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25
26 /**
27 * Defines channels, which represent connections to entities that are capable of
28 * performing I/O operations, such as files and sockets; defines selectors, for
29 * multiplexed, non-blocking I/O operations.
30 *
31 * <a name="channels"></a>
32 *
33 * <blockquote><table cellspacing=1 cellpadding=0 summary="Lists channels and their descriptions">
34 * <tr><th align="left">Channels</th><th align="left">Description</th></tr>
35 * <tr><td valign=top><i>{@link java.nio.channels.Channel}</i></td>
36 * <td>A nexus for I/O operations</td></tr>
37 * <tr><td valign=top> <i>{@link java.nio.channels.ReadableByteChannel}</i></td>
38 * <td>Can read into a buffer</td></tr>
39 * <tr><td valign=top> <i>{@link java.nio.channels.ScatteringByteChannel} </i></td>
40 * <td>Can read into a sequence of buffers</td></tr>
41 * <tr><td valign=top> <i>{@link java.nio.channels.WritableByteChannel}</i></td>
42 * <td>Can write from a buffer</td></tr>
43 * <tr><td valign=top> <i>{@link java.nio.channels.GatheringByteChannel}</i></td>
44 * <td>Can write from a sequence of buffers</td></tr>
45 * <tr><td valign=top> <i>{@link java.nio.channels.ByteChannel}</i></td>
46 * <td>Can read/write to/from a buffer</td></tr>
47 * <tr><td valign=top> <i>{@link java.nio.channels.SeekableByteChannel}</i></td>
48 * <td>A {@code ByteChannel} connected to an entity that contains a variable-length sequence of bytes</td></tr>
49 * <tr><td valign=top> <i>{@link java.nio.channels.AsynchronousChannel}</i></td>
50 * <td>Supports asynchronous I/O operations.</td></tr>
51 * <tr><td valign=top> <i>{@link java.nio.channels.AsynchronousByteChannel}</i></td>
52 * <td>Can read and write bytes asynchronously</td></tr>
53 * <tr><td valign=top> <i>{@link java.nio.channels.NetworkChannel}</i></td>
54 * <td>A channel to a network socket</td></tr>
55 * <tr><td valign=top> <i>{@link java.nio.channels.MulticastChannel}</i></td>
56 * <td>Can join Internet Protocol (IP) multicast groups</td></tr>
57 * <tr><td valign=top>{@link java.nio.channels.Channels}</td>
58 * <td>Utility methods for channel/stream interoperation</td></tr>
59 * </table></blockquote>
60 *
61 * <p> A <i>channel</i> represents an open connection to an entity such as a
62 * hardware device, a file, a network socket, or a program component that is
63 * capable of performing one or more distinct I/O operations, for example reading
64 * or writing. As specified in the {@link java.nio.channels.Channel} interface,
65 * channels are either open or closed, and they are both <i>asynchronously
66 * closeable</i> and <i>interruptible</i>.
67 *
68 * <p> The {@link java.nio.channels.Channel} interface is extended by several
69 * other interfaces.
70 *
71 * <p> The {@link java.nio.channels.ReadableByteChannel} interface specifies a
72 * {@link java.nio.channels.ReadableByteChannel#read read} method that reads bytes
73 * from the channel into a buffer; similarly, the {@link
74 * java.nio.channels.WritableByteChannel} interface specifies a {@link
75 * java.nio.channels.WritableByteChannel#write write} method that writes bytes
76 * from a buffer to the channel. The {@link java.nio.channels.ByteChannel}
77 * interface unifies these two interfaces for the common case of channels that can
78 * both read and write bytes. The {@link java.nio.channels.SeekableByteChannel}
79 * interface extends the {@code ByteChannel} interface with methods to {@link
80 * java.nio.channels.SeekableByteChannel#position() query} and {@link
81 * java.nio.channels.SeekableByteChannel#position(long) modify} the channel's
82 * current position, and its {@link java.nio.channels.SeekableByteChannel#size
83 * size}.
