SelectionKey
public
abstract
class
SelectionKey
extends Object
java.lang.Object | |
↳ | java.nio.channels.SelectionKey |
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. A selection operation will always use the interest-set value that was current at the moment that the operation began.
See also:
Summary
Constants | |
---|---|
int |
OP_ACCEPT
Operation-set bit for socket-accept operations. |
int |
OP_CONNECT
Operation-set bit for socket-connect operations. |
int |
OP_READ
Operation-set bit for read operations. |
int |
OP_WRITE
Operation-set bit for write operations. |
Protected constructors | |
---|---|
SelectionKey()
Constructs an instance of this class. |
Public methods | |
---|---|
final
Object
|
attach(Object ob)
Attaches the given object to this key. |
final
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. |
int
|
interestOpsAnd(int ops)
Atomically sets this key's interest set to the bitwise intersection ("and") of the existing interest set and the given value. |
int
|
interestOpsOr(int ops)
Atomically sets this key's interest set to the bitwise union ("or") of the existing interest set and the given value. |
final
boolean
|
isAcceptable()
Tests whether this key's channel is ready to accept a new socket connection. |
final
boolean
|
isConnectable()
Tests whether this key's channel has either finished, or failed to finish, its socket-connection operation. |
final
boolean
|
isReadable()
Tests whether this key's channel is ready for reading. |
abstract
boolean
|
isValid()
Tells whether or not this key is valid. |
final
boolean
|
isWritable()
Tests whether this key's channel is ready for writing. |
abstract
int
|
readyOps()
Retrieves this key's ready-operation set. |
abstract
Selector
|
selector()
Returns the selector for which this key was created. |
Inherited methods | |
---|---|
Constants
OP_ACCEPT
public static final int OP_ACCEPT
Operation-set bit for socket-accept operations.
Suppose that a selection key's interest set contains
OP_ACCEPT
at the start of a selection operation. If the selector
detects that the corresponding server-socket channel is ready to accept
another connection, or has an error pending, then it will add
OP_ACCEPT
to the key's ready set.
Constant Value: 16 (0x00000010)
OP_CONNECT
public static final int OP_CONNECT
Operation-set bit for socket-connect operations.
Suppose that a selection key's interest set contains
OP_CONNECT
at the start of a selection operation. If the selector
detects that the corresponding socket channel is ready to complete its
connection sequence, or has an error pending, then it will add
OP_CONNECT
to the key's ready set.
Constant Value: 8 (0x00000008)
OP_READ
public static final int OP_READ
Operation-set bit for read operations.
Suppose that a selection key's interest set contains
OP_READ
at the start of a selection operation. If the selector
detects that the corresponding channel is ready for reading, has reached
end-of-stream, has been remotely shut down for further reading, or has
an error pending, then it will add OP_READ
to the key's
ready-operation set.
Constant Value: 1 (0x00000001)
OP_WRITE
public static final int OP_WRITE
Operation-set bit for write operations.
Suppose that a selection key's interest set contains
OP_WRITE
at the start of a selection operation. If the selector
detects that the corresponding channel is ready for writing, has been
remotely shut down for further writing, or has an error pending, then it
will add OP_WRITE
to the key's ready set.
Constant Value: 4 (0x00000004)
Protected constructors
Public methods
attach
public final Object attach (Object ob)
Attaches the given object to this key.
An attached object may later be retrieved via the attachment
method. Only one object may be attached at a time; invoking
this method causes any previous attachment to be discarded. The current
attachment may be discarded by attaching null
.
Parameters | |
---|---|
ob |
Object : The object to be attached; may be null |
Returns | |
---|---|
Object |
The previously-attached object, if any,
otherwise null |
attachment
public final Object attachment ()
Retrieves the current attachment.
Returns | |
---|---|
Object |
The object currently attached to this key,
or null if there is no attachment |
cancel
public abstract void cancel ()
Requests that the registration of this key's channel with its selector be cancelled. Upon return the key will be invalid and will have been added to its selector's cancelled-key set. The key will be removed from all of the selector's key sets during the next selection operation.
If this key has already been cancelled then invoking this method has no effect. Once cancelled, a key remains forever invalid.
This method may be invoked at any time. It synchronizes on the selector's cancelled-key set, and therefore may block briefly if invoked concurrently with a cancellation or selection operation involving the same selector.
channel
public abstract SelectableChannel channel ()
Returns the channel for which this key was created. This method will continue to return the channel even after the key is cancelled.
Returns | |
---|---|
SelectableChannel |
This key's channel |
interestOps
public abstract int interestOps ()
Retrieves this key's interest set.
It is guaranteed that the returned set will only contain operation bits that are valid for this key's channel.
