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package java.nio.channels;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.io.IOException;
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.
Author: Mark Reinhold, JSR-51 Expert Group See Also: Since: 1.4
/**
* A token representing the registration of a {@link SelectableChannel} with a
* {@link Selector}.
*
* <p> A selection key is created each time a channel is registered with a
* selector. A key remains valid until it is <i>cancelled</i> by invoking its
* {@link #cancel 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 <a
* href="Selector.html#ks"><i>cancelled-key set</i></a> for removal during the
* next selection operation. The validity of a key may be tested by invoking
* its {@link #isValid isValid} method.
*
* <a name="opsets"></a>
*
* <p> A selection key contains two <i>operation sets</i> represented as
* integer values. Each bit of an operation set denotes a category of
* selectable operations that are supported by the key's channel.
*
* <ul>
*
* <li><p> The <i>interest set</i> 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 {@link
* #interestOps(int)} method. </p></li>
*
* <li><p> The <i>ready set</i> 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. </p></li>
*
* </ul>
*
* <p> 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.
*
* <p> 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 {@link SelectableChannel} defines an {@link
* SelectableChannel#validOps() 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.
*
* <p> 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
* <i>attachment</i> of a single arbitrary object to a key. An object can be
* attached via the {@link #attach attach} method and then later retrieved via
* the {@link #attachment() attachment} method.
*
* <p> 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. </p>
*
*
* @author Mark Reinhold
* @author JSR-51 Expert Group
* @since 1.4
*
* @see SelectableChannel
* @see Selector
*/
public abstract class SelectionKey {
Constructs an instance of this class.
/**
* Constructs an instance of this class.
*/
protected SelectionKey() { }
// -- Channel and selector operations --
Returns the channel for which this key was created. This method will
continue to return the channel even after the key is cancelled.
Returns: This key's channel
/**
* Returns the channel for which this key was created. This method will
* continue to return the channel even after the key is cancelled.
*
* @return This key's channel
*/
public abstract SelectableChannel channel();
Returns the selector for which this key was created. This method will
continue to return the selector even after the key is cancelled.
Returns: This key's selector
/**
* Returns the selector for which this key was created. This method will
* continue to return the selector even after the key is cancelled.
*
* @return This key's selector
*/
public abstract Selector selector();
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: true if, and only if, this key is valid
/**
* Tells whether or not this key is valid.
*
* <p> A key is valid upon creation and remains so until it is cancelled,
* its channel is closed, or its selector is closed. </p>
*
* @return <tt>true</tt> if, and only if, this key is valid
*/
public abstract boolean isValid();
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.
/**
* 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.
*
* <p> If this key has already been cancelled then invoking this method has
* no effect. Once cancelled, a key remains forever invalid. </p>
*
* <p> 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. </p>
*/
public abstract void cancel();
// -- Operation-set accessors --
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.
This method may be invoked at any time. Whether or not it blocks,
and for how long, is implementation-dependent.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: This key's interest set
/**
* Retrieves this key's interest set.
*
* <p> It is guaranteed that the returned set will only contain operation
* bits that are valid for this key's channel.
*
* <p> This method may be invoked at any time. Whether or not it blocks,
* and for how long, is implementation-dependent. </p>
*
* @return This key's interest set
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public abstract int interestOps();
Sets this key's interest set to the given value.
This method may be invoked at any time. Whether or not it blocks,
and for how long, is implementation-dependent.
Params: - ops – The new 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
Returns: This selection key
/**
* Sets this key's interest set to the given value.
*
* <p> This method may be invoked at any time. Whether or not it blocks,
* and for how long, is implementation-dependent. </p>
*
* @param ops The new interest set
*
* @return 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
* {@code (ops & ~channel().validOps()) != 0}
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public abstract SelectionKey interestOps(int ops);
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.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: This key's ready-operation set
/**
* Retrieves this key's ready-operation set.
*
* <p> It is guaranteed that the returned set will only contain operation
* bits that are valid for this key's channel. </p>
*
* @return This key's ready-operation set
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public abstract int readyOps();
// -- Operation bits and bit-testing convenience methods --
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 and add the key to its selected-key set.
/**
* Operation-set bit for read operations.
*
* <p> Suppose that a selection key's interest set contains
* <tt>OP_READ</tt> at the start of a <a
* href="Selector.html#selop">selection operation</a>. 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 <tt>OP_READ</tt> to the key's
* ready-operation set and add the key to its selected-key set. </p>
*/
public static final int OP_READ = 1 << 0;
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 and add the key to its
selected-key set.
/**
* Operation-set bit for write operations.
*
* <p> Suppose that a selection key's interest set contains
* <tt>OP_WRITE</tt> at the start of a <a
* href="Selector.html#selop">selection operation</a>. 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 <tt>OP_WRITE</tt> to the key's ready set and add the key to its
* selected-key set. </p>
*/
public static final int OP_WRITE = 1 << 2;
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 and add the key to its
selected-key set.
