341 lines
18 KiB
Java
341 lines
18 KiB
Java
/*
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* Copyright (c) 2001, 2017, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/**
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* Defines channels, which represent connections to entities that are capable of
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* performing I/O operations, such as files and sockets; defines selectors, for
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* multiplexed, non-blocking I/O operations.
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*
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* <a id="channels"></a>
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*
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* <table class="striped" style="text-align:left; margin-left:2em">
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* <caption style="display:none">Lists channels and their descriptions</caption>
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* <thead>
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* <tr><th scope="col">Channels</th>
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* <th scope="col">Description</th></tr>
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* </thead>
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* <tbody>
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* <tr><th scope="row"><i>{@link java.nio.channels.Channel}</i></th>
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* <td>A nexus for I/O operations</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:1em"><i>{@link java.nio.channels.ReadableByteChannel}</i></span></th>
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* <td>Can read into a buffer</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em"><i>{@link java.nio.channels.ScatteringByteChannel}</i></span></th>
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* <td>Can read into a sequence of buffers</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:1em"><i>{@link java.nio.channels.WritableByteChannel}</i></span></th>
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* <td>Can write from a buffer</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em"><i>{@link java.nio.channels.GatheringByteChannel}</i></span></th>
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* <td>Can write from a sequence of buffers</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:1em"><i>{@link java.nio.channels.ByteChannel}</i></span></th>
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* <td>Can read/write to/from a buffer</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em"><i>{@link java.nio.channels.SeekableByteChannel}</i></span></th>
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* <td>A {@code ByteChannel} connected to an entity that contains a variable-length
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* sequence of bytes</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:1em"><i>{@link java.nio.channels.AsynchronousChannel}</i></span></th>
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* <td>Supports asynchronous I/O operations.</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em"><i>{@link java.nio.channels.AsynchronousByteChannel}</i></span></th>
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* <td>Can read and write bytes asynchronously</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:1em"><i>{@link java.nio.channels.NetworkChannel}</i></span></th>
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* <td>A channel to a network socket</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em"><i>{@link java.nio.channels.MulticastChannel}</i></span></th>
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* <td>Can join Internet Protocol (IP) multicast groups</td></tr>
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* <tr><th scope="row">{@link java.nio.channels.Channels}</th>
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* <td>Utility methods for channel/stream interoperation</td></tr>
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* </tbody>
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* </table>
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*
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* <p> A <i>channel</i> represents an open connection to an entity such as a
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* hardware device, a file, a network socket, or a program component that is
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* capable of performing one or more distinct I/O operations, for example reading
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* or writing. As specified in the {@link java.nio.channels.Channel} interface,
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* channels are either open or closed, and they are both <i>asynchronously
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* closeable</i> and <i>interruptible</i>.
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*
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* <p> The {@link java.nio.channels.Channel} interface is extended by several
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* other interfaces.
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*
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* <p> The {@link java.nio.channels.ReadableByteChannel} interface specifies a
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* {@link java.nio.channels.ReadableByteChannel#read read} method that reads bytes
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* from the channel into a buffer; similarly, the {@link
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* java.nio.channels.WritableByteChannel} interface specifies a {@link
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* java.nio.channels.WritableByteChannel#write write} method that writes bytes
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* from a buffer to the channel. The {@link java.nio.channels.ByteChannel}
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* interface unifies these two interfaces for the common case of channels that can
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* both read and write bytes. The {@link java.nio.channels.SeekableByteChannel}
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* interface extends the {@code ByteChannel} interface with methods to {@link
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* java.nio.channels.SeekableByteChannel#position() query} and {@link
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* java.nio.channels.SeekableByteChannel#position(long) modify} the channel's
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* current position, and its {@link java.nio.channels.SeekableByteChannel#size
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* size}.
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*
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* <p> The {@link java.nio.channels.ScatteringByteChannel} and {@link
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* java.nio.channels.GatheringByteChannel} interfaces extend the {@link
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* java.nio.channels.ReadableByteChannel} and {@link
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* java.nio.channels.WritableByteChannel} interfaces, respectively, adding {@link
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* java.nio.channels.ScatteringByteChannel#read read} and {@link
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* java.nio.channels.GatheringByteChannel#write write} methods that take a
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* sequence of buffers rather than a single buffer.
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*
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* <p> The {@link java.nio.channels.NetworkChannel} interface specifies methods
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* to {@link java.nio.channels.NetworkChannel#bind bind} the channel's socket,
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* obtain the address to which the socket is bound, and methods to {@link
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* java.nio.channels.NetworkChannel#getOption get} and {@link
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* java.nio.channels.NetworkChannel#setOption set} socket options. The {@link
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* java.nio.channels.MulticastChannel} interface specifies methods to join
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* Internet Protocol (IP) multicast groups.
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*
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* <p> The {@link java.nio.channels.Channels} utility class defines static methods
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* that support the interoperation of the stream classes of the {@link
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* java.io} package with the channel classes of this package. An appropriate
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* channel can be constructed from an {@link java.io.InputStream} or an {@link
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* java.io.OutputStream}, and conversely an {@link java.io.InputStream} or an
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* {@link java.io.OutputStream} can be constructed from a channel. A {@link
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* java.io.Reader} can be constructed that uses a given charset to decode bytes
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* from a given readable byte channel, and conversely a {@link java.io.Writer} can
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* be constructed that uses a given charset to encode characters into bytes and
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* write them to a given writable byte channel.
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*
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* <table class="striped" style="margin-left:2em; text-align:left">
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* <caption style="display:none">
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* Lists file channels and their descriptions</caption>
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* <thead>
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* <tr><th scope="col">File channels</th>
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* <th scope="col">Description</th></tr>
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* </thead>
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* <tbody>
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* <tr><th scope="row">
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* {@link java.nio.channels.FileChannel}</th>
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* <td>Reads, writes, maps, and manipulates files</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.channels.FileLock}</th>
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* <td>A lock on a (region of a) file</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.MappedByteBuffer}</th>
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* <td>A direct byte buffer mapped to a region of a file</td></tr>
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* </tbody>
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* </table>
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*
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* <p> The {@link java.nio.channels.FileChannel} class supports the usual
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* operations of reading bytes from, and writing bytes to, a channel connected to
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* a file, as well as those of querying and modifying the current file position
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* and truncating the file to a specific size. It defines methods for acquiring
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* locks on the whole file or on a specific region of a file; these methods return
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* instances of the {@link java.nio.channels.FileLock} class. Finally, it defines
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* methods for forcing updates to the file to be written to the storage device that
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* contains it, for efficiently transferring bytes between the file and other
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* channels, and for mapping a region of the file directly into memory.
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*
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* <p> A {@code FileChannel} is created by invoking one of its static {@link
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* java.nio.channels.FileChannel#open open} methods, or by invoking the {@code
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* getChannel} method of a {@link java.io.FileInputStream}, {@link
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* java.io.FileOutputStream}, or {@link java.io.RandomAccessFile} to return a
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* file channel connected to the same underlying file as the {@link java.io}
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* class.
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*
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* <a id="multiplex"></a>
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* <table class="striped" style="margin-left:2em; text-align:left">
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* <caption style="display:none">
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* Lists multiplexed, non-blocking channels and their descriptions</caption>
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* <thead>
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* <tr><th scope="col">Multiplexed, non-blocking I/O</th>
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* <th scope="col">Description</th></tr>
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* </thead>
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* <tbody>
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* <tr><th scope="row">{@link java.nio.channels.SelectableChannel}</th>
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* <td>A channel that can be multiplexed</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em">{@link java.nio.channels.DatagramChannel}</span></th>
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* <td>A channel to a datagram-oriented socket</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em">{@link java.nio.channels.Pipe.SinkChannel}</span></th>
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* <td>The write end of a pipe</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em">{@link java.nio.channels.Pipe.SourceChannel}</span></th>
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* <td>The read end of a pipe</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em">{@link java.nio.channels.ServerSocketChannel}</span></th>
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* <td>A channel to a stream-oriented listening socket</td></tr>
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* <tr><th scope="row">
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* <span style="padding-left:2em">{@link java.nio.channels.SocketChannel}</span></th>
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* <td>A channel for a stream-oriented connecting socket</td></tr>
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* <tr><th scope="row">{@link java.nio.channels.Selector}</th>
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* <td>A multiplexor of selectable channels</td></tr>
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* <tr><th scope="row">{@link java.nio.channels.SelectionKey}</th>
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* <td>A token representing the registration of a channel
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* with a selector</td></tr>
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* <tr><th scope="row">{@link java.nio.channels.Pipe}</th>
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* <td>Two channels that form a unidirectional pipe</td></tr>
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* </tbody>
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* </table>
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*
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* <p> Multiplexed, non-blocking I/O, which is much more scalable than
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* thread-oriented, blocking I/O, is provided by <i>selectors</i>, <i>selectable
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* channels</i>, and <i>selection keys</i>.
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*
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* <p> A <a href="Selector.html"><i>selector</i></a> is a multiplexor of <a
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* href="SelectableChannel.html"><i>selectable channels</i></a>, which in turn are
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* a special type of channel that can be put into <a
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* href="SelectableChannel.html#bm"><i>non-blocking mode</i></a>. To perform
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* multiplexed I/O operations, one or more selectable channels are first created,
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* put into non-blocking mode, and {@link
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* java.nio.channels.SelectableChannel#register <i>registered</i>}
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* with a selector. Registering a channel specifies the set of I/O operations
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* that will be tested for readiness by the selector, and returns a <a
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* href="SelectionKey.html"><i>selection key</i></a> that represents the
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* registration.
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*
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* <p> Once some channels have been registered with a selector, a <a
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* href="Selector.html#selop"><i>selection operation</i></a> can be performed in
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* order to discover which channels, if any, have become ready to perform one or
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* more of the operations in which interest was previously declared. If a channel
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* is ready then the key returned when it was registered will be added to the
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* selector's <i>selected-key set</i>. The key set, and the keys within it, can
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* be examined in order to determine the operations for which each channel is
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* ready. From each key one can retrieve the corresponding channel in order to
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* perform whatever I/O operations are required.
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*
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* <p> That a selection key indicates that its channel is ready for some operation
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* is a hint, but not a guarantee, that such an operation can be performed by a
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* thread without causing the thread to block. It is imperative that code that
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* performs multiplexed I/O be written so as to ignore these hints when they prove
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* to be incorrect.
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*
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* <p> This package defines selectable-channel classes corresponding to the {@link
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* java.net.DatagramSocket}, {@link java.net.ServerSocket}, and {@link
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* java.net.Socket} classes defined in the {@link java.net} package.
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* Minor changes to these classes have been made in order to support sockets that
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* are associated with channels. This package also defines a simple class that
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* implements unidirectional pipes. In all cases, a new selectable channel is
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* created by invoking the static {@code open} method of the corresponding class.
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* If a channel needs an associated socket then a socket will be created as a side
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* effect of this operation.
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*
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* <p> The implementation of selectors, selectable channels, and selection keys
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* can be replaced by "plugging in" an alternative definition or instance of the
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* {@link java.nio.channels.spi.SelectorProvider} class defined in the {@link
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* java.nio.channels.spi} package. It is not expected that many developers
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* will actually make use of this facility; it is provided primarily so that
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* sophisticated users can take advantage of operating-system-specific
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* I/O-multiplexing mechanisms when very high performance is required.
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*
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* <p> Much of the bookkeeping and synchronization required to implement the
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* multiplexed-I/O abstractions is performed by the {@link
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* java.nio.channels.spi.AbstractInterruptibleChannel}, {@link
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* java.nio.channels.spi.AbstractSelectableChannel}, {@link
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* java.nio.channels.spi.AbstractSelectionKey}, and {@link
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* java.nio.channels.spi.AbstractSelector} classes in the {@link
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* java.nio.channels.spi} package. When defining a custom selector provider,
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* only the {@link java.nio.channels.spi.AbstractSelector} and {@link
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* java.nio.channels.spi.AbstractSelectionKey} classes should be subclassed
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* directly; custom channel classes should extend the appropriate {@link
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* java.nio.channels.SelectableChannel} subclasses defined in this package.
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*
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* <a id="async"></a>
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*
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* <table class="striped" style="padding-left:2em; text-align:left">
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* <caption style="display:none">
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* Lists asynchronous channels and their descriptions</caption>
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* <thead>
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* <tr><th scope="col">Asynchronous I/O</th>
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* <th scope="col">Description</th></tr>
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* </thead>
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* <tbody>
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* <tr><th scope="row">
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* {@link java.nio.channels.AsynchronousFileChannel}</th>
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* <td>An asynchronous channel for reading, writing, and manipulating a file</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.channels.AsynchronousSocketChannel}</th>
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* <td>An asynchronous channel to a stream-oriented connecting socket</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.channels.AsynchronousServerSocketChannel}</th>
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* <td>An asynchronous channel to a stream-oriented listening socket</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.channels.CompletionHandler}</th>
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* <td>A handler for consuming the result of an asynchronous operation</td></tr>
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* <tr><th scope="row">
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* {@link java.nio.channels.AsynchronousChannelGroup}</th>
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* <td>A grouping of asynchronous channels for the purpose of resource sharing</td></tr>
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* </tbody>
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* </table>
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*
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* <p> {@link java.nio.channels.AsynchronousChannel Asynchronous channels} are a
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* special type of channel capable of asynchronous I/O operations. Asynchronous
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* channels are non-blocking and define methods to initiate asynchronous
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* operations, returning a {@link java.util.concurrent.Future} representing the
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* pending result of each operation. The {@code Future} can be used to poll or
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* wait for the result of the operation. Asynchronous I/O operations can also
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* specify a {@link java.nio.channels.CompletionHandler} to invoke when the
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* operation completes. A completion handler is user provided code that is executed
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* to consume the result of I/O operation.
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*
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* <p> This package defines asynchronous-channel classes that are connected to
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* a stream-oriented connecting or listening socket, or a datagram-oriented socket.
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* It also defines the {@link java.nio.channels.AsynchronousFileChannel} class
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* for asynchronous reading, writing, and manipulating a file. As with the {@link
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* java.nio.channels.FileChannel} it supports operations to truncate the file
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* to a specific size, force updates to the file to be written to the storage
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* device, or acquire locks on the whole file or on a specific region of the file.
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* Unlike the {@code FileChannel} it does not define methods for mapping a
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* region of the file directly into memory. Where memory mapped I/O is required,
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* then a {@code FileChannel} can be used.
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*
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* <p> Asynchronous channels are bound to an asynchronous channel group for the
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* purpose of resource sharing. A group has an associated {@link
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* java.util.concurrent.ExecutorService} to which tasks are submitted to handle
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* I/O events and dispatch to completion handlers that consume the result of
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* asynchronous operations performed on channels in the group. The group can
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* optionally be specified when creating the channel or the channel can be bound
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* to a <em>default group</em>. Sophisticated users may wish to create their
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* own asynchronous channel groups or configure the {@code ExecutorService}
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* that will be used for the default group.
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*
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* <p> As with selectors, the implementation of asynchronous channels can be
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* replaced by "plugging in" an alternative definition or instance of the {@link
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* java.nio.channels.spi.AsynchronousChannelProvider} class defined in the
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* {@link java.nio.channels.spi} package. It is not expected that many
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* developers will actually make use of this facility; it is provided primarily
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* so that sophisticated users can take advantage of operating-system-specific
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* asynchronous I/O mechanisms when very high performance is required.
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*
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* <p> Unless otherwise noted, passing a {@code null} argument to a constructor
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* or method in any class or interface in this package will cause a {@link
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* java.lang.NullPointerException NullPointerException} to be thrown.
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*
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* @since 1.4
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* @author Mark Reinhold
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* @author JSR-51 Expert Group
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*/
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package java.nio.channels;
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