OpenJDK 27 · java.util.concurrent.Semaphore · APIdia, the JavaDoc alternative

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:

 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 java.util.concurrent.locks.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.

Modifier and Type	Class and Description
pack-priv static class	Semaphore.FairSync Fair version
pack-priv static class	Semaphore.NonfairSync NonFair version
pack-priv abstract static class	Semaphore.Sync Synchronization implementation for semaphore.

Modifier and Type	Field and Description
private static final long	serialVersionUID
private final Semaphore.Sync	sync

Constructor Summary

Access	Constructor and Description
public	Semaphore(int the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. permits) Creates a `Semaphore` with the given number of permits and nonfair fairness setting.
public	Semaphore(int the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. permits, boolean `true` if this semaphore will guarantee first-in first-out granting of permits under contention, else `false` fair) Creates a `Semaphore` with the given number of permits and the given fairness setting.

Method Summary

Modifier and Type	Method and Description
public void	acquire() Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted.
public void	acquire(int the number of permits to acquire permits) Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is interrupted.
public void	acquireUninterruptibly() Acquires a permit from this semaphore, blocking until one is available.
public void	acquireUninterruptibly(int the number of permits to acquire permits) Acquires the given number of permits from this semaphore, blocking until all are available.
public int	Returns: the number of permits available in this semaphore availablePermits() Returns the current number of permits available in this semaphore.
public int	Returns: the number of permits acquired or, if negative, the number released drainPermits() Acquires and returns all permits that are immediately available, or if negative permits are available, releases them.
protected Collection<Thread>	Returns: the collection of threads getQueuedThreads() Returns a collection containing threads that may be waiting to acquire.
public final int	Returns: the estimated number of threads waiting for this lock getQueueLength() Returns an estimate of the number of threads waiting to acquire.
public final boolean	Returns: `true` if there may be other threads waiting to acquire the lock hasQueuedThreads() Queries whether any threads are waiting to acquire.
public boolean	Returns: `true` if this semaphore has fairness set true isFair() Returns `true` if this semaphore has fairness set true.
protected void	reducePermits(int the number of permits to remove 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 the number of permits to release permits) Releases the given number of permits, returning them to the semaphore.
public String	Returns: a string identifying this semaphore, as well as its state toString() Overrides java.lang.Object.toString. Returns a string identifying this semaphore, as well as its state.
public boolean	Returns: `true` if a permit was acquired and `false` otherwise tryAcquire() Acquires a permit from this semaphore, only if one is available at the time of invocation.
public boolean	Returns: `true` if a permit was acquired and `false` if the waiting time elapsed before a permit was acquired tryAcquire(long the maximum time to wait for a permit timeout, TimeUnit the time unit of the `timeout` argument unit) Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been interrupted.
public boolean	Returns: `true` if the permits were acquired and `false` otherwise tryAcquire(int the number of permits to acquire permits) Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.
public boolean	Returns: `true` if all permits were acquired and `false` if the waiting time elapsed before all permits were acquired tryAcquire(int the number of permits to acquire permits, long the maximum time to wait for the permits timeout, TimeUnit the time unit of the `timeout` argument 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.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll wait wait wait

Field Detail

serialVersionUID	back to summary
private static final long serialVersionUID

sync	back to summary
private final Semaphore.Sync sync

Constructor Detail

Semaphore back to summary

Semaphore	back to summary
public 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.

public 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 back to summary

Semaphore	back to summary
public 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`

public 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

Method Detail

acquire back to summary

acquire	back to summary
public void acquire() throws InterruptedException 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: Some other thread invokes the `release` method for this semaphore and the current thread is next to be assigned a permit; or Some other thread interrupts the current thread. If the current thread: has its interrupted status set on entry to this method; or is interrupted while waiting for a permit, then `InterruptedException` is thrown and the current thread's interrupted status is cleared. Exceptions InterruptedException: if the current thread is interrupted

public void acquire() throws InterruptedException

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:

Some other thread invokes the release method for this semaphore and the current thread is next to be assigned a permit; or
Some other thread interrupts the current thread.

If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting for a permit,

then InterruptedException is thrown and the current thread's interrupted status is cleared.

Exceptions

InterruptedException:: if the current thread is interrupted

acquire back to summary

acquire	back to summary
public void acquire(int permits) throws InterruptedException 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: 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; or Some other thread interrupts the current thread. If the current thread: has its interrupted status set on entry to this method; or is 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 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 Exceptions InterruptedException: if the current thread is interrupted IllegalArgumentException: if `permits` is negative

public void acquire(int permits) throws InterruptedException

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:

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; or
Some other thread interrupts the current thread.

If the current thread:

has its interrupted status set on entry to this method; or
is 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 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

Exceptions

InterruptedException:: if the current thread is interrupted
IllegalArgumentException:: if permits is negative

acquireUninterruptibly back to summary

acquireUninterruptibly	back to summary
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 `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 interrupted status will be set.

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 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 interrupted status will be set.

acquireUninterruptibly back to summary

acquireUninterruptibly	back to summary
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. 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 interrupted status will be set. Parameters permits:int the number of permits to acquire Exceptions IllegalArgumentException: if `permits` is negative

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. 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 interrupted status will be set.

Parameters

permits:int: the number of permits to acquire

Exceptions

IllegalArgumentException:: if permits is negative

availablePermits	back to summary
public 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

availablePermits

back to summary

public 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

drainPermits	back to summary
public int drainPermits() Acquires and returns all permits that are immediately available, or if negative permits are available, releases them. Upon return, zero permits are available. Returns:int the number of permits acquired or, if negative, the number released

drainPermits

back to summary

public int drainPermits()

Acquires and returns all permits that are immediately available, or if negative permits are available, releases them. Upon return, zero permits are available.

Returns:int: the number of permits acquired or, if negative, the number released

getQueuedThreads	back to summary
protected 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

getQueuedThreads

back to summary

protected 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

getQueueLength	back to summary
public final 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

getQueueLength

back to summary

public final 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

hasQueuedThreads back to summary

hasQueuedThreads	back to summary
public final 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

public final 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

isFair back to summary

isFair	back to summary
public boolean isFair() Returns `true` if this semaphore has fairness set true. Returns:boolean `true` if this semaphore has fairness set true

public boolean isFair()

Returns true if this semaphore has fairness set true.

Returns:boolean: true if this semaphore has fairness set true

reducePermits back to summary

reducePermits	back to summary
protected 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 Exceptions IllegalArgumentException: if `reduction` is negative

protected 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

Exceptions

IllegalArgumentException:: if reduction is negative

release back to summary

release	back to summary
public 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.

public 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.

release back to summary

release	back to summary
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 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 Exceptions IllegalArgumentException: if `permits` is negative

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 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.

Parameters

permits:int: the number of permits to release

Exceptions

IllegalArgumentException:: if permits is negative

toString back to summary

toString	back to summary
public String toString() Overrides java.lang.Object.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

public String toString()

Overrides java.lang.Object.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

tryAcquire back to summary

tryAcquire	back to summary
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. 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

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. 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

tryAcquire back to summary

tryAcquire	back to summary
public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException 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: Some other thread invokes the `release` method for this semaphore and the current thread is next to be assigned a permit; or Some other thread 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 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: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 Exceptions InterruptedException: if the current thread is interrupted

public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException

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:

Some other thread invokes the release method for this semaphore and the current thread is next to be assigned a permit; or
Some other thread 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 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: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

Exceptions

InterruptedException:: if the current thread is interrupted

tryAcquire back to summary

tryAcquire	back to summary
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. 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 Exceptions IllegalArgumentException: if `permits` is negative

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. 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

Exceptions

IllegalArgumentException:: if permits is negative

tryAcquire back to summary

tryAcquire	back to summary
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException 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: 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; or Some other thread 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 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 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 Exceptions InterruptedException: if the current thread is interrupted IllegalArgumentException: if `permits` is negative

public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException

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:

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; or
Some other thread 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 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 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

Exceptions

InterruptedException:: if the current thread is interrupted
IllegalArgumentException:: if permits is negative

Modifier and Type	Method and Description
pack-priv final int	drainPermits()
pack-priv final int	getPermits()
pack-priv final int	nonfairTryAcquireShared(int acquires)
pack-priv final void	reducePermits(int reductions)
protected final boolean	tryReleaseShared(int releases) Overrides java.util.concurrent.locks.AbstractQueuedSynchronizer.tryReleaseShared. Attempts to set the state to reflect a release in shared mode.

public Class Semaphore

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pack-priv final Class Semaphore.FairSync

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pack-priv final Class Semaphore.NonfairSync

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pack-priv abstract Class Semaphore.Sync

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