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JavaTM 2 Platform Std. Ed. v1.6.0
java.util.concurrent
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Constructor Summary | |
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Semaphore(int permits)
Creates a Semaphore with the given number of
permits and nonfair fairness setting. |
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Semaphore(int permits,
boolean fair)
Creates a Semaphore with the given number of
permits and the given fairness setting. |
Method Summary | |
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void |
acquire()
Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted. |
void |
acquire(int permits)
Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is interrupted. |
void |
acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is available. |
void |
acquireUninterruptibly(int permits)
Acquires the given number of permits from this semaphore, blocking until all are available. |
int |
availablePermits()
Returns the current number of permits available in this semaphore. |
int |
drainPermits()
Acquires and returns all permits that are immediately available. |
protected Collection<Thread> |
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire. |
int |
getQueueLength()
Returns an estimate of the number of threads waiting to acquire. |
boolean |
hasQueuedThreads()
Queries whether any threads are waiting to acquire. |
boolean |
isFair()
Returns true if this semaphore has fairness set true. |
protected void |
reducePermits(int reduction)
Shrinks the number of available permits by the indicated reduction. |
void |
release()
Releases a permit, returning it to the semaphore. |
void |
release(int permits)
Releases the given number of permits, returning them to the semaphore. |
String |
toString()
Returns a string identifying this semaphore, as well as its state. |
boolean |
tryAcquire()
Acquires a permit from this semaphore, only if one is available at the time of invocation. |
boolean |
tryAcquire(int permits)
Acquires the given number of permits from this semaphore, only if all are available at the time of invocation. |
boolean |
tryAcquire(int permits,
long timeout,
TimeUnit unit)
Acquires the given number of permits from this semaphore, if all become available within the given waiting time and the current thread has not been interrupted. |
boolean |
tryAcquire(long timeout,
TimeUnit unit)
Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been interrupted. |
Methods inherited from class java.lang.Object |
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clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait |
Constructor Detail |
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public Semaphore(int permits)
Semaphore
with the given number of
permits and nonfair fairness setting.
permits
- the initial number of permits available.
This value may be negative, in which case releases
must occur before any acquires will be granted.public Semaphore(int permits, boolean fair)
Semaphore
with the given number of
permits and the given fairness setting.
permits
- the initial number of permits available.
This value may be negative, in which case releases
must occur before any acquires will be granted.fair
- true
if this semaphore will guarantee
first-in first-out granting of permits under contention,
else false
Method Detail |
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public void acquire() throws InterruptedException
Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one.
If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
release()
method for this
semaphore and the current thread is next to be assigned a permit; or
If the current thread:
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
InterruptedException
- if the current thread is interruptedpublic void acquireUninterruptibly()
Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one.
If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
some other thread invokes the release()
method for this
semaphore and the current thread is next to be assigned a permit.
If the current thread is interrupted while waiting for a permit then it will continue to wait, but the time at which the thread is assigned a permit may change compared to the time it would have received the permit had no interruption occurred. When the thread does return from this method its interrupt status will be set.
public boolean tryAcquire()
Acquires a permit, if one is available and returns immediately,
with the value true
,
reducing the number of available permits by one.
If no permit is available then this method will return
immediately with the value false
.
Even when this semaphore has been set to use a
fair ordering policy, a call to tryAcquire()
will
immediately acquire a permit if one is available, whether or not
other threads are currently waiting.
This "barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to honor
the fairness setting, then use
tryAcquire(0, TimeUnit.SECONDS)
which is almost equivalent (it also detects interruption).
true
if a permit was acquired and false
otherwisepublic boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException
Acquires a permit, if one is available and returns immediately,
with the value true
,
reducing the number of available permits by one.
If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:
release()
method for this
semaphore and the current thread is next to be assigned a permit; or
If a permit is acquired then the value true
is returned.
If the current thread:
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
If the specified waiting time elapses then the value false
is returned. If the time is less than or equal to zero, the method
will not wait at all.
timeout
- the maximum time to wait for a permitunit
- the time unit of the timeout
argument
true
if a permit was acquired and false
if the waiting time elapsed before a permit was acquired
InterruptedException
- if the current thread is interruptedpublic void release()
Releases a permit, increasing the number of available permits by one. If any threads are trying to acquire a permit, then one is selected and given the permit that was just released. That thread is (re)enabled for thread scheduling purposes.
