package java.nio.channels

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.

Lists channels and their descriptions
Channels Description
java.nio.channels.Channel A nexus for I/O operations
  java.nio.channels.ReadableByteChannel Can read into a buffer
    java.nio.channels.ScatteringByteChannel   Can read into a sequence of buffers
  java.nio.channels.WritableByteChannel Can write from a buffer
    java.nio.channels.GatheringByteChannel Can write from a sequence of buffers
  java.nio.channels.ByteChannel Can read/write to/from a buffer
    java.nio.channels.SeekableByteChannel A ByteChannel connected to an entity that contains a variable-length sequence of bytes
  java.nio.channels.AsynchronousChannel Supports asynchronous I/O operations.
    java.nio.channels.AsynchronousByteChannel Can read and write bytes asynchronously
  java.nio.channels.NetworkChannel A channel to a network socket
    java.nio.channels.MulticastChannel Can join Internet Protocol (IP) multicast groups
java.nio.channels.Channels Utility methods for channel/stream interoperation

A channel 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 java.nio.channels.Channel interface, channels are either open or closed, and they are both asynchronously closeable and interruptible.

The java.nio.channels.Channel interface is extended by several other interfaces.

The java.nio.channels.ReadableByteChannel interface specifies a read method that reads bytes from the channel into a buffer; similarly, the java.nio.channels.WritableByteChannel interface specifies a write method that writes bytes from a buffer to the channel. The java.nio.channels.ByteChannel interface unifies these two interfaces for the common case of channels that can both read and write bytes. The java.nio.channels.SeekableByteChannel interface extends the ByteChannel interface with methods to query and modify the channel's current position, and its size.

The java.nio.channels.ScatteringByteChannel and java.nio.channels.GatheringByteChannel interfaces extend the java.nio.channels.ReadableByteChannel and java.nio.channels.WritableByteChannel interfaces, respectively, adding read and write methods that take a sequence of buffers rather than a single buffer.

The java.nio.channels.NetworkChannel interface specifies methods to bind the channel's socket, obtain the address to which the socket is bound, and methods to get and set socket options. The java.nio.channels.MulticastChannel interface specifies methods to join Internet Protocol (IP) multicast groups.

The java.nio.channels.Channels utility class defines static methods that support the interoperation of the stream classes of the java.io package with the channel classes of this package. An appropriate channel can be constructed from an java.io.InputStream or an java.io.OutputStream, and conversely an java.io.InputStream or an java.io.OutputStream can be constructed from a channel. A java.io.Reader can be constructed that uses a given charset to decode bytes from a given readable byte channel, and conversely a 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.

Lists file channels and their descriptions
File channels Description
java.nio.channels.FileChannel Reads, writes, maps, and manipulates files
java.nio.channels.FileLock A lock on a (region of a) file
java.nio.MappedByteBuffer   A direct byte buffer mapped to a region of a file

The 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 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.

A FileChannel is created by invoking one of its static open methods, or by invoking the getChannel method of a java.io.FileInputStream, java.io.FileOutputStream, or java.io.RandomAccessFile to return a file channel connected to the same underlying file as the java.io class.

Lists multiplexed, non-blocking channels and their descriptions
Multiplexed, non-blocking I/O Description
java.nio.channels.SelectableChannel A channel that can be multiplexed
  java.nio.channels.DatagramChannel A channel to a datagram-oriented socket
  java.nio.channels.Pipe.SinkChannel The write end of a pipe
  java.nio.channels.Pipe.SourceChannel The read end of a pipe
  java.nio.channels.ServerSocketChannel   A channel to a stream-oriented listening socket
  java.nio.channels.SocketChannel A channel for a stream-oriented connecting socket
java.nio.channels.Selector A multiplexor of selectable channels
java.nio.channels.SelectionKey A token representing the registration
of a channel with a selector
java.nio.channels.Pipe Two channels that form a unidirectional pipe

Multiplexed, non-blocking I/O, which is much more scalable than thread-oriented, blocking I/O, is provided by selectors, selectable channels, and selection keys.

A selector is a multiplexor of selectable channels, which in turn are a special type of channel that can be put into non-blocking mode. To perform multiplexed I/O operations, one or more selectable channels are first created, put into non-blocking mode, and registered 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 selection key that represents the registration.

Once some channels have been registered with a selector, a selection operation 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 selected-key set. 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.

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.

This package defines selectable-channel classes corresponding to the java.net.DatagramSocket, java.net.ServerSocket, and java.net.Socket classes defined in the java.net 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 open 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.

The implementation of selectors, selectable channels, and selection keys can be replaced by "plugging in" an alternative definition or instance of the java.nio.channels.spi.SelectorProvider class defined in the java.nio.channels.spi 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.

Much of the bookkeeping and synchronization required to implement the multiplexed-I/O abstractions is performed by the java.nio.channels.spi.AbstractInterruptibleChannel, java.nio.channels.spi.AbstractSelectableChannel, java.nio.channels.spi.AbstractSelectionKey, and java.nio.channels.spi.AbstractSelector classes in the java.nio.channels.spi package. When defining a custom selector provider, only the java.nio.channels.spi.AbstractSelector and java.nio.channels.spi.AbstractSelectionKey classes should be subclassed directly; custom channel classes should extend the appropriate java.nio.channels.SelectableChannel subclasses defined in this package.

Lists asynchronous channels and their descriptions
Asynchronous I/ODescription
java.nio.channels.AsynchronousFileChannel An asynchronous channel for reading, writing, and manipulating a file
java.nio.channels.AsynchronousSocketChannel An asynchronous channel to a stream-oriented connecting socket
java.nio.channels.AsynchronousServerSocketChannel   An asynchronous channel to a stream-oriented listening socket
java.nio.channels.CompletionHandler A handler for consuming the result of an asynchronous operation
java.nio.channels.AsynchronousChannelGroup A grouping of asynchronous channels for the purpose of resource sharing

Asynchronous channels are a special type of channel capable of asynchronous I/O operations. Asynchronous channels are non-blocking and define methods to initiate asynchronous operations, returning a java.util.concurrent.Future representing the pending result of each operation. The Future can be used to poll or wait for the result of the operation. Asynchronous I/O operations can also specify a java.nio.channels.CompletionHandler to invoke when the operation completes. A completion handler is user provided code that is executed to consume the result of I/O operation.

This package defines asynchronous-channel classes that are connected to a stream-oriented connecting or listening socket, or a datagram-oriented socket. It also defines the java.nio.channels.AsynchronousFileChannel class for asynchronous reading, writing, and manipulating a file. As with the java.nio.channels.FileChannel it supports operations to truncate the file to a specific size, force updates to the file to be written to the storage device, or acquire locks on the whole file or on a specific region of the file. Unlike the FileChannel it does not define methods for mapping a region of the file directly into memory. Where memory mapped I/O is required, then a FileChannel can be used.

Asynchronous channels are bound to an asynchronous channel group for the purpose of resource sharing. A group has an associated java.util.concurrent.ExecutorService to which tasks are submitted to handle I/O events and dispatch to completion handlers that consume the result of asynchronous operations performed on channels in the group. The group can optionally be specified when creating the channel or the channel can be bound to a default group. Sophisticated users may wish to create their own asynchronous channel groups or configure the ExecutorService that will be used for the default group.

As with selectors, the implementation of asynchronous channels can be replaced by "plugging in" an alternative definition or instance of the java.nio.channels.spi.AsynchronousChannelProvider class defined in the java.nio.channels.spi 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 asynchronous I/O mechanisms when very high performance is required.

Unless otherwise noted, passing a null argument to a constructor or method in any class or interface in this package will cause a NullPointerException to be thrown.

Since:  1.4