| java.lang.Object
 java.util.concurrent.Semaphore
| Semaphore | | public class Semaphore implements java.io.Serializable(Code) | | A counting semaphore. Conceptually, a semaphore maintains a set of
permits. Each
Semaphore.acquire blocks if necessary until a permit is
available, and then takes it. Each
Semaphore.release adds a permit,
potentially releasing a blocking acquirer.
However, no actual permit objects are used; the Semaphore just
keeps a count of the number available and acts accordingly.
Semaphores are often used to restrict the number of threads than can
access some (physical or logical) resource. For example, here is
a class that uses a semaphore to control access to a pool of items:
class Pool {
private static final MAX_AVAILABLE = 100;
private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
public Object getItem() throws InterruptedException {
available.acquire();
return getNextAvailableItem();
}
public void putItem(Object x) {
if (markAsUnused(x))
available.release();
}
// Not a particularly efficient data structure; just for demo
protected Object[] items = ... whatever kinds of items being managed
protected boolean[] used = new boolean[MAX_AVAILABLE];
protected synchronized Object getNextAvailableItem() {
for (int i = 0; i < MAX_AVAILABLE; ++i) {
if (!used[i]) {
used[i] = true;
return items[i];
}
}
return null; // not reached
}
protected synchronized boolean markAsUnused(Object item) {
for (int i = 0; i < MAX_AVAILABLE; ++i) {
if (item == items[i]) {
if (used[i]) {
used[i] = false;
return true;
} else
return false;
}
}
return false;
}
}
Before obtaining an item each thread must acquire a permit from
the semaphore, guaranteeing that an item is available for use. When
the thread has finished with the item it is returned back to the
pool and a permit is returned to the semaphore, allowing another
thread to acquire that item. Note that no synchronization lock is
held when
Semaphore.acquire is called as that would prevent an item
from being returned to the pool. The semaphore encapsulates the
synchronization needed to restrict access to the pool, separately
from any synchronization needed to maintain the consistency of the
pool itself.
A semaphore initialized to one, and which is used such that it
only has at most one permit available, can serve as a mutual
exclusion lock. This is more commonly known as a binary
semaphore, because it only has two states: one permit
available, or zero permits available. When used in this way, the
binary semaphore has the property (unlike many
Lock implementations), that the "lock" can be released by a
thread other than the owner (as semaphores have no notion of
ownership). This can be useful in some specialized contexts, such
as deadlock recovery.
The constructor for this class optionally accepts a
fairness parameter. When set false, this class makes no
guarantees about the order in which threads acquire permits. In
particular, barging is permitted, that is, a thread
invoking
Semaphore.acquire can be allocated a permit ahead of a
thread that has been waiting. When fairness is set true, the
semaphore guarantees that threads invoking any of the
Semaphore.acquire() acquire methods are allocated permits in the order in
which their invocation of those methods was processed
(first-in-first-out; FIFO). Note that FIFO ordering necessarily
applies to specific internal points of execution within these
methods. So, it is possible for one thread to invoke
acquire before another, but reach the ordering point after
the other, and similarly upon return from the method.
Generally, semaphores used to control resource access should be
initialized as fair, to ensure that no thread is starved out from
accessing a resource. When using semaphores for other kinds of
synchronization control, the throughput advantages of non-fair
ordering often outweigh fairness considerations.
This class also provides convenience methods to
Semaphore.acquire(int) acquire and
Semaphore.release(int) release multiple
permits at a time. Beware of the increased risk of indefinite
postponement when these methods are used without fairness set true.
since:
1.5
author:
Doug Lea |
| Inner Class :final static class NonfairSync extends Sync | |
| Inner Class :final static class FairSync extends Sync | |
| Constructor Summary | |
| public | Semaphore(int permits)
Creates a Semaphore with the given number of
permits and nonfair fairness setting.
Parameters:
permits - the initial number of permits available. | | public | Semaphore(int permits, boolean fair)
Creates a Semaphore with the given number of
permits and the given fairness setting.
Parameters:
permits - the initial number of permits available. |
| Method Summary | |
| public void | acquire()
Acquires a permit from this semaphore, blocking until one is
available, or the thread is
Thread.interrupt interrupted . | | public void | acquire(int permits)
Acquires the given number of permits from this semaphore,
blocking until all are available,
or the thread is
Thread.interrupt interrupted .
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:
- Some other thread invokes one of the
Semaphore.release() release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
- Some other thread
Thread.interrupt interrupts the current
thread.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting
for a permit,
then
InterruptedException is thrown and the current thread's
interrupted status is cleared. | | public void | acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is
available.
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
Semaphore.release method for this
semaphore and the current thread is next to be assigned a permit.
If the current thread
is
Thread.interrupt 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. | | public void | acquireUninterruptibly(int permits)
Acquires the given number of permits from this semaphore,
blocking until all are available.