84 *
85 * <p> The {@link java.nio.channels.ScatteringByteChannel} and {@link
86 * java.nio.channels.GatheringByteChannel} interfaces extend the {@link
87 * java.nio.channels.ReadableByteChannel} and {@link
88 * java.nio.channels.WritableByteChannel} interfaces, respectively, adding {@link
89 * java.nio.channels.ScatteringByteChannel#read read} and {@link
90 * java.nio.channels.GatheringByteChannel#write write} methods that take a
91 * sequence of buffers rather than a single buffer.
92 *
93 * <p> The {@link java.nio.channels.NetworkChannel} interface specifies methods
94 * to {@link java.nio.channels.NetworkChannel#bind bind} the channel's socket,
95 * obtain the address to which the socket is bound, and methods to {@link
96 * java.nio.channels.NetworkChannel#getOption get} and {@link
97 * java.nio.channels.NetworkChannel#setOption set} socket options. The {@link
98 * java.nio.channels.MulticastChannel} interface specifies methods to join
99 * Internet Protocol (IP) multicast groups.
100 *
101 * <p> The {@link java.nio.channels.Channels} utility class defines static methods
102 * that support the interoperation of the stream classes of the {@link
103 * java.io} package with the channel classes of this package. An appropriate
104 * channel can be constructed from an {@link java.io.InputStream} or an {@link
105 * java.io.OutputStream}, and conversely an {@link java.io.InputStream} or an
106 * {@link java.io.OutputStream} can be constructed from a channel. A {@link
107 * java.io.Reader} can be constructed that uses a given charset to decode bytes
108 * from a given readable byte channel, and conversely a {@link java.io.Writer} can
109 * be constructed that uses a given charset to encode characters into bytes and
110 * write them to a given writable byte channel.
111 *
112 * <blockquote><table cellspacing=1 cellpadding=0 summary="Lists file channels and their descriptions">
113 * <tr><th align="left">File channels</th><th align="left">Description</th></tr>
114 * <tr><td valign=top>{@link java.nio.channels.FileChannel}</td>
115 * <td>Reads, writes, maps, and manipulates files</td></tr>
116 * <tr><td valign=top>{@link java.nio.channels.FileLock}</td>
117 * <td>A lock on a (region of a) file</td></tr>
118 * <tr><td valign=top>{@link java.nio.MappedByteBuffer} </td>
119 * <td>A direct byte buffer mapped to a region of a file</td></tr>
120 * </table></blockquote>
121 *
122 * <p> The {@link java.nio.channels.FileChannel} class supports the usual
123 * operations of reading bytes from, and writing bytes to, a channel connected to
124 * a file, as well as those of querying and modifying the current file position
125 * and truncating the file to a specific size. It defines methods for acquiring
126 * locks on the whole file or on a specific region of a file; these methods return
127 * instances of the {@link java.nio.channels.FileLock} class. Finally, it defines
128 * methods for forcing updates to the file to be written to the storage device that
129 * contains it, for efficiently transferring bytes between the file and other
130 * channels, and for mapping a region of the file directly into memory.
131 *
132 * <p> A {@code FileChannel} is created by invoking one of its static {@link
133 * java.nio.channels.FileChannel#open open} methods, or by invoking the {@code
134 * getChannel} method of a {@link java.io.FileInputStream}, {@link
135 * java.io.FileOutputStream}, or {@link java.io.RandomAccessFile} to return a
136 * file channel connected to the same underlying file as the {@link java.io}
137 * class.
138 *
139 * <a name="multiplex"></a>
140 * <blockquote><table cellspacing=1 cellpadding=0 summary="Lists multiplexed, non-blocking channels and their descriptions">
141 * <tr><th align="left">Multiplexed, non-blocking I/O</th><th align="left"><p>Description</th></tr>
142 * <tr><td valign=top>{@link java.nio.channels.SelectableChannel}</td>
143 * <td>A channel that can be multiplexed</td></tr>
144 * <tr><td valign=top> {@link java.nio.channels.DatagramChannel}</td>
145 * <td>A channel to a datagram-oriented socket</td></tr>
146 * <tr><td valign=top> {@link java.nio.channels.Pipe.SinkChannel}</td>
147 * <td>The write end of a pipe</td></tr>
148 * <tr><td valign=top> {@link java.nio.channels.Pipe.SourceChannel}</td>
149 * <td>The read end of a pipe</td></tr>
150 * <tr><td valign=top> {@link java.nio.channels.ServerSocketChannel} </td>
151 * <td>A channel to a stream-oriented listening socket</td></tr>
152 * <tr><td valign=top> {@link java.nio.channels.SocketChannel}</td>
153 * <td>A channel for a stream-oriented connecting socket</td></tr>
154 * <tr><td valign=top>{@link java.nio.channels.Selector}</td>
155 * <td>A multiplexor of selectable channels</td></tr>
156 * <tr><td valign=top>{@link java.nio.channels.SelectionKey}</td>
157 * <td>A token representing the registration <br> of a channel
158 * with a selector</td></tr>
159 * <tr><td valign=top>{@link java.nio.channels.Pipe}</td>
160 * <td>Two channels that form a unidirectional pipe</td></tr>
161 * </table></blockquote>
162 *
163 * <p> Multiplexed, non-blocking I/O, which is much more scalable than
164 * thread-oriented, blocking I/O, is provided by <i>selectors</i>, <i>selectable
165 * channels</i>, and <i>selection keys</i>.
