jdk.util.concurrent.Semaphore — clojure-interop/java.util.concurrent 1.0.5

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 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. 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.

->semaphore clj

(->semaphore permits)

(->semaphore permits fair)

Constructor.

Creates a Semaphore with the given number of permits and the given fairness setting.

permits - the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. - int fair - true if this semaphore will guarantee first-in first-out granting of permits under contention, else false - boolean

acquire clj

(acquire this)

(acquire this permits)

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.

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, 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().

permits - the number of permits to acquire - int

throws: java.lang.InterruptedException - if the current thread is interrupted

acquire-uninterruptibly clj

(acquire-uninterruptibly this)

(acquire-uninterruptibly this 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.

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 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.

permits - the number of permits to acquire - int

throws: java.lang.IllegalArgumentException - if permits is negative

available-permits clj

(available-permits this)

Returns the current number of permits available in this semaphore.

This method is typically used for debugging and testing purposes.

returns: the number of permits available in this semaphore - int

drain-permits clj

(drain-permits this)

Acquires and returns all permits that are immediately available.

returns: the number of permits acquired - int

fair? clj

(fair? this)

Returns true if this semaphore has fairness set true.

returns: true if this semaphore has fairness set true - boolean

get-queue-length clj

(get-queue-length this)

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 of the system state, not for synchronization control.

returns: the estimated number of threads waiting for this lock - int

has-queued-threads? clj

(has-queued-threads? this)

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: true if there may be other threads waiting to acquire the lock - boolean

release clj

(release this)

(release this 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 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.

permits - the number of permits to release - int

throws: java.lang.IllegalArgumentException - if permits is negative

to-string clj

(to-string this)

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 - java.lang.String

try-acquire clj

(try-acquire this)

(try-acquire this permits)

(try-acquire this timeout unit)

(try-acquire this permits timeout 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.

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 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().

permits - the number of permits to acquire - int timeout - the maximum time to wait for the permits - long unit - the time unit of the timeout argument - java.util.concurrent.TimeUnit

returns: true if all permits were acquired and false if the waiting time elapsed before all permits were acquired - boolean

throws: java.lang.InterruptedException - if the current thread is interrupted