java.nio.channels

Class SelectionKey

java.lang.Object

java.nio.channels.SelectionKey

Direct Known Subclasses:

AbstractSelectionKey

public abstract class SelectionKey
extends Object

A token representing the registration of a SelectableChannel with a Selector.

A selection key is created each time a channel is registered with a selector. A key remains valid until it is cancelled by invoking its cancel method, by closing its channel, or by closing its selector. Cancelling a key does not immediately remove it from its selector; it is instead added to the selector's cancelled-key set for removal during the next selection operation. The validity of a key may be tested by invoking its isValid method.

A selection key contains two operation sets represented as integer values. Each bit of an operation set denotes a category of selectable operations that are supported by the key's channel.

• The interest set determines which operation categories will be tested for readiness the next time one of the selector's selection methods is invoked. The interest set is initialized with the value given when the key is created; it may later be changed via the interestOps(int) method.

• The ready set identifies the operation categories for which the key's channel has been detected to be ready by the key's selector. The ready set is initialized to zero when the key is created; it may later be updated by the selector during a selection operation, but it cannot be updated directly.

That a selection key's ready set indicates that its channel is ready for some operation category is a hint, but not a guarantee, that an operation in such a category may be performed by a thread without causing the thread to block. A ready set is most likely to be accurate immediately after the completion of a selection operation. It is likely to be made inaccurate by external events and by I/O operations that are invoked upon the corresponding channel.

This class defines all known operation-set bits, but precisely which bits are supported by a given channel depends upon the type of the channel. Each subclass of SelectableChannel defines an validOps() method which returns a set identifying just those operations that are supported by the channel. An attempt to set or test an operation-set bit that is not supported by a key's channel will result in an appropriate run-time exception.

It is often necessary to associate some application-specific data with a selection key, for example an object that represents the state of a higher-level protocol and handles readiness notifications in order to implement that protocol. Selection keys therefore support the attachment of a single arbitrary object to a key. An object can be attached via the attach method and then later retrieved via the attachment method.

Selection keys are safe for use by multiple concurrent threads. The operations of reading and writing the interest set will, in general, be synchronized with certain operations of the selector. Exactly how this synchronization is performed is implementation-dependent: In a naive implementation, reading or writing the interest set may block indefinitely if a selection operation is already in progress; in a high-performance implementation, reading or writing the interest set may block briefly, if at all. In any case, a selection operation will always use the interest-set value that was current at the moment that the operation began.

Since:

1.4

See Also:

SelectableChannel, Selector

Field Summary

Fields

Modifier and Type	Field and Description
static int	OP_ACCEPT Operation-set bit for socket-accept operations.
static int	OP_CONNECT Operation-set bit for socket-connect operations.
static int	OP_READ Operation-set bit for read operations.
static int	OP_WRITE Operation-set bit for write operations.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	SelectionKey() Constructs an instance of this class.

Method Summary

Methods

Modifier and Type	Method and Description
Object	attach(Object ob) Attaches the given object to this key.
Object	attachment() Retrieves the current attachment.
abstract void	cancel() Requests that the registration of this key's channel with its selector be cancelled.
abstract SelectableChannel	channel() Returns the channel for which this key was created.
abstract int	interestOps() Retrieves this key's interest set.
abstract SelectionKey	interestOps(int ops) Sets this key's interest set to the given value.
boolean	isAcceptable() Tests whether this key's channel is ready to accept a new socket connection.
boolean