166 *
167 * <p> A <a href="Selector.html"><i>selector</i></a> is a multiplexor of <a
168 * href="SelectableChannel.html"><i>selectable channels</i></a>, which in turn are
169 * a special type of channel that can be put into <a
170 * href="SelectableChannel.html#bm"><i>non-blocking mode</i></a>. To perform
171 * multiplexed I/O operations, one or more selectable channels are first created,
172 * put into non-blocking mode, and {@link
173 * java.nio.channels.SelectableChannel#register <i>registered</i>}
174 * with a selector. Registering a channel specifies the set of I/O operations
175 * that will be tested for readiness by the selector, and returns a <a
176 * href="SelectionKey.html"><i>selection key</i></a> that represents the
177 * registration.
178 *
179 * <p> Once some channels have been registered with a selector, a <a
180 * href="Selector.html#selop"><i>selection operation</i></a> can be performed in
181 * order to discover which channels, if any, have become ready to perform one or
182 * more of the operations in which interest was previously declared. If a channel
183 * is ready then the key returned when it was registered will be added to the
184 * selector's <i>selected-key set</i>. The key set, and the keys within it, can
185 * be examined in order to determine the operations for which each channel is
186 * ready. From each key one can retrieve the corresponding channel in order to
187 * perform whatever I/O operations are required.
188 *
189 * <p> That a selection key indicates that its channel is ready for some operation
190 * is a hint, but not a guarantee, that such an operation can be performed by a
191 * thread without causing the thread to block. It is imperative that code that
192 * performs multiplexed I/O be written so as to ignore these hints when they prove
193 * to be incorrect.
194 *
195 * <p> This package defines selectable-channel classes corresponding to the {@link
196 * java.net.DatagramSocket}, {@link java.net.ServerSocket}, and {@link
197 * java.net.Socket} classes defined in the {@link java.net} package.
198 * Minor changes to these classes have been made in order to support sockets that
199 * are associated with channels. This package also defines a simple class that
200 * implements unidirectional pipes. In all cases, a new selectable channel is
201 * created by invoking the static {@code open} method of the corresponding class.
202 * If a channel needs an associated socket then a socket will be created as a side
203 * effect of this operation.
204 *
205 * <p> The implementation of selectors, selectable channels, and selection keys
206 * can be replaced by "plugging in" an alternative definition or instance of the
207 * {@link java.nio.channels.spi.SelectorProvider} class defined in the {@link
208 * java.nio.channels.spi} package. It is not expected that many developers
209 * will actually make use of this facility; it is provided primarily so that
210 * sophisticated users can take advantage of operating-system-specific
211 * I/O-multiplexing mechanisms when very high performance is required.
212 *
213 * <p> Much of the bookkeeping and synchronization required to implement the
214 * multiplexed-I/O abstractions is performed by the {@link
215 * java.nio.channels.spi.AbstractInterruptibleChannel}, {@link
216 * java.nio.channels.spi.AbstractSelectableChannel}, {@link
217 * java.nio.channels.spi.AbstractSelectionKey}, and {@link
218 * java.nio.channels.spi.AbstractSelector} classes in the {@link
219 * java.nio.channels.spi} package. When defining a custom selector provider,
220 * only the {@link java.nio.channels.spi.AbstractSelector} and {@link
221 * java.nio.channels.spi.AbstractSelectionKey} classes should be subclassed
222 * directly; custom channel classes should extend the appropriate {@link
223 * java.nio.channels.SelectableChannel} subclasses defined in this package.
