edu.emory.mathcs.backport.java.util.concurrent
public class Semaphore extends Object implements Serializable
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 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 Semaphore 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 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
{@code 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. BACKPORT NOTE: currently, these methods are only
supported for FAIR semaphores.
Memory consistency effects: Actions in a thread prior to calling a "release" method such as {@code release()} happen-before actions following a successful "acquire" method such as {@code acquire()} in another thread.
Since: 1.5
Constructor Summary | |
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Semaphore(int permits)
Creates a {@code Semaphore} with the given number of
permits and nonfair fairness setting.
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Semaphore(int permits, boolean fair)
Creates a {@code Semaphore} with the given number of
permits and the given fairness setting.
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Method Summary | |
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void | acquire()
Acquires a permit from this semaphore, blocking until one is
available, or the thread is Thread#interrupt interrupted.
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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.
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void | acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is
available.
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void | acquireUninterruptibly(int permits)
Acquires the given number of permits from this semaphore,
blocking until all are available.
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int | availablePermits()
Returns the current number of permits available in this semaphore.
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int | drainPermits()
Acquires and returns all permits that are immediately available.
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protected Collection | getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire.
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int | getQueueLength()
Returns an estimate of the number of threads waiting to acquire.
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boolean | hasQueuedThreads()
Queries whether any threads are waiting to acquire. |
boolean | isFair()
Returns {@code true} if this semaphore has fairness set true.
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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.
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void | release(int permits)
Releases the given number of permits, returning them to the semaphore.
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String | toString()
Returns a string identifying this semaphore, as well as its state.
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boolean | tryAcquire()
Acquires a permit from this semaphore, only if one is available at the
time of invocation.
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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.
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boolean | tryAcquire(int permits)
Acquires the given number of permits from this semaphore, only
if all are available at the time of invocation.
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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.
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Parameters: permits the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted.
Parameters: 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 {@code true} if this semaphore will guarantee first-in first-out granting of permits under contention, else {@code false}
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:
If the current thread:
Throws: InterruptedException if the current 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.
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:
Parameters: permits the number of permits to acquire
Throws: InterruptedException if the current thread is interrupted IllegalArgumentException if {@code permits} is negative
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 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.
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 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 {@code permits} is negative
This method is typically used for debugging and testing purposes.
Returns: the number of permits available in this semaphore
Returns: the number of permits acquired
Returns: the collection of threads
Returns: the estimated number of threads waiting for this lock
Returns: {@code true} if there may be other threads waiting to acquire the lock
Returns: {@code true} if this semaphore has fairness set true
Parameters: reduction the number of permits to remove
Throws: IllegalArgumentException if {@code reduction} is negative
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 Semaphore. Correct usage of a semaphore is established by programming convention in the application.
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.
Parameters: permits the number of permits to release
Throws: IllegalArgumentException if {@code permits} is negative
Returns: a string identifying this semaphore, as well as its state
Acquires a permit, if one is available and returns immediately, with the value {@code true}, reducing the number of available permits by one.
If no permit is available then this method will return immediately with the value {@code false}.
Even when this semaphore has been set to use a
fair ordering policy, a call to {@code 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: {@code true} if a permit was acquired and {@code false} otherwise
Acquires a permit, if one is available and returns immediately, with the value {@code 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:
If a permit is acquired then the value {@code true} is returned.
If the current thread:
If the specified waiting time elapses then the value {@code 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 unit the time unit of the {@code timeout} argument
Returns: {@code true} if a permit was acquired and {@code false} if the waiting time elapsed before a permit was acquired
Throws: InterruptedException if the current thread is interrupted
Acquires the given number of permits, if they are available, and returns immediately, with the value {@code 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 {@code 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 {@code 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 the number of permits to acquire
Returns: {@code true} if the permits were acquired and {@code false} otherwise
Throws: IllegalArgumentException if {@code permits} is negative
Acquires the given number of permits, if they are available and returns immediately, with the value {@code 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 {@code true} is returned.
If the current thread:
If the specified waiting time elapses then the value {@code 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 the number of permits to acquire timeout the maximum time to wait for the permits unit the time unit of the {@code timeout} argument
Returns: {@code true} if all permits were acquired and {@code false} if the waiting time elapsed before all permits were acquired
Throws: InterruptedException if the current thread is interrupted IllegalArgumentException if {@code permits} is negative