+java.io}</tt> package with the channel classes of this package.  An appropriate
+channel can be constructed from an {@link java.io.InputStream} or an {@link
+java.io.OutputStream}, and conversely an {@link java.io.InputStream} or an
+{@link java.io.OutputStream} can be constructed from a channel.  A {@link
+java.io.Reader} can be constructed that uses a given charset to decode bytes
+from a given readable byte channel, and conversely a {@link java.io.Writer} can
+be constructed that uses a given charset to encode characters into bytes and
+write them to a given writable byte channel.
+<blockquote><table cellspacing=1 cellpadding=0 summary="Lists file channels and their descriptions">
+<tr><th><p align="left">File channels</p></th><th><p align="left">Description</p></th></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.FileChannel}</tt></td>
+      <td>Reads, writes, maps, and manipulates files</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.FileLock}</tt></td>
+      <td>A lock on a (region of a) file</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.MappedByteBuffer}&nbsp;&nbsp;</tt></td>
+      <td>A direct byte buffer mapped to a region of a&nbsp;file</td></tr>
+<p> The {@link java.nio.channels.FileChannel} class supports the usual
+operations of reading bytes from, and writing bytes to, a channel connected to
+a file, as well as those of querying and modifying the current file position
+and truncating the file to a specific size.  It defines methods for acquiring
+locks on the whole file or on a specific region of a file; these methods return
+instances of the {@link java.nio.channels.FileLock} class.  Finally, it defines
+methods for forcing updates to the file to be written to the storage device that
+contains it, for efficiently transferring bytes between the file and other
+channels, and for mapping a region of the file directly into memory.  This last
+operation creates an instance of the {@link java.nio.MappedByteBuffer}
+class, which extends the {@link java.nio.ByteBuffer} class with several
+file-related operations.
+<p> A <tt>getChannel</tt> method has been added to each of the {@link
+java.io.FileInputStream#getChannel FileInputStream}, {@link
+java.io.FileOutputStream#getChannel FileOutputStream}, and {@link
+java.io.RandomAccessFile#getChannel RandomAccessFile} classes of the <tt>{@link
+java.io java.io}</tt> package.  Invoking this method upon an instance of one of
+these classes will return a file channel connected to the underlying file.
+<a name="multiplex">
+<blockquote><table cellspacing=1 cellpadding=0 summary="Lists multiplexed, non-blocking channels and their descriptions">
+  <tr><th><p align="left">Multiplexed, non-blocking I/O</p></th><th><p align="left">Description</p></th></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.SelectableChannel}</tt></td>
+      <td>A channel that can be multiplexed</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;{@link java.nio.channels.DatagramChannel}</tt></td>
+      <td>A channel for a {@link java.net.DatagramSocket java.net.DatagramSocket}</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;{@link java.nio.channels.Pipe.SinkChannel}</tt></td>
+      <td>The write end of a pipe</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;{@link java.nio.channels.Pipe.SourceChannel}</tt></td>
+      <td>The read end of a pipe</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;{@link java.nio.channels.ServerSocketChannel}&nbsp;&nbsp;</tt></td>
+      <td>A channel for a {@link java.net.ServerSocket java.net.ServerSocket}</td></tr>
+  <tr><td valign=top><tt>&nbsp;&nbsp;{@link java.nio.channels.SocketChannel}</tt></td>
+      <td>A channel for a {@link java.net.Socket java.net.Socket}</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.Selector}</tt></td>
+      <td>A multiplexor of selectable channels</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.SelectionKey}</tt></td>
+      <td>A token representing the registration <br> of a channel
+          with&nbsp;a&nbsp;selector</td></tr>
+  <tr><td valign=top><tt>{@link java.nio.channels.Pipe}</tt></td>
+      <td>Two channels that form a unidirectional&nbsp;pipe</td></tr>
+<p> Multiplexed, non-blocking I/O, which is much more scalable than
+thread-oriented, blocking I/O, is provided by <i>selectors</i>, <i>selectable
+channels</i>, and <i>selection keys</i>.
+<p> A <a href="Selector.html"><i>selector</i></a> is a multiplexor of <a
+href="SelectableChannel.html"><i>selectable channels</i></a>, which in turn are
+a special type of channel that can be put into <a
+href="SelectableChannel.html#bm"><i>non-blocking mode</i></a>.  To perform
+multiplexed I/O operations, one or more selectable channels are first created,
+put into non-blocking mode, and {@link
+java.nio.channels.SelectableChannel#register </code><i>registered</i><code>}
+with a selector.  Registering a channel specifies the set of I/O operations
+that will be tested for readiness by the selector, and returns a <a
+href="SelectionKey.html"><i>selection key</i></a> that represents the
+<p> Once some channels have been registered with a selector, a <a
+href="Selector.html#selop"><i>selection operation</i></a> can be performed in
+order to discover which channels, if any, have become ready to perform one or
+more of the operations in which interest was previously declared.  If a channel
+is ready then the key returned when it was registered will be added to the
+selector's <i>selected-key set</i>.  The key set, and the keys within it, can
+be examined in order to determine the operations for which each channel is
+ready.  From each key one can retrieve the corresponding channel in order to
+perform whatever I/O operations are required.
+<p> That a selection key indicates that its channel is ready for some operation
+is a hint, but not a guarantee, that such an operation can be performed by a
+thread without causing the thread to block.  It is imperative that code that
+performs multiplexed I/O be written so as to ignore these hints when they prove
+to be incorrect.
+<p> This package defines selectable-channel classes corresponding to the {@link
+java.net.DatagramSocket}, {@link java.net.ServerSocket}, and {@link
+java.net.Socket} classes defined in the <tt>{@link java.net}</tt> package.
+Minor changes to these classes have been made in order to support sockets that
+are associated with channels.  This package also defines a simple class that
+implements unidirectional pipes.  In all cases, a new selectable channel is
+created by invoking the static <tt>open</tt> method of the corresponding class.
+If a channel needs an associated socket then a socket will be created as a side
+effect of this operation.
+<p> The implementation of selectors, selectable channels, and selection keys
+can be replaced by "plugging in" an alternative definition or instance of the
+{@link java.nio.channels.spi.SelectorProvider} class defined in the <tt>{@link
+java.nio.channels.spi}</tt> package.  It is not expected that many developers
+will actually make use of this facility; it is provided primarily so that
+sophisticated users can take advantage of operating-system-specific
+I/O-multiplexing mechanisms when very high performance is required.
+<p> Much of the bookkeeping and synchronization required to implement the
+multiplexed-I/O abstractions is performed by the {@link
+java.nio.channels.spi.AbstractInterruptibleChannel}, {@link
+java.nio.channels.spi.AbstractSelectableChannel}, {@link
+java.nio.channels.spi.AbstractSelectionKey}, and {@link
+java.nio.channels.spi.AbstractSelector} classes in the <tt>{@link
+java.nio.channels.spi}</tt> package.  When defining a custom selector provider,
+only the {@link java.nio.channels.spi.AbstractSelector} and {@link
+java.nio.channels.spi.AbstractSelectionKey} classes should be subclassed
+directly; custom channel classes should extend the appropriate {@link
+java.nio.channels.SelectableChannel} subclasses defined in this package.
+<p> Unless otherwise noted, passing a <tt>null</tt> argument to a constructor
+or method in any class or interface in this package will cause a {@link
+java.lang.NullPointerException NullPointerException} to be thrown.
+@since 1.4
+@author Mark Reinhold
+@author JSR-51 Expert Group