com.hazelcast.core
Interface ISemaphore

All Superinterfaces:

Instance

public interface ISemaphore
extends Instance

ISemaphore is a backed-up distributed implementation of java.util.concurrent.Semaphore.

Hazelcast's 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. So, 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.

The Hazelcast semaphore also allows you to attach()/detach() permits to the caller address. This provides a safety mechanism in case that address becomes disconnected from the cluster. Attached permits will automatically be released to the semaphore if a disconnection occurs. An address can also have an excess number of detached permits and is represented by a negative attached permit count. This is the number of permits that the semaphore will automatically be reduced by if a disconnection occurs.

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.

Since:

1.9.3

Nested Class Summary

Nested classes/interfaces inherited from interface com.hazelcast.core.Instance

Instance.InstanceType

Method Summary

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.
Future	acquireAsync() Asynchronously acquires a permit.
Future	acquireAsync(int permits) Asynchronously acquires a given number of permits.
void	acquireAttach() Acquires and attaches a permit to the caller's address.
void	acquireAttach(int permits) Acquires and attaches the given number of permits to the caller's address.
Future	acquireAttachAsync() Asynchronously acquires and attaches a permit to the caller's address.
Future	acquireAttachAsync(int permits) Asynchronously acquires and attaches the given number of permits to the caller's address.
void	attach() Attaches a permit to the caller's address.
void	attach(int permits) Attaches the given number of permits to the caller's address.
int	attachedPermits() Returns the current number of permits attached to the caller's address.
int	availablePermits() Returns the current number of permits currently available in this semaphore.
void	detach() Detaches a permit from the caller's address.
void	detach(int permits) Detaches the given number of permits from the caller's address.
int	drainPermits() Acquires and returns all permits that are immediately available.
LocalSemaphoreStats	getLocalSemaphoreStats()
String	getName() Returns the name of this ISemaphore instance.
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.
void	releaseDetach() Detaches a permit from the caller's address and returns it to the semaphore.
void	releaseDetach(int permits) Detaches the given number of permits from the caller's address and returns them to the semaphore.
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.
boolean	tryAcquireAttach() Acquires a permit from this semaphore and attaches it to the calling member, only if one is available at the time of invocation.
boolean	tryAcquireAttach(int permits) Acquires the given number of permits from this semaphore and attaches them to the calling member, only if all are available at the time of invocation.
boolean	tryAcquireAttach(int permits, long timeout, TimeUnit unit) Acquires the given number of permits from this semaphore and attaches them to the calling member, only if all become available within the given waiting time and the current thread has not been interrupted or the instance destroyed.
boolean	tryAcquireAttach(long timeout, TimeUnit unit) Acquires a permit from this semaphore and attaches it to the calling member, only if one becomes available within the given waiting time and the current thread has not been interrupted or the instance destroyed.

Methods inherited from interface com.hazelcast.core.Instance

destroy, getId, getInstanceType

Method Detail

getName

String getName()

Returns the name of this ISemaphore instance.

Returns:

name of this instance

acquire

void acquire()
             throws InstanceDestroyedException,
                    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 ISemaphore instance is destroyed while the thread is waiting then InstanceDestroyedException will be thrown.

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:

InstanceDestroyedException - if the instance is destroyed while waiting

InterruptedException - if the current thread is interrupted

IllegalStateException - if hazelcast instance is shutdown while waiting

acquire

void acquire(int permits)
             throws InstanceDestroyedException,
                    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 ISemaphore instance is destroyed while waiting then InstanceDestroyedException will be thrown.

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:

InstanceDestroyedException - if the instance is destroyed while waiting

InterruptedException - if the current thread is interrupted

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting

acquireAsync

Future acquireAsync()

Asynchronously acquires a permit.

 Future future = semaphore.acquireAsync();
 // do some other stuff, when ready get the result
 Object value = future.get();
 

Future.get() will block until the actual acquire() completes. If the application requires timely response, then Future.get(timeout, timeunit) can be used.

 Future future = semaphore.acquireAsync();
 try{
   Object value = future.get(40, TimeUnit.MILLISECOND);
 }catch (TimeoutException t) {
   // time wasn't enough
 }
 

This method itself does not throw any exceptions. Exceptions occur during the future.get() operation.

If insufficient permits are available when calling future.get() then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four 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;
• The get method is called with a waiting time that elapses; or
• Some other thread interrupts the current thread.

If the ISemaphore instance is destroyed while the future.get() is waiting then ExecutionException will be thrown with InstanceDestroyedException set as it's cause.

If the Hazelcast instance is shutdows while the future.get() is waiting then IllegalStateException will be thrown.

If when calling future.get() a timeout is specified and the a permit cannot be acquired within the timeout period TimeoutException will be thrown.

If when calling future.get() 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.

If the thread future.get() throws an InterruptedException, the acquire request is still outstanding and future.get() may be called again. To cancel the actual acquire, future.cancel() must be called. If the cancel method returns false then it was too late to cancel the request and the permit was acquired. The future.cancel() mayInterruptIfRunning argument is ignored.

Returns:

Future from which the acquire result can be obtained.

See Also:

Future

acquireAsync

Future acquireAsync(int permits)

Asynchronously acquires a given number of permits.

See acquireAsync().

Parameters:

permits - the number of permits to acquire

Throws:

IllegalArgumentException - if permits is negative

acquireAttach

void acquireAttach()
                   throws InstanceDestroyedException,
                          InterruptedException

Acquires and attaches a permit to the caller's address.

See acquire() and attach().

Throws:

IllegalStateException - if hazelcast instance is shutdown while waiting

InstanceDestroyedException

InterruptedException

acquireAttach

void acquireAttach(int permits)
                   throws InstanceDestroyedException,
                          InterruptedException

Acquires and attaches the given number of permits to the caller's address.

See acquire() and attach().

Parameters:

permits - the number of permits to acquire and attach

Throws:

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting

InstanceDestroyedException

InterruptedException

acquireAttachAsync

Future acquireAttachAsync()

Asynchronously acquires and attaches a permit to the caller's address.

See acquireAsync() and attach().

acquireAttachAsync

Future acquireAttachAsync(int permits)

Asynchronously acquires and attaches the given number of permits to the caller's address.

See acquireAsync() and attach().

Parameters:

permits - the number of permits to acquire and attach

Throws:

IllegalArgumentException - if permits is negative

attach

void attach()

Attaches a permit to the caller's address.

See attachedPermits().

attach

void attach(int permits)

Attaches the given number of permits to the caller's address.

See attachedPermits().

Parameters:

permits - the number of permits to attach

Throws:

IllegalArgumentException - if permits is negative

attachedPermits

int attachedPermits()

Returns the current number of permits attached to the caller's address. This value has no affect on the semaphores operation unless the caller address actually becomes disconnected.

Positive value is the number of permits that will automatically be released to the semaphore if the caller address becomes disconnected from the cluster.

Negative value represents the number of permits that the semaphore will automatically be reduced by if the caller address becomes disconnected from the cluster.

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

Returns:

the number of permits attached to caller addresses

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

detach

void detach()

Detaches a permit from the caller's address.

See attachedPermits().

detach

void detach(int permits)

Detaches the given number of permits from the caller's address.

See attachedPermits().

Parameters:

permits - the number of permits to detach

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

releaseDetach

void releaseDetach()

Detaches a permit from the caller's address and returns it to the semaphore.

See release() and detach().

releaseDetach

void releaseDetach(int permits)

Detaches the given number of permits from the caller's address and returns them to the semaphore.

See release(int) and detach().

Parameters:

permits - the number of permits to release and detach

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 InstanceDestroyedException,
                          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 ISemaphore instance is destroyed while the thread is waiting then InstanceDestroyedException will be thrown.

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:

InstanceDestroyedException - if the instance is destroyed while waiting

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 InstanceDestroyedException,
                          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 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 ISemaphore instance is destroyed while the thread is waiting then InstanceDestroyedException will be thrown.

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 and false if the waiting time elapsed before all permits could be acquired

Throws:

InstanceDestroyedException - if the instance is destroyed while waiting

InterruptedException - if the current thread is interrupted

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting

tryAcquireAttach

boolean tryAcquireAttach()

Acquires a permit from this semaphore and attaches it to the calling member, only if one is available at the time of invocation.

See tryAcquire() and attach().

Returns:

true if permit was acquired and attached to the caller's address

Throws:

IllegalArgumentException - if permits is negative

tryAcquireAttach

boolean tryAcquireAttach(int permits)

Acquires the given number of permits from this semaphore and attaches them to the calling member, only if all are available at the time of invocation.

See tryAcquire(int permits) and attach(int).

Parameters:

permits - the number of permits to try to acquire and attach

Returns:

true if permit(s) were acquired and attached to the caller's address

Throws:

IllegalArgumentException - if permits is negative

tryAcquireAttach

boolean tryAcquireAttach(long timeout,
                         TimeUnit unit)
                         throws InstanceDestroyedException,
                                InterruptedException

Acquires a permit from this semaphore and attaches it to the calling member, only if one becomes available within the given waiting time and the current thread has not been interrupted or the instance destroyed.

If the ISemaphore instance is destroyed while the thread is waiting then InstanceDestroyedException will be thrown.

See tryAcquire(long timeout, TimeUnit unit) and attach().

Parameters:

timeout - the maximum time to wait for a permit

unit - 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 could be acquired

Throws:

InstanceDestroyedException - if the instance is destroyed while waiting

InterruptedException - if the current thread is