224 *
225 * <a name="async"></a>
226 *
227 * <blockquote><table cellspacing=1 cellpadding=0 summary="Lists asynchronous channels and their descriptions">
228 * <tr><th align="left">Asynchronous I/O</th><th align="left">Description</th></tr>
229 * <tr><td valign=top>{@link java.nio.channels.AsynchronousFileChannel}</td>
230 * <td>An asynchronous channel for reading, writing, and manipulating a file</td></tr>
231 * <tr><td valign=top>{@link java.nio.channels.AsynchronousSocketChannel}</td>
232 * <td>An asynchronous channel to a stream-oriented connecting socket</td></tr>
233 * <tr><td valign=top>{@link java.nio.channels.AsynchronousServerSocketChannel} </td>
234 * <td>An asynchronous channel to a stream-oriented listening socket</td></tr>
235 * <tr><td valign=top>{@link java.nio.channels.CompletionHandler}</td>
236 * <td>A handler for consuming the result of an asynchronous operation</td></tr>
237 * <tr><td valign=top>{@link java.nio.channels.AsynchronousChannelGroup}</td>
238 * <td>A grouping of asynchronous channels for the purpose of resource sharing</td></tr>
239 * </table></blockquote>
240 *
241 * <p> {@link java.nio.channels.AsynchronousChannel Asynchronous channels} are a
242 * special type of channel capable of asynchronous I/O operations. Asynchronous
243 * channels are non-blocking and define methods to initiate asynchronous
244 * operations, returning a {@link java.util.concurrent.Future} representing the
245 * pending result of each operation. The {@code Future} can be used to poll or
246 * wait for the result of the operation. Asynchronous I/O operations can also
247 * specify a {@link java.nio.channels.CompletionHandler} to invoke when the
248 * operation completes. A completion handler is user provided code that is executed
249 * to consume the result of I/O operation.
250 *
251 * <p> This package defines asynchronous-channel classes that are connected to
252 * a stream-oriented connecting or listening socket, or a datagram-oriented socket.
253 * It also defines the {@link java.nio.channels.AsynchronousFileChannel} class
254 * for asynchronous reading, writing, and manipulating a file. As with the {@link
255 * java.nio.channels.FileChannel} it supports operations to truncate the file
256 * to a specific size, force updates to the file to be written to the storage
257 * device, or acquire locks on the whole file or on a specific region of the file.
258 * Unlike the {@code FileChannel} it does not define methods for mapping a
259 * region of the file directly into memory. Where memory mapped I/O is required,
260 * then a {@code FileChannel} can be used.
261 *
262 * <p> Asynchronous channels are bound to an asynchronous channel group for the
263 * purpose of resource sharing. A group has an associated {@link
264 * java.util.concurrent.ExecutorService} to which tasks are submitted to handle
265 * I/O events and dispatch to completion handlers that consume the result of
266 * asynchronous operations performed on channels in the group. The group can
267 * optionally be specified when creating the channel or the channel can be bound
268 * to a <em>default group</em>. Sophisticated users may wish to create their
269 * own asynchronous channel groups or configure the {@code ExecutorService}
270 * that will be used for the default group.
271 *
272 * <p> As with selectors, the implementation of asynchronous channels can be
273 * replaced by "plugging in" an alternative definition or instance of the {@link
274 * java.nio.channels.spi.AsynchronousChannelProvider} class defined in the
275 * {@link java.nio.channels.spi} package. It is not expected that many
276 * developers will actually make use of this facility; it is provided primarily
277 * so that sophisticated users can take advantage of operating-system-specific
278 * asynchronous I/O mechanisms when very high performance is required.
279 *
280 * <hr width="80%">
281 * <p> Unless otherwise noted, passing a {@code null} argument to a constructor
282 * or method in any class or interface in this package will cause a {@link
283 * java.lang.NullPointerException NullPointerException} to be thrown.
284 *
285 * @since 1.4
286 * @author Mark Reinhold
287 * @author JSR-51 Expert Group
288 */
289
290 package java.nio.channels;