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
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.
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.
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.
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.
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.
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.
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.
The equals method implements an equivalence relation
on non-null object references:
x, x.equals(x) should return
true.
x and y, x.equals(y)
should return true if and only if
y.equals(x) returns true.
x, y, and z, if
x.equals(y) returns true and
y.equals(z) returns true, then
x.equals(z) should return true.
x and y, multiple invocations of
x.equals(y) consistently return true
or consistently return false, provided no
information used in equals comparisons on the
objects is modified.
x,
x.equals(null) should return false.
The equals method for class Object implements
the most discriminating possible equivalence relation on objects;
that is, for any non-null reference values x and
y, this method returns true if and only
if x and y refer to the same object
(x == y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
java.util.Hashtable.
The general contract of hashCode is:
hashCode method on each of
the two objects must produce the same integer result.
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
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.
This method may be invoked at any time. Whether or not it blocks, and for how long, is implementation-dependent.
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.
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.
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.
A key is valid upon creation and remains so until it is cancelled, its channel is closed, or its selector is closed.
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.
wait methods.
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
synchronized statement
that synchronizes on the object.
Class, by executing a
synchronized static method of that class.
Only one thread at a time can own an object's monitor.
wait methods.
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
It is guaranteed that the returned set will only contain operation bits that are valid for this key's channel.
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up
either through a call to the notify method or the
notifyAll method. The thread then waits until it can
re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.The current thread must own this object's monitor.
This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Thread T becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic, see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's "Effective Java Programming
Language Guide" (Addison-Wesley, 2001).
If the current thread is interrupted by another thread while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
This method is similar to the wait method of one
argument, but it allows finer control over the amount of time to
wait for a notification before giving up. The amount of real time,
measured in nanoseconds, is given by:
1000000*timeout+nanos
In all other respects, this method does the same thing as the method of one argument. In particular, wait(0, 0) means the same thing as wait(0).
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
notify method
or the notifyAll method.
timeout
milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout, nanos);
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.