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| <!doctype html public "-//IETF//DTD HTML//EN"> |
| <html> |
| <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> <i>{@link java.nio.channels.ReadableByteChannel}</i></tt></td> |
| <td>Can read into a buffer</td></tr> |
| <tr><td valign=top><tt> <i>{@link java.nio.channels.ScatteringByteChannel} </i></tt></td> |
| <td>Can read into a sequence of buffers</td></tr> |
| <tr><td valign=top><tt> <i>{@link java.nio.channels.WritableByteChannel}</i></tt></td> |
| <td>Can write from a buffer</td></tr> |
| <tr><td valign=top><tt> <i>{@link java.nio.channels.GatheringByteChannel}</i></tt></td> |
| <td>Can write from a sequence of buffers</td></tr> |
| <tr><td valign=top><tt> <i>{@link java.nio.channels.ByteChannel}</i></tt></td> |
| <td>Can read/write to/from a buffer</td></tr> |
| <tr><td valign=top><tt>{@link java.nio.channels.Channels}</tt></td> |
| <td>Utility methods for channel/stream interoperation</td></tr> |
| </table></blockquote> |
| |
| <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} </tt></td> |
| <td>A direct byte buffer mapped to a region of a file</td></tr> |
| </table></blockquote> |
| |
| <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> {@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> {@link java.nio.channels.Pipe.SinkChannel}</tt></td> |
| <td>The write end of a pipe</td></tr> |
| <tr><td valign=top><tt> {@link java.nio.channels.Pipe.SourceChannel}</tt></td> |
| <td>The read end of a pipe</td></tr> |
| <tr><td valign=top><tt> {@link java.nio.channels.ServerSocketChannel} </tt></td> |
| <td>A channel for a {@link java.net.ServerSocket java.net.ServerSocket}</td></tr> |
| <tr><td valign=top><tt> {@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 a selector</td></tr> |
| <tr><td valign=top><tt>{@link java.nio.channels.Pipe}</tt></td> |
| <td>Two channels that form a unidirectional pipe</td></tr> |
| </table></blockquote> |
| |
| <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 |
| registration. |
| |
| <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 |
| |
| </body> |
| </html> |