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 * {@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 cancelled-key set for removal during the * next selection operation. The validity of a key may be tested by invoking * its {@link #isValid 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 {@link * #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 {@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. * *
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 {@link #attach attach} method and then later retrieved via * the {@link #attachment() 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 * @author JSR-51 Expert Group * @since 1.4 * * @see SelectableChannel * @see Selector */ public abstract class SelectionKey { /** * 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. * * @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. * * @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.
* * @return true 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.
*/ 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.
* * @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.
* * @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.
* * @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.
*/ 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.
*/ 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.
*/ 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.
*/ 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 * *
{@code
* k.readyOps() & OP_READ != 0
* }If this key's channel does not support read operations then this * method always returns false.
* * @return true 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 * *
{@code
* k.readyOps() & OP_WRITE != 0
* }If this key's channel does not support write operations then this * method always returns false.
* * @return true 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 * *
{@code
* k.readyOps() & OP_CONNECT != 0
* }If this key's channel does not support socket-connect operations * then this method always returns false.
* * @return true 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 * *
{@code
* k.readyOps() & OP_ACCEPT != 0
* }If this key's channel does not support socket-accept operations then * this method always returns false.
* * @return true 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 AtomicReferenceFieldUpdaterAn 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 null.
* * @param ob * The object to be attached; may be null * * @return The previously-attached object, if any, * otherwise null */ public final Object attach(Object ob) { return attachmentUpdater.getAndSet(this, ob); } /** * Retrieves the current attachment. * * @return The object currently attached to this key, * or null if there is no attachment */ public final Object attachment() { return attachment; } }