Returns | |
---|---|
int |
This key's interest set |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
interestOps
public abstract SelectionKey interestOps (int ops)
Sets this key's interest set to the given value.
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 change to the key's interest set will be seen by the next selection operation.
Parameters | |
---|---|
ops |
int : The new interest set |
Returns | |
---|---|
SelectionKey |
This selection key |
Throws | |
---|---|
IllegalArgumentException |
If a bit in the set does not correspond to an operation that
is supported by this key's channel, that is, if
(ops & ~channel().validOps()) != 0 |
CancelledKeyException |
If this key has been cancelled |
interestOpsAnd
public int interestOpsAnd (int ops)
Atomically sets this key's interest set to the bitwise intersection ("and")
of the existing interest set and the given value. This method is guaranteed
to be atomic with respect to other concurrent calls to this method or to
interestOpsOr(int)
.
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 change to the key's interest set will be seen by the next selection operation.
API Note:
- Unlike the
interestOps(int)
andinterestOpsOr(int)
methods, this method does not throwIllegalArgumentException
when invoked with bits in the interest set that do not correspond to an operation that is supported by this key's channel. This is to allow operation bits in the interest set to be cleared using bitwise complement values, e.g.,interestOpsAnd(~SelectionKey.OP_READ)
will remove theOP_READ
from the interest set without affecting other bits.
Implementation Requirements:
- The default implementation synchronizes on this key and invokes
interestOps()
andinterestOps(int)
to retrieve and set this key's interest set.
Parameters | |
---|---|
ops |
int : The interest set to apply |
Returns | |
---|---|
int |
The previous interest set |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
interestOpsOr
public int interestOpsOr (int ops)
Atomically sets this key's interest set to the bitwise union ("or") of
the existing interest set and the given value. This method is guaranteed
to be atomic with respect to other concurrent calls to this method or to
interestOpsAnd(int)
.
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 change to the key's interest set will be seen by the next selection operation.
Implementation Requirements:
- The default implementation synchronizes on this key and invokes
interestOps()
andinterestOps(int)
to retrieve and set this key's interest set.
Parameters | |
---|---|
ops |
int : The interest set to apply |
Returns | |
---|---|
int |
The previous interest set |
Throws | |
---|---|
IllegalArgumentException |
If a bit in the set does not correspond to an operation that
is supported by this key's channel, that is, if
(ops & ~channel().validOps()) != 0 |
CancelledKeyException |
If this key has been cancelled |
isAcceptable
public final boolean isAcceptable ()
Tests whether this key's channel is ready to accept a new socket connection.
An invocation of this method of the form k.isAcceptable()
behaves in exactly the same way as the expression
k.readyOps() & OP_ACCEPT != 0
If this key's channel does not support socket-accept operations then
this method always returns false
.
Returns | |
---|---|
boolean |
true if, and only if,
readyOps() & OP_ACCEPT is nonzero |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
isConnectable
public final boolean isConnectable ()
Tests whether this key's channel has either finished, or failed to finish, its socket-connection operation.
An invocation of this method of the form k.isConnectable()
behaves in exactly the same way as the expression
k.readyOps() & OP_CONNECT != 0
If this key's channel does not support socket-connect operations
then this method always returns false
.
Returns | |
---|---|
boolean |
true if, and only if,
readyOps() & OP_CONNECT is nonzero |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
isReadable
public final boolean isReadable ()
Tests whether this key's channel is ready for reading.
An invocation of this method of the form k.isReadable()
behaves in exactly the same way as the expression
k.readyOps() & OP_READ != 0
If this key's channel does not support read operations then this
method always returns false
.
Returns | |
---|---|
boolean |
true if, and only if,
readyOps() & OP_READ is nonzero |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
isValid
public abstract boolean isValid ()
Tells whether or not this key is valid.
A key is valid upon creation and remains so until it is cancelled, its channel is closed, or its selector is closed.
Returns | |
---|---|
boolean |
true if, and only if, this key is valid |
isWritable
public final boolean isWritable ()
Tests whether this key's channel is ready for writing.
An invocation of this method of the form k.isWritable()
behaves in exactly the same way as the expression
k.readyOps() & OP_WRITE != 0
If this key's channel does not support write operations then this
method always returns false
.
Returns | |
---|---|
boolean |
true if, and only if,
readyOps() & OP_WRITE is nonzero |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
readyOps
public abstract int readyOps ()
Retrieves this key's ready-operation set.
It is guaranteed that the returned set will only contain operation bits that are valid for this key's channel.
Returns | |
---|---|
int |
This key's ready-operation set |
Throws | |
---|---|
CancelledKeyException |
If this key has been cancelled |
selector
public abstract Selector selector ()
Returns the selector for which this key was created. This method will continue to return the selector even after the key is cancelled.
Returns | |
---|---|
Selector |
This key's selector |