Semaphore JDK 1.6)

Description

public class Semaphore

A counting semaphore.

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each Semaphore.acquire() blocks if necessary until a permit is available, and then takes it. Each Semaphore.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 Semaphore.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 Semaphore.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. Beware of the increased risk of indefinite postponement when these methods are used without fairness set true.

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.

See also:

Methods

Hide/Show inherited methods

public void acquire () throws InterruptedException

Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted.

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.

public void acquireUninterruptibly ()

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

public void acquireUninterruptibly (int permits)

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

public int availablePermits ()

Returns the current number of permits available in this semaphore.

public int drainPermits ()

Acquires and returns all permits that are immediately available.

protected Collection<Thread> getQueuedThreads ()

Returns a collection containing threads that may be waiting to acquire.

publicfinal int getQueueLength ()

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

publicfinal boolean hasQueuedThreads ()

Queries whether any threads are waiting to acquire.

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: {@code true} if there may be other threads waiting to acquire the lock

public boolean isFair ()

Returns true if this semaphore has fairness set true.

Returns true if this semaphore has fairness set true.
Returns: {@code true} if this semaphore has fairness set true

protected void reducePermits (int reduction)

Shrinks the number of available permits by the indicated 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 - the number of permits to remove
Throws:
- IllegalArgumentException - if {@code reduction} is negative

public void release ()

Releases a permit, returning it to the semaphore.

public void release (int permits)

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

public String toString ()

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

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: a string identifying this semaphore, as well as its state

public boolean tryAcquire ()

Acquires a permit from this semaphore, only if one is available at the time of invocation.

public boolean tryAcquire (int permits)

Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.

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, 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 Semaphore.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 Semaphore.release().
Returns: {@code true} if all permits were acquired and {@code false} if the waiting time elapsed before all permits were acquired
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
Throws:
- InterruptedException - if the current thread is interrupted
- IllegalArgumentException - if {@code permits} is negative

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.