public
class
Semaphore
extends Object
implements
Serializable
| java.lang.Object | |
| ↳ | java.util.concurrent.Semaphore |
A counting semaphore. Conceptually, a semaphore maintains a set of
permits. Each acquire() blocks if necessary until a permit is
available, and then takes it. Each 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:
Developers
class Pool {
private static final int 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 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 acquire() can be allocated a permit ahead of a
thread that has been waiting - logically the new thread places itself at
the head of the queue of waiting threads. When fairness is set true, the
semaphore guarantees that threads invoking any of the acquire methods are selected to obtain 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.
Also note that the untimed tryAcquire methods do not
honor the fairness setting, but will take any permits that are
available.
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 acquire and release multiple
permits at a time. These methods are generally more efficient and
effective than loops. However, they do not establish any preference
order. For example, if thread A invokes s.acquire(3) and
thread B invokes s.acquire(2), and two permits become
available, then there is no guarantee that thread B will obtain
them unless its acquire came first and Semaphore s is in
fair mode.
Memory consistency effects: Actions in a thread prior to calling
a "release" method such as release()
happen-before
actions following a successful "acquire" method such as acquire()
in another thread.
Public constructors | |
|---|---|
Semaphore(int permits)
Creates a |
|
Semaphore(int permits, boolean fair)
Creates a |
|
Public methods | |
|---|---|
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(int permits)
Acquires the given number of permits from this semaphore, blocking until all are available. |
void
|
acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is available. |
int
|
availablePermits()
Returns the current number of permits available in this semaphore. |
int
|
drainPermits()
Acquires and returns all permits that are immediately available. |
final
int
|
getQueueLength()
Returns an estimate of the number of threads waiting to acquire. |
final
boolean
|
hasQueuedThreads()
Queries whether any threads are waiting to acquire. |
boolean
|
isFair()
Returns |
void
|
release(int permits)
Releases the given number of permits, returning them to the semaphore. |
void
|
release()
Releases a permit, returning it to the semaphore. |
String
|
toString()
Returns a string identifying this semaphore, as well as its state. |
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. |
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()
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. |
Protected methods | |
|---|---|
Collection<Thread>
|
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire. |
void
|
reducePermits(int reduction)
Shrinks the number of available permits by the indicated reduction. |
Inherited methods | |
|---|---|
 From
class
java.lang.Object
| |
Semaphore (int permits)
Creates a Semaphore with the given number of
permits and nonfair fairness setting.
| Parameters | |
|---|---|
permits |
int:
the initial number of permits available.
This value may be negative, in which case releases
must occur before any acquires will be granted.
|
Semaphore (int permits,
boolean fair)
Creates a Semaphore with the given number of
permits and the given fairness setting.
| Parameters | |
|---|---|
permits |
int:
the initial number of permits available.
This value may be negative, in which case releases
must occur before any acquires will be granted. |
fair |
boolean:
true if this semaphore will guarantee
first-in first-out granting of permits under contention,
else false
|
void acquire ()
Acquires a permit from this semaphore, blocking until one is available, or the thread is 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:
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.
| Throws | |
|---|---|
InterruptedException |
if the current 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.
Acquires the given number of permits, if they are available,
and returns immediately, reducing the number of available permits
by the given amount. This method has the same effect as the
loop for (int i = 0; i < permits; ++i) acquire(); except
that it atomically acquires the permits all at once:
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 and 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().
| Parameters | |
|---|---|
permits |
int:
the number of permits to acquire |
| Throws | |
|---|---|
InterruptedException |
if the current thread is interrupted |
IllegalArgumentException |
if permits is negative
|
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. This method has the same effect as the
loop for (int i = 0; i < permits; ++i) acquireUninterruptibly();
except that it atomically acquires the permits all at once:
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 and 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.
| Parameters | |
|---|---|
permits |
int:
the number of permits to acquire |
| Throws | |
|---|---|
IllegalArgumentException |
if permits is negative
|
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 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.
int availablePermits ()
Returns the current number of permits available in this semaphore.
This method is typically used for debugging and testing purposes.
| Returns | |
|---|---|
int |
the number of permits available in this semaphore |
int drainPermits ()
Acquires and returns all permits that are immediately available.
| Returns | |
|---|---|
int |
the number of permits acquired |
int getQueueLength ()
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 system state, not for synchronization control.
| Returns | |
|---|---|
int |
the estimated number of threads waiting for this lock |
boolean hasQueuedThreads ()
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.
| Returns | |
|---|---|
boolean |
true if there may be other threads waiting to
acquire the lock
|
boolean isFair ()
Returns true if this semaphore has fairness set true.
| Returns | |
|---|---|
boolean |
true if this semaphore has fairness set true
|
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 trying to acquire permits, then one thread 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.
| Parameters | |
|---|---|
permits |
int:
the number of permits to release |
| Throws | |
|---|---|
IllegalArgumentException |
if permits is negative
|
void release ()
Releases a permit, returning it to the semaphore.
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.
String toString ()
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.
| Returns | |
|---|---|
String |
a string identifying this semaphore, as well as its state |
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.
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.
| Parameters | |
|---|---|
timeout |
long:
the maximum time to wait for a permit |
unit |
TimeUnit:
the time unit of the timeout argument |
| Returns | |
|---|---|
boolean |
true if a permit was acquired and false
if the waiting time elapsed before a permit was acquired |
| Throws | |
|---|---|
InterruptedException |
if the current thread is interrupted |
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.
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 and 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().
| Parameters | |
|---|---|
permits |
int:
the number of permits to acquire |
timeout |
long:
the maximum time to wait for the permits |
unit |
TimeUnit:
the time unit of the timeout argument |
| Returns | |
|---|---|
boolean |
true if all permits were acquired and false
if the waiting time elapsed before all permits were acquired |
| Throws | |
|---|---|
InterruptedException |
if the current thread is interrupted |
IllegalArgumentException |
if permits is negative
|
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.
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).
| Returns | |
|---|---|
boolean |
true if a permit was acquired and false
otherwise
|
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. 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).
| Parameters | |
|---|---|
permits |
int:
the number of permits to acquire |
| Returns | |
|---|---|
boolean |
true if the permits were acquired and
false otherwise |
| Throws | |
|---|---|
IllegalArgumentException |
if permits is negative
|
Collection<Thread> getQueuedThreads ()
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.
| Returns | |
|---|---|
Collection<Thread> |
the collection of threads |
void reducePermits (int reduction)
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 |
int:
the number of permits to remove |
| Throws | |
|---|---|
IllegalArgumentException |
if reduction is negative
|