Java™ Platform
Standard Ed. 6

java.nio.channels
Class SelectableChannel

java.lang.Object
  extended by java.nio.channels.spi.AbstractInterruptibleChannel
      extended by java.nio.channels.SelectableChannel
All Implemented Interfaces:
Closeable, Channel, InterruptibleChannel
Direct Known Subclasses:
AbstractSelectableChannel

public abstract class SelectableChannel
extends AbstractInterruptibleChannel
implements Channel

A channel that can be multiplexed via a Selector.

In order to be used with a selector, an instance of this class must first be registered via the register method. This method returns a new SelectionKey object that represents the channel's registration with the selector.

Once registered with a selector, a channel remains registered until it is deregistered. This involves deallocating whatever resources were allocated to the channel by the selector.

A channel cannot be deregistered directly; instead, the key representing its registration must be cancelled. Cancelling a key requests that the channel be deregistered during the selector's next selection operation. A key may be cancelled explicitly by invoking its cancel method. All of a channel's keys are cancelled implicitly when the channel is closed, whether by invoking its close method or by interrupting a thread blocked in an I/O operation upon the channel.

If the selector itself is closed then the channel will be deregistered, and the key representing its registration will be invalidated, without further delay.

A channel may be registered at most once with any particular selector.

Whether or not a channel is registered with one or more selectors may be determined by invoking the isRegistered method.

Selectable channels are safe for use by multiple concurrent threads.

Blocking mode

A selectable channel is either in blocking mode or in non-blocking mode. In blocking mode, every I/O operation invoked upon the channel will block until it completes. In non-blocking mode an I/O operation will never block and may transfer fewer bytes than were requested or possibly no bytes at all. The blocking mode of a selectable channel may be determined by invoking its
isBlocking method.

Newly-created selectable channels are always in blocking mode. Non-blocking mode is most useful in conjunction with selector-based multiplexing. A channel must be placed into non-blocking mode before being registered with a selector, and may not be returned to blocking mode until it has been deregistered.

Since:
1.4
See Also:
SelectionKey, Selector

Constructor Summary
protected SelectableChannel()
          Initializes a new instance of this class.
 
Method Summary
abstract  Object blockingLock()
          Retrieves the object upon which the configureBlocking and register methods synchronize.
abstract  SelectableChannel configureBlocking(boolean block)
          Adjusts this channel's blocking mode.
abstract  boolean isBlocking()
          Tells whether or not every I/O operation on this channel will block until it completes.
abstract  boolean isRegistered()
          Tells whether or not this channel is currently registered with any selectors.
abstract  SelectionKey keyFor(Selector sel)
          Retrieves the key representing the channel's registration with the given selector.
abstract  SelectorProvider provider()
          Returns the provider that created this channel.
 SelectionKey register(Selector sel, int ops)
          Registers this channel with the given selector, returning a selection key.
abstract  SelectionKey register(Selector sel, int ops, Object att)
          Registers this channel with the given selector, returning a selection key.
abstract  int validOps()
          Returns an operation set identifying this channel's supported operations.
 
Methods inherited from class java.nio.channels.spi.AbstractInterruptibleChannel
begin, close, end, implCloseChannel, isOpen
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 
Methods inherited from interface java.nio.channels.Channel
close, isOpen
 

Constructor Detail

SelectableChannel

protected SelectableChannel()
Initializes a new instance of this class.

Method Detail

provider

public abstract SelectorProvider provider()
Returns the provider that created this channel.

Returns:
The provider that created this channel

validOps

public abstract int validOps()
Returns an operation set identifying this channel's supported operations. The bits that are set in this integer value denote exactly the operations that are valid for this channel. This method always returns the same value for a given concrete channel class.

Returns:
The valid-operation set

isRegistered

public abstract boolean isRegistered()
Tells whether or not this channel is currently registered with any selectors. A newly-created channel is not registered.

Due to the inherent delay between key cancellation and channel deregistration, a channel may remain registered for some time after all of its keys have been cancelled. A channel may also remain registered for some time after it is closed.

