Semaphore Class

Reference

Definition

Namespace:: Java.Util.Concurrent

Assembly:: Mono.Android.dll

Important

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A counting semaphore.

[Android.Runtime.Register("java/util/concurrent/Semaphore", DoNotGenerateAcw=true)]
public class Semaphore : Java.Lang.Object, IDisposable, Java.Interop.IJavaPeerable, Java.IO.ISerializable

[<Android.Runtime.Register("java/util/concurrent/Semaphore", DoNotGenerateAcw=true)>]
type Semaphore = class
    inherit Object
    interface ISerializable
    interface IJavaObject
    interface IDisposable
    interface IJavaPeerable

Inheritance: Object

Object
Semaphore

Attributes: RegisterAttribute

Implements: IJavaObject IJavaPeerable ISerializable IDisposable

Remarks

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 < 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() 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() 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(int) acquire and #release(int) 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.

Added in 1.5.

Java documentation for java.util.concurrent.Semaphore.

Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.

Constructors

Semaphore(Int32, Boolean)	Creates a `Semaphore` with the given number of permits and the given fairness setting.
Semaphore(Int32)	Creates a `Semaphore` with the given number of permits and nonfair fairness setting.
Semaphore(IntPtr, JniHandleOwnership)	A constructor used when creating managed representations of JNI objects; called by the runtime.

Properties

Class	Returns the runtime class of this `Object`. (Inherited from Object)
Handle	The handle to the underlying Android instance. (Inherited from Object)
HasQueuedThreads	Queries whether any threads are waiting to acquire.
IsFair	Returns `true` if this semaphore has fairness set true.
JniIdentityHashCode	(Inherited from Object)
JniPeerMembers
PeerReference	(Inherited from Object)
QueuedThreads	Returns a collection containing threads that may be waiting to acquire.
QueueLength	Returns an estimate of the number of threads waiting to acquire.
ThresholdClass	This API supports the Mono for Android infrastructure and is not intended to be used directly from your code.
ThresholdType	This API supports the Mono for Android infrastructure and is not intended to be used directly from your code.

Methods

Acquire()	Acquires a permit from this semaphore, blocking until one is available, or the thread is Thread#interrupt interrupted.
Acquire(Int32)	Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is Thread#interrupt interrupted.
AcquireUninterruptibly()	Acquires a permit from this semaphore, blocking until one is available.
AcquireUninterruptibly(Int32)	Acquires the given number of permits from this semaphore, blocking until all are available.
AvailablePermits()	Returns the current number of permits available in this semaphore.
Clone()	Creates and returns a copy of this object. (Inherited from Object)
Dispose()	(Inherited from Object)
Dispose(Boolean)	(Inherited from Object)
DrainPermits()	Acquires and returns all permits that are immediately available, or if negative permits are available, releases them.
Equals(Object)	Indicates whether some other object is "equal to" this one. (Inherited from Object)
GetHashCode()	Returns a hash code value for the object. (Inherited from Object)
JavaFinalize()	Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. (Inherited from Object)
Notify()	Wakes up a single thread that is waiting on this object's monitor. (Inherited from Object)
NotifyAll()	Wakes up all threads that are waiting on this object's monitor. (Inherited from Object)
ReducePermits(Int32)	Shrinks the number of available permits by the indicated reduction.
Release()	Releases a permit, returning it to the semaphore.
Release(Int32)	Releases the given number of permits, returning them to the semaphore.
SetHandle(IntPtr, JniHandleOwnership)	Sets the Handle property. (Inherited from Object)
ToArray<T>()	(Inherited from Object)
ToString()	Returns a string representation of the object. (Inherited from Object)
TryAcquire()	Acquires a permit from this semaphore, only if one is available at the time of invocation.
TryAcquire(Int32, Int64, 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 Thread#interrupt interrupted.
TryAcquire(Int32)	Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.
TryAcquire(Int64, TimeUnit)	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.
UnregisterFromRuntime()	(Inherited from Object)
Wait()	Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>. (Inherited from Object)
Wait(Int64, Int32)	Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object)
Wait(Int64)	Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object)

Explicit Interface Implementations

IJavaPeerable.Disposed()	(Inherited from Object)
IJavaPeerable.DisposeUnlessReferenced()	(Inherited from Object)
IJavaPeerable.Finalized()	(Inherited from Object)
IJavaPeerable.JniManagedPeerState	(Inherited from Object)
IJavaPeerable.SetJniIdentityHashCode(Int32)	(Inherited from Object)
IJavaPeerable.SetJniManagedPeerState(JniManagedPeerStates)	(Inherited from Object)
IJavaPeerable.SetPeerReference(JniObjectReference)	(Inherited from Object)

Extension Methods

JavaCast<TResult>(IJavaObject)	Performs an Android runtime-checked type conversion.
JavaCast<TResult>(IJavaObject)
GetJniTypeName(IJavaPeerable)

Applies to

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