There is no requirement that a thread that releases a permit must
have acquired that permit by calling acquire()
.
Correct usage of a semaphore is established by programming convention
in the application.
public void acquire(int permits) throws InterruptedException
Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount.
If insufficient permits are available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
If the current thread:
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
Any permits that were to be assigned to this thread are instead
assigned to other threads trying to acquire permits, as if
permits had been made available by a call to release()
.
permits
- the number of permits to acquire
InterruptedException
- if the current thread is interrupted
IllegalArgumentException
- if permits
is negativepublic void acquireUninterruptibly(int permits)
Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount.
If insufficient permits are available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
some other thread invokes one of the release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request.
If the current thread is interrupted while waiting for permits then it will continue to wait and its position in the queue is not affected. When the thread does return from this method its interrupt status will be set.
permits
- the number of permits to acquire
IllegalArgumentException
- if permits
is negativepublic boolean tryAcquire(int permits)
Acquires the given number of permits, if they are available, and
returns immediately, with the value true
,
reducing the number of available permits by the given amount.
If insufficient permits are available then this method will return
immediately with the value false
and the number of available
permits is unchanged.
Even when this semaphore has been set to use a fair ordering
policy, a call to tryAcquire
will
immediately acquire a permit if one is available, whether or
not other threads are currently waiting. This
"barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to
honor the fairness setting, then use tryAcquire(permits, 0, TimeUnit.SECONDS)
which is almost equivalent (it also detects interruption).
permits
- the number of permits to acquire
true
if the permits were acquired and
false
otherwise
IllegalArgumentException
- if permits
is negativepublic boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException
Acquires the given number of permits, if they are available and
returns immediately, with the value true
,
reducing the number of available permits by the given amount.
If insufficient permits are available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:
release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
If the permits are acquired then the value true
is returned.
If the current thread:
InterruptedException
is thrown and the current thread's
interrupted status is cleared.
Any permits that were to be assigned to this thread, are instead
assigned to other threads trying to acquire permits, as if
the permits had been made available by a call to release()
.
If the specified waiting time elapses then the value false
is returned. If the time is less than or equal to zero, the method
will not wait at all. Any permits that were to be assigned to this
thread, are instead assigned to other threads trying to acquire
permits, as if the permits had been made available by a call to
release()
.
permits
- the number of permits to acquiretimeout
- the maximum time to wait for the permitsunit
- the time unit of the timeout
argument
true
if all permits were acquired and false
if the waiting time elapsed before all permits were acquired
InterruptedException
- if the current thread is interrupted
IllegalArgumentException
- if permits
is negativepublic void release(int permits)
Releases the given number of permits, increasing the number of available permits by that amount. If any threads are trying to acquire permits, then one is selected and given the permits that were just released. If the number of available permits satisfies that thread's request then that thread is (re)enabled for thread scheduling purposes; otherwise the thread will wait until sufficient permits are available. If there are still permits available after this thread's request has been satisfied, then those permits are assigned in turn to other threads trying to acquire permits.
There is no requirement that a thread that releases a permit must
have acquired that permit by calling acquire
.
Correct usage of a semaphore is established by programming convention
in the application.
permits
- the number of permits to release
IllegalArgumentException
- if permits
is negativepublic int availablePermits()
This method is typically used for debugging and testing purposes.
public int drainPermits()
protected void reducePermits(int reduction)
acquire
in that it does not block
waiting for permits to become available.
reduction
- the number of permits to remove
IllegalArgumentException
- if reduction
is negativepublic boolean isFair()
true
if this semaphore has fairness set true.
true
if this semaphore has fairness set truepublic final boolean hasQueuedThreads()
true
return does not guarantee that any other thread will ever
acquire. This method is designed primarily for use in
monitoring of the system state.
true
if there may be other threads waiting to
acquire the lockpublic final int getQueueLength()
protected Collection<Thread> getQueuedThreads()
public String toString()
"Permits ="
followed by the number of permits.