OpenJDK 27 · java.nio.channels.SelectableChannel · APIdia, the JavaDoc alternative

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.

Access	Constructor and Description
protected	SelectableChannel() Initializes a new instance of this class.

Method Summary

Modifier and Type	Method and Description
public abstract Object	Returns: The blocking-mode lock object blockingLock() Retrieves the object upon which the `configureBlocking` and `register` methods synchronize.
public abstract SelectableChannel	Returns: This selectable channel configureBlocking(boolean If `true` then this channel will be placed in blocking mode; if `false` then it will be placed non-blocking mode block) Adjusts this channel's blocking mode.
public abstract boolean	Returns: `true` if, and only if, this channel is in blocking mode isBlocking() Tells whether or not every I/O operation on this channel will block until it completes.
public abstract boolean	Returns: `true` if, and only if, this channel is registered isRegistered() Tells whether or not this channel is currently registered with any selectors.
public abstract SelectionKey	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 keyFor(Selector The selector sel) Retrieves the key representing the channel's registration with the given selector.
public abstract SelectorProvider	Returns: The provider that created this channel provider() Returns the provider that created this channel.
public abstract SelectionKey	Returns: A key representing the registration of this channel with the given selector register(Selector The selector with which this channel is to be registered sel, int The interest set for the resulting key ops, Object The attachment for the resulting key; may be `null` att) Registers this channel with the given selector, returning a selection key.
public final SelectionKey	Returns: A key representing the registration of this channel with the given selector register(Selector The selector with which this channel is to be registered sel, int The interest set for the resulting key ops) Registers this channel with the given selector, returning a selection key.
public abstract int	Returns: The valid-operation set validOps() Returns an operation set identifying this channel's supported operations.

Inherited from java.nio.channels.spi.AbstractInterruptibleChannel:: begin close end implCloseChannel isOpen

Constructor Detail

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

Method Detail

blockingLock back to summary

blockingLock	back to summary
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:Object The blocking-mode lock object

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:Object: The blocking-mode lock object

configureBlocking back to summary

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

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:boolean: If true then this channel will be placed in blocking mode; if false then it will be placed non-blocking mode

Returns:SelectableChannel

This selectable channel

Exceptions

IOException:: If an I/O error occurs
ClosedChannelException:: If this channel is closed
IllegalBlockingModeException:: If block is true and this channel is registered with one or more selectors

isBlocking back to summary

isBlocking	back to summary
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:boolean `true` if, and only if, this channel is in blocking mode

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

isRegistered back to summary

isRegistered	back to summary
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:boolean `true` if, and only if, this channel is registered

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

keyFor back to summary

keyFor	back to summary
public abstract SelectionKey keyFor(Selector sel) Retrieves the key representing the channel's registration with the given selector. Parameters sel:Selector The selector Returns:SelectionKey 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

public abstract SelectionKey keyFor(Selector sel)

Retrieves the key representing the channel's registration with the given selector.

Parameters

sel:Selector: The selector

Returns:SelectionKey

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

provider	back to summary
public abstract SelectorProvider provider() Returns the provider that created this channel. Returns:SelectorProvider The provider that created this channel

provider

back to summary

public abstract SelectorProvider provider()

Returns the provider that created this channel.

Returns:SelectorProvider: The provider that created this channel

register	back to summary
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 a selection operation is in progress then it has no effect upon that operation; the new registration or change to the key's interest set will be seen by the next selection operation. If this method is invoked while an invocation of `configureBlocking` is in progress then it will block until the channel's blocking mode has been adjusted. 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:Selector The selector with which this channel is to be registered ops:int The interest set for the resulting key att:Object The attachment for the resulting key; may be `null` Returns:SelectionKey A key representing the registration of this channel with the given selector Exceptions ClosedChannelException: If this channel is closed ClosedSelectorException: If the selector 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`

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 a selection operation is in progress then it has no effect upon that operation; the new registration or change to the key's interest set will be seen by the next selection operation. If this method is invoked while an invocation of configureBlocking is in progress then it will block until the channel's blocking mode has been adjusted.

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:Selector: The selector with which this channel is to be registered
ops:int: The interest set for the resulting key
att:Object: The attachment for the resulting key; may be null

Returns:SelectionKey

A key representing the registration of this channel with the given selector

Exceptions

ClosedChannelException:: If this channel is closed
ClosedSelectorException:: If the selector 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	back to summary
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:Selector The selector with which this channel is to be registered ops:int The interest set for the resulting key Returns:SelectionKey A key representing the registration of this channel with the given selector Exceptions ClosedChannelException: If this channel is closed ClosedSelectorException: If the selector 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`

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:Selector: The selector with which this channel is to be registered
ops:int: The interest set for the resulting key

Returns:SelectionKey

A key representing the registration of this channel with the given selector

Exceptions

ClosedChannelException:: If this channel is closed
ClosedSelectorException:: If the selector 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

validOps	back to summary
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:int The valid-operation set

validOps

back to summary

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:int: The valid-operation set

public abstract Class SelectableChannel

Blocking mode

Constructor Summary

Method Summary

Constructor Detail

Method Detail