/**
* Operation-set bit for socket-connect operations.
*
* <p> Suppose that a selection key's interest set contains
* <tt>OP_CONNECT</tt> at the start of a <a
* href="Selector.html#selop">selection operation</a>. 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
* <tt>OP_CONNECT</tt> to the key's ready set and add the key to its
* selected-key set. </p>
*/
public static final int OP_CONNECT = 1 << 3;
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 and add the key to its
selected-key set.
/**
* Operation-set bit for socket-accept operations.
*
* <p> Suppose that a selection key's interest set contains
* <tt>OP_ACCEPT</tt> at the start of a <a
* href="Selector.html#selop">selection operation</a>. 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
* <tt>OP_ACCEPT</tt> to the key's ready set and add the key to its
* selected-key set. </p>
*/
public static final int OP_ACCEPT = 1 << 4;
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.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: true if, and only if, readyOps() & OP_READ
is nonzero
/**
* Tests whether this key's channel is ready for reading.
*
* <p> An invocation of this method of the form <tt>k.isReadable()</tt>
* behaves in exactly the same way as the expression
*
* <blockquote><pre>{@code
* k.readyOps() & OP_READ != 0
* }</pre></blockquote>
*
* <p> If this key's channel does not support read operations then this
* method always returns <tt>false</tt>. </p>
*
* @return <tt>true</tt> if, and only if,
{@code readyOps() & OP_READ} is nonzero
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public final boolean isReadable() {
return (readyOps() & OP_READ) != 0;
}
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.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: true if, and only if, readyOps() & OP_WRITE
is nonzero
/**
* Tests whether this key's channel is ready for writing.
*
* <p> An invocation of this method of the form <tt>k.isWritable()</tt>
* behaves in exactly the same way as the expression
*
* <blockquote><pre>{@code
* k.readyOps() & OP_WRITE != 0
* }</pre></blockquote>
*
* <p> If this key's channel does not support write operations then this
* method always returns <tt>false</tt>. </p>
*
* @return <tt>true</tt> if, and only if,
* {@code readyOps() & OP_WRITE} is nonzero
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public final boolean isWritable() {
return (readyOps() & OP_WRITE) != 0;
}
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.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: true if, and only if, readyOps() & OP_CONNECT
is nonzero
/**
* Tests whether this key's channel has either finished, or failed to
* finish, its socket-connection operation.
*
* <p> An invocation of this method of the form <tt>k.isConnectable()</tt>
* behaves in exactly the same way as the expression
*
* <blockquote><pre>{@code
* k.readyOps() & OP_CONNECT != 0
* }</pre></blockquote>
*
* <p> If this key's channel does not support socket-connect operations
* then this method always returns <tt>false</tt>. </p>
*
* @return <tt>true</tt> if, and only if,
* {@code readyOps() & OP_CONNECT} is nonzero
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public final boolean isConnectable() {
return (readyOps() & OP_CONNECT) != 0;
}
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.
Throws: - CancelledKeyException –
If this key has been cancelled
Returns: true if, and only if, readyOps() & OP_ACCEPT
is nonzero
/**
* Tests whether this key's channel is ready to accept a new socket
* connection.
*
* <p> An invocation of this method of the form <tt>k.isAcceptable()</tt>
* behaves in exactly the same way as the expression
*
* <blockquote><pre>{@code
* k.readyOps() & OP_ACCEPT != 0
* }</pre></blockquote>
*
* <p> If this key's channel does not support socket-accept operations then
* this method always returns <tt>false</tt>. </p>
*
* @return <tt>true</tt> if, and only if,
* {@code readyOps() & OP_ACCEPT} is nonzero
*
* @throws CancelledKeyException
* If this key has been cancelled
*/
public final boolean isAcceptable() {
return (readyOps() & OP_ACCEPT) != 0;
}
// -- Attachments --
private volatile Object attachment = null;
private static final AtomicReferenceFieldUpdater<SelectionKey,Object>
attachmentUpdater = AtomicReferenceFieldUpdater.newUpdater(
SelectionKey.class, Object.class, "attachment"
);
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.
Params: - ob –
The object to be attached; may be null
Returns: The previously-attached object, if any,
otherwise null
/**
* Attaches the given object to this key.
*
* <p> An attached object may later be retrieved via the {@link #attachment()
* 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 <tt>null</tt>. </p>
*
* @param ob
* The object to be attached; may be <tt>null</tt>
*
* @return The previously-attached object, if any,
* otherwise <tt>null</tt>
*/
public final Object attach(Object ob) {
return attachmentUpdater.getAndSet(this, ob);
}
Retrieves the current attachment.
Returns: The object currently attached to this key,
or null if there is no attachment
/**
* Retrieves the current attachment.
*
* @return The object currently attached to this key,
* or <tt>null</tt> if there is no attachment
*/
public final Object attachment() {
return attachment;
}
}