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
Semaphore.release() 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
Thread.interrupt interrupted while waiting
for permits then it will continue to wait and its position in the
queue is not affected. | | public int | availablePermits()
Returns the current number of permits available in this semaphore. | | public int | drainPermits()
Acquire and return all permits that are immediately available. | | final public int | getQueueLength()
Returns an estimate of the number of threads waiting to
acquire. | | protected Collection<Thread> | getQueuedThreads()
Returns a collection containing threads that may be waiting to
acquire. | | final public boolean | hasQueuedThreads()
Queries whether any threads are waiting to acquire. | | public 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. | | public void | release()
Releases a permit, returning it to the semaphore. | | public void | release(int permits)
Releases the given number of permits, returning them to the semaphore.
Releases the given number of permits, increasing the number of
available permits by that amount.
If any threads are blocking trying to acquire permits, then the
one that has been waiting the longest
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 continues to wait. | | public String | toString()
Returns a string identifying this semaphore, as well as its state. | | public boolean | tryAcquire()
Acquires a permit from this semaphore, only if one is available at the
time of invocation.
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. | | public 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
Thread.interrupt interrupted .
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:
- Some other thread invokes the
Semaphore.release method for this
semaphore and the current thread is next to be assigned a permit; or
- Some other thread
Thread.interrupt interrupts the current
thread; or
- The specified waiting time elapses.
If a permit is acquired then the value true is returned.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting to acquire
a permit,
then
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.
Parameters:
timeout - the maximum time to wait for a permit
Parameters:
unit - the time unit of the timeout argument. | | public boolean | tryAcquire(int permits)
Acquires the given number of permits from this semaphore, only
if all are available at the time of invocation.
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. | | public 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
Thread.interrupt interrupted .
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:
- Some other thread invokes one of the
Semaphore.release() release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
- Some other thread
Thread.interrupt interrupts the current
thread; or
- The specified waiting time elapses.
If the permits are acquired then the value true is returned.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting to acquire
the permits,
then
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 the next waiting thread(s), as if
they had been made available by a call to
Semaphore.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 the next waiting thread(s), as if
they had been made available by a call to
Semaphore.release() .
Parameters:
permits - the number of permits to acquire
Parameters:
timeout - the maximum time to wait for the permits
Parameters:
unit - the time unit of the timeout argument. |
| Semaphore | | public Semaphore(int permits)(Code) | | Creates a Semaphore with the given number of
permits and nonfair fairness setting.
Parameters:
permits - the initial number of permits available. Thisvalue may be negative, in which case releases mustoccur before any acquires will be granted. |
| Semaphore | | public Semaphore(int permits, boolean fair)(Code) | | Creates a Semaphore with the given number of
permits and the given fairness setting.
Parameters:
permits - the initial number of permits available. Thisvalue may be negative, in which case releases mustoccur before any acquires will be granted.
Parameters:
fair - true if this semaphore will guarantee first-infirst-out granting of permits under contention, else false. |
| acquire | | public void acquire() throws InterruptedException(Code) | | Acquires a permit from this semaphore, blocking until one is
available, or the thread is
Thread.interrupt interrupted .
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:
- Some other thread invokes the
Semaphore.release method for this
semaphore and the current thread is next to be assigned a permit; or
- Some other thread
Thread.interrupt interrupts the current
thread.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting
for a permit,
then
InterruptedException is thrown and the current thread's
interrupted status is cleared.
throws:
InterruptedException - if the current thread is interrupted
See Also: Thread.interrupt |
| acquire | | public void acquire(int permits) throws InterruptedException(Code) | | Acquires the given number of permits from this semaphore,
blocking until all are available,
or the thread is
Thread.interrupt interrupted .
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:
- Some other thread invokes one of the
Semaphore.release() release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
- Some other thread
Thread.interrupt interrupts the current
thread.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting
for a permit,
then
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 the next waiting thread(s), as if
they had been made available by a call to
Semaphore.release() .
Parameters:
permits - the number of permits to acquire
throws:
InterruptedException - if the current thread is interrupted
throws:
IllegalArgumentException - if permits less than zero.
See Also: Thread.interrupt |
| acquireUninterruptibly | | public void acquireUninterruptibly()(Code) | | Acquires a permit from this semaphore, blocking until one is
available.
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
Semaphore.release method for this
semaphore and the current thread is next to be assigned a permit.
If the current thread
is
Thread.interrupt 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.
|
| acquireUninterruptibly | | public void acquireUninterruptibly(int permits)(Code) | | Acquires the given number of permits from this semaphore,
blocking until all are available.
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
Semaphore.release() 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
Thread.interrupt 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.
Parameters:
permits - the number of permits to acquire
throws:
IllegalArgumentException - if permits less than zero. |
| availablePermits | | public int availablePermits()(Code) | | Returns the current number of permits available in this semaphore.
This method is typically used for debugging and testing purposes.
the number of permits available in this semaphore. |
| drainPermits | | public int drainPermits()(Code) | | Acquire and return all permits that are immediately available.
the number of permits |
| getQueueLength | | final public int getQueueLength()(Code) | | Returns an estimate of the number of threads waiting to
acquire. The value is only an estimate because the number of
threads may change dynamically while this method traverses
internal data structures. This method is designed for use in
monitoring of the system state, not for synchronization
control.
the estimated number of threads waiting for this lock |
| getQueuedThreads | | protected Collection<Thread> getQueuedThreads()(Code) | | Returns a collection containing threads that may be waiting to
acquire. Because the actual set of threads may change
dynamically while constructing this result, the returned
collection is only a best-effort estimate. The elements of the
returned collection are in no particular order. This method is
designed to facilitate construction of subclasses that provide
more extensive monitoring facilities.
the collection of threads |
| hasQueuedThreads | | final public boolean hasQueuedThreads()(Code) | | Queries whether any threads are waiting to acquire. Note that
because cancellations may occur at any time, a 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 acquirethe lock. |
| isFair | | public boolean isFair()(Code) | | Returns true if this semaphore has fairness set true.
true if this semaphore has fairness set true. |
| reducePermits | | protected void reducePermits(int reduction)(Code) | | Shrinks the number of available permits by the indicated
reduction. This method can be useful in subclasses that use
semaphores to track resources that become unavailable. This
method differs from acquire in that it does not block
waiting for permits to become available.
Parameters:
reduction - the number of permits to remove
throws:
IllegalArgumentException - if reduction is negative |
| release | | public void release()(Code) | | Releases a permit, returning it to the semaphore.
Releases a permit, increasing the number of available permits
by one.
If any threads are blocking 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
Semaphore.acquire .
Correct usage of a semaphore is established by programming convention
in the application.
|
| release | | public void release(int permits)(Code) | | Releases the given number of permits, returning them to the semaphore.
Releases the given number of permits, increasing the number of
available permits by that amount.
If any threads are blocking trying to acquire permits, then the
one that has been waiting the longest
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 continues to wait. If there are still permits available
after the first thread's request has been satisfied, then those permits
are assigned to the next waiting thread. If it is satisfied then it is
re-enabled for thread scheduling purposes. This continues until there
are insufficient permits to satisfy the next waiting thread, or there
are no more waiting threads.
There is no requirement that a thread that releases a permit must
have acquired that permit by calling
Semaphore.acquire acquire .
Correct usage of a semaphore is established by programming convention
in the application.
Parameters:
permits - the number of permits to release
throws:
IllegalArgumentException - if permits less than zero. |
| toString | | public String toString()(Code) | | Returns a string identifying this semaphore, as well as its state.
The state, in brackets, includes the String
"Permits =" followed by the number of permits.
a string identifying this semaphore, as well as itsstate |
| tryAcquire | | public boolean tryAcquire()(Code) | | Acquires a permit from this semaphore, only if one is available at the
time of invocation.
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
Semaphore.tryAcquire(long,TimeUnit) tryAcquire(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption).
true if a permit was acquired and falseotherwise. |
| tryAcquire | | public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException(Code) | | Acquires a permit from this semaphore, if one becomes available
within the given waiting time and the
current thread has not been
Thread.interrupt interrupted .
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:
- Some other thread invokes the
Semaphore.release method for this
semaphore and the current thread is next to be assigned a permit; or
- Some other thread
Thread.interrupt interrupts the current
thread; or
- The specified waiting time elapses.
If a permit is acquired then the value true is returned.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting to acquire
a permit,
then
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.
Parameters:
timeout - the maximum time to wait for a permit
Parameters:
unit - the time unit of the timeout argument. true if a permit was acquired and falseif the waiting time elapsed before a permit was acquired.
throws:
InterruptedException - if the current thread is interrupted
See Also: Thread.interrupt |
| tryAcquire | | public boolean tryAcquire(int permits)(Code) | | Acquires the given number of permits from this semaphore, only
if all are available at the time of invocation.
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
Semaphore.tryAcquire(int,long,TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption).
Parameters:
permits - the number of permits to acquire true if the permits were acquired and falseotherwise.
throws:
IllegalArgumentException - if permits less than zero. |
| tryAcquire | | public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException(Code) | | 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
Thread.interrupt interrupted .
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:
- Some other thread invokes one of the
Semaphore.release() release
methods for this semaphore, the current thread is next to be assigned
permits and the number of available permits satisfies this request; or
- Some other thread
Thread.interrupt interrupts the current
thread; or
- The specified waiting time elapses.
If the permits are acquired then the value true is returned.
If the current thread:
- has its interrupted status set on entry to this method; or
- is
Thread.interrupt interrupted while waiting to acquire
the permits,
then
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 the next waiting thread(s), as if
they had been made available by a call to
Semaphore.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 the next waiting thread(s), as if
they had been made available by a call to
Semaphore.release() .
Parameters:
permits - the number of permits to acquire
Parameters:
timeout - the maximum time to wait for the permits
Parameters:
unit - the time unit of the timeout argument. true if all permits were acquired and falseif the waiting time elapsed before all permits were acquired.
throws:
InterruptedException - if the current thread is interrupted
throws:
IllegalArgumentException - if permits less than zero.
See Also: Thread.interrupt |
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