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.

  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.

Channels	Description
Channel	A nexus for I/O operations
ReadableByteChannel	Can read into a buffer
ScatteringByteChannel	Can read into a sequence of buffers
WritableByteChannel	Can write from a buffer
GatheringByteChannel	Can write from a sequence of buffers
ByteChannel	Can read/write to/from a buffer
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 Channel interface, channels are either open or closed, and they are both asynchronously closeable and interruptible.

The Channel interface is extended by several other interfaces, each of which specifies a new I/O operation.

The ReadableByteChannel interface specifies a read method that reads bytes from the channel into a buffer; similarly, the WritableByteChannel interface specifies a write method that writes bytes from a buffer to the channel. The ByteChannel interface unifies these two interfaces for the common case of channels that can both read and write bytes.

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

The 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 InputStream or an OutputStream, and conversely an InputStream or an OutputStream can be constructed from a channel. A Reader can be constructed that uses a given charset to decode bytes from a given readable byte channel, and conversely a Writer can be constructed that uses a given charset to encode characters into bytes and write them to a given writable byte channel.

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

The 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 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 MappedByteBuffer class, which extends the ByteBuffer class with several file-related operations.

A getChannel method has been added to each of the FileInputStream, FileOutputStream, and RandomAccessFile classes of the java.io package. Invoking this method upon an instance of one of these classes will return a file channel connected to the underlying file.

Multiplexed, non-blocking I/O	Description
SelectableChannel	A channel that can be multiplexed
DatagramChannel	A channel for a java.net.DatagramSocket
Pipe.SinkChannel	The write end of a pipe
Pipe.SourceChannel	The read end of a pipe
ServerSocketChannel	A channel for a java.net.ServerSocket
SocketChannel	A channel for a java.net.Socket
Selector	A multiplexor of selectable channels
SelectionKey	A token representing the registration of a channel with a selector
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 DatagramSocket, ServerSocket, and 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 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 AbstractInterruptibleChannel, AbstractSelectableChannel, AbstractSelectionKey, and AbstractSelector classes in the java.nio.channels.spi package. When defining a custom selector provider, only the AbstractSelector and AbstractSelectionKey classes should be subclassed directly; custom channel classes should extend the appropriate SelectableChannel subclasses defined in this package.

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.