Returns:
true if, and only if, this channel is registered

keyFor

public abstract SelectionKey keyFor(Selector sel)
Retrieves the key representing the channel's registration with the given selector.

Returns:
The key returned when this channel was last registered with the given selector, or null if this channel is not currently registered with that selector

register

public abstract SelectionKey register(Selector sel,
                                      int ops,
                                      Object att)
                               throws ClosedChannelException
Registers this channel with the given selector, returning a selection key.

If this channel is currently registered with the given selector then the selection key representing that registration is returned. The key's interest set will have been changed to ops, as if by invoking the interestOps(int) method. If the att argument is not null then the key's attachment will have been set to that value. A CancelledKeyException will be thrown if the key has already been cancelled.

Otherwise this channel has not yet been registered with the given selector, so it is registered and the resulting new key is returned. The key's initial interest set will be ops and its attachment will be att.

This method may be invoked at any time. If this method is invoked while another invocation of this method or of the configureBlocking method is in progress then it will first block until the other operation is complete. This method will then synchronize on the selector's key set and therefore may block if invoked concurrently with another registration or selection operation involving the same selector.

If this channel is closed while this operation is in progress then the key returned by this method will have been cancelled and will therefore be invalid.

Parameters:
sel - The selector with which this channel is to be registered
ops - The interest set for the resulting key
att - The attachment for the resulting key; may be null
Returns:
A key representing the registration of this channel with the given selector
Throws:
ClosedChannelException - If this channel is closed
IllegalBlockingModeException - If this channel is in blocking mode
IllegalSelectorException - If this channel was not created by the same provider as the given selector
CancelledKeyException - If this channel is currently registered with the given selector but the corresponding key has already been cancelled
IllegalArgumentException - If a bit in the ops set does not correspond to an operation that is supported by this channel, that is, if set & ~validOps() != 0

register

public final SelectionKey register(Selector sel,
                                   int ops)
                            throws ClosedChannelException
Registers this channel with the given selector, returning a selection key.

An invocation of this convenience method of the form

sc.register(sel, ops)
behaves in exactly the same way as the invocation
sc.register(sel, ops, null)

Parameters:
sel - The selector with which this channel is to be registered
ops - The interest set for the resulting key
Returns:
A key representing the registration of this channel with the given selector
Throws:
ClosedChannelException - If this channel is closed
IllegalBlockingModeException - If this channel is in blocking mode
IllegalSelectorException - If this channel was not created by the same provider as the given selector
CancelledKeyException - If this channel is currently registered with the given selector but the corresponding key has already been cancelled
IllegalArgumentException - If a bit in ops does not correspond to an operation that is supported by this channel, that is, if set & ~validOps() != 0

configureBlocking

public abstract SelectableChannel configureBlocking(boolean block)
                                             throws IOException
Adjusts this channel's blocking mode.

If this channel is registered with one or more selectors then an attempt to place it into blocking mode will cause an IllegalBlockingModeException to be thrown.

This method may be invoked at any time. The new blocking mode will only affect I/O operations that are initiated after this method returns. For some implementations this may require blocking until all pending I/O operations are complete.

If this method is invoked while another invocation of this method or of the register method is in progress then it will first block until the other operation is complete.

Parameters:
block - If true then this channel will be placed in blocking mode; if false then it will be placed non-blocking mode
Returns:
This selectable channel
Throws:
ClosedChannelException - If this channel is closed
IllegalBlockingModeException - If block is true and this channel is registered with one or more selectors
IOException - If an I/O error occurs

isBlocking

public abstract boolean isBlocking()
Tells whether or not every I/O operation on this channel will block until it completes. A newly-created channel is always in blocking mode.

If this channel is closed then the value returned by this method is not specified.

Returns:
true if, and only if, this channel is in blocking mode

blockingLock

public abstract Object blockingLock()
Retrieves the object upon which the configureBlocking and register methods synchronize. This is often useful in the implementation of adaptors that require a specific blocking mode to be maintained for a short period of time.

Returns:
The blocking-mode lock object

Java™ Platform
Standard Ed. 6

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