com.hazelcast.core

Interface ISemaphore

All Superinterfaces:

DistributedObject

All Known Implementing Classes:

ClientSemaphoreProxy, SemaphoreProxy

public interface ISemaphore
extends DistributedObject

ISemaphore is a backed-up distributed alternative to the Semaphore.

ISemaphore is a cluster-wide counting semaphore. Conceptually, it 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.

The Hazelcast distributed semaphore implementation 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 the cluster. Therefore, it is possible for one member to invoke acquire before another, but reach the ordering point after the other, and similarly upon return from the method.

This class also provides convenience methods to acquire and release multiple permits at a time. Beware of the increased risk of indefinite postponement when using the multiple acquire. If a single permit is released to a semaphore that is currently blocking, a thread waiting for one permit will acquire it before a thread waiting for multiple permits regardless of the call order.

• Correct usage of a semaphore is established by programming convention in the application.

During a split-brain scenario, atomicity is lost. The same ISemaphore can exist within each cluster partition and therefore the permit counters within will diverge. When the cluster heals, Hazelcast will choose one of the ISemaphores, by default this is selected from the largest partitioned cluster (by number of members). If the cluster sizes are all equal then a random ISemaphore will be chosen.

Method Summary

Modifier and Type	Method and Description
void	acquire() Acquires a permit if one is available, and returns immediately, reducing the number of available permits by one.
void	acquire(int permits) Acquires the given number of permits if they are available, and returns immediately, reducing the number of available permits by the given amount.
int	availablePermits() Returns the current number of permits currently available in this semaphore.
int	drainPermits() Acquires and returns all permits that are immediately available.
String	getName() Returns the name of this ISemaphore instance.
boolean	init(int permits) Try to initialize this ISemaphore instance with the given permit count
void	reducePermits(int reduction) Shrinks the number of available permits by the indicated reduction.
void	release() Releases a permit, increasing the number of available permits by one.
void	release(int permits) Releases the given number of permits, increasing the number of available permits by that amount.
boolean	tryAcquire() Acquires a permit, if one is available and returns immediately, with the value true, reducing the number of available permits by one.
boolean	tryAcquire(int permits) 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.
boolean	tryAcquire(int permits, long timeout, TimeUnit unit) 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.
boolean	tryAcquire(long timeout, TimeUnit unit) Acquires a permit from this semaphore if one becomes available within the given waiting time and the current thread has not been interrupted.

Methods inherited from interface com.hazelcast.core.DistributedObject

destroy, getPartitionKey, getServiceName

Method Detail

getName

String getName()

Returns the name of this ISemaphore instance.

Specified by:

getName in interface DistributedObject

Returns:

name of this instance

init

boolean init(int permits)

Try to initialize this ISemaphore instance with the given permit count

Parameters:

permits - the given permit count

Returns:

true if initialization success

acquire

void acquire()
      throws InterruptedException

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 three things happens:

• some other thread invokes one of the release() methods for this semaphore and the current thread is next to be assigned a permit,
• this ISemaphore instance is destroyed, 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.

Throws:

InterruptedException - if the current thread is interrupted

IllegalStateException - if hazelcast instance is shutdown while waiting

acquire

void acquire(int permits)
      throws InterruptedException

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 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,
• this ISemaphore instance is destroyed, 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.

Parameters:

permits - the number of permits to acquire

Throws:

InterruptedException - if the current thread is interrupted

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting

availablePermits

int availablePermits()

Returns the current number of permits currently available in this semaphore.

• This method is typically used for debugging and testing purposes.

Returns:

the number of permits available in this semaphore

drainPermits

int drainPermits()

Acquires and returns all permits that are immediately available.

Returns:

the number of permits drained

reducePermits

void reducePermits(int reduction)

Shrinks the number of available permits by the indicated reduction. 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 reduction is negative

release

void release()

Releases a permit, increasing the number of available permits by one. If any threads in the cluster are trying to acquire a permit, then one is selected and given the permit that was just released.

There is no requirement that a thread that releases a permit must have acquired that permit by calling one of the acquire methods. Correct usage of a semaphore is established by programming convention in the application.

release

void release(int permits)

Releases the given number of permits, increasing the number of available permits by that amount.

There is no requirement that a thread that releases a permit must have acquired that permit by calling one of the acquire methods. Correct usage of a semaphore is established by programming convention in the application.

Parameters:

permits - the number of permits to release

Throws:

IllegalArgumentException - if permits is negative

tryAcquire

boolean tryAcquire()

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.

Returns:

true if a permit was acquired and false otherwise

tryAcquire

boolean tryAcquire(int permits)

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.

Parameters:

permits - the number of permits to acquire

Returns:

true if the permits were acquired and false otherwise

Throws:

IllegalArgumentException - if permits is negative

tryAcquire

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 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.

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.

Parameters:

timeout - the maximum time to wait for a permit

unit - the time unit of the timeout argument

Returns:

true if a permit was acquired and false if the waiting time elapsed before a permit was acquired

Throws:

InterruptedException - if the current thread is interrupted

IllegalStateException - if hazelcast instance is shutdown while waiting

tryAcquire

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

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 true is returned.

If the specified waiting time elapses then false is returned. If the time is less than or equal to zero, the method will not wait at all.

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.

Parameters:

permits - the number of permits to acquire

timeout - the maximum time to wait for the permits

unit - the time unit of the timeout argument

Returns:

true if all permits were acquired, false if the waiting time elapsed before all permits could be acquired

Throws:

InterruptedException - if the current thread is interrupted

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting