Added in API level 1

Semaphore

open class Semaphore : Serializable
kotlin.Any
   ↳ 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: 

<code>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 &lt; 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 &lt; MAX_AVAILABLE; ++i) {
        if (item == items[i]) {
          if (used[i]) {
            used[i] = false;
            return true;
          } else
            return false;
        }
      }
      return false;
    }
  }</code>

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.

Summary

Public constructors
Semaphore(permits: Int)

Creates a Semaphore with the given number of permits and nonfair fairness setting.
Semaphore(permits: Int, fair: Boolean)

Creates a Semaphore with the given number of permits and the given fairness setting.
Public methods
open Unit

Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted.
open Unit
acquire(permits: Int)

Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is interrupted.
open Unit

Acquires a permit from this semaphore, blocking until one is available.
open Unit

Acquires the given number of permits from this semaphore, blocking until all are available.
open Int

Returns the current number of permits available in this semaphore.
open Int

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

Returns an estimate of the number of threads waiting to acquire.
Boolean

Queries whether any threads are waiting to acquire.
open Boolean

Returns true if this semaphore has fairness set true.
open Unit

Releases a permit, returning it to the semaphore.
open Unit
release(permits: Int)

Releases the given number of permits, returning them to the semaphore.
open String

Returns a string identifying this semaphore, as well as its state.
open Boolean

Acquires a permit from this semaphore, only if one is available at the time of invocation.
open Boolean
tryAcquire(timeout: Long, unit: TimeUnit!)

Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been interrupted.
open Boolean
tryAcquire(permits: Int)

Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.
open Boolean
tryAcquire(permits: Int, timeout: Long, unit: TimeUnit!)

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
Protected methods
open MutableCollection<Thread!>!

Returns a collection containing threads that may be waiting to acquire.
open Unit
reducePermits(reduction: Int)

Shrinks the number of available permits by the indicated reduction.

Public constructors

Semaphore

Added in API level 1
Semaphore(permits: Int)

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

Added in API level 1
Semaphore(
    permits: Int, 
    fair: Boolean)

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 methods

acquire

Added in API level 1
open fun acquire(): Unit

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
java.lang.InterruptedException if the current thread is interrupted

acquire

Added in API level 1
open fun acquire(permits: Int): Unit

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
java.lang.InterruptedException if the current thread is interrupted
java.lang.IllegalArgumentException if permits is negative

acquireUninterruptibly

Added in API level 1
open fun acquireUninterruptibly(): Unit

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.

acquireUninterruptibly

Added in API level 1
open fun acquireUninterruptibly(permits: Int): Unit

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
Exceptions
java.lang.IllegalArgumentException if permits is negative

availablePermits

Added in API level 1
open fun availablePermits(): Int

Returns the current number of permits available in this semaphore.

This method is typically used for debugging and testing purposes.

Return
Int the number of permits available in this semaphore

drainPermits

Added in API level 1
open fun drainPermits(): Int

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

Return
Int the number of permits acquired or, if negative, the number released

getQueueLength

Added in API level 1
fun getQueueLength(): Int

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.

Return
Int the estimated number of threads waiting for this lock

hasQueuedThreads

Added in API level 1
fun hasQueuedThreads(): Boolean

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.

Return
Boolean true if there may be other threads waiting to acquire the lock

isFair

Added in API level 1
open fun isFair(): Boolean

Returns true if this semaphore has fairness set true.

Return
Boolean true if this semaphore has fairness set true

release

Added in API level 1
open fun release(): Unit

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

Added in API level 1
open fun release(permits: Int): Unit

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 java.util.concurrent.Semaphore#acquire. Correct usage of a semaphore is established by programming convention in the application.

Parameters
permits Int: the number of permits to release
Exceptions
java.lang.IllegalArgumentException if permits is negative

toString

Added in API level 1
open fun toString(): String

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.

Return
String a string identifying this semaphore, as well as its state

tryAcquire

Added in API level 1
open fun tryAcquire(): Boolean

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).

Return
Boolean true if a permit was acquired and false otherwise

tryAcquire

Added in API level 1
open fun tryAcquire(
    timeout: Long, 
    unit: TimeUnit!
): Boolean

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
Return
Boolean true if a permit was acquired and false if the waiting time elapsed before a permit was acquired
Exceptions
java.lang.InterruptedException if the current thread is interrupted

tryAcquire

Added in API level 1
open fun tryAcquire(permits: Int): Boolean

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
Return
Boolean true if the permits were acquired and false otherwise
Exceptions
java.lang.IllegalArgumentException if permits is negative

tryAcquire

Added in API level 1
open fun tryAcquire(
    permits: Int, 
    timeout: Long, 
    unit: TimeUnit!
): Boolean

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
Return
Boolean true if all permits were acquired and false if the waiting time elapsed before all permits were acquired
Exceptions
java.lang.InterruptedException if the current thread is interrupted
java.lang.IllegalArgumentException if permits is negative

Protected methods

getQueuedThreads

Added in API level 1
protected open fun getQueuedThreads(): MutableCollection<Thread!>!

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.

Return
MutableCollection<Thread!>! the collection of threads

reducePermits

Added in API level 1
protected open fun reducePermits(reduction: Int): Unit

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
java.lang.IllegalArgumentException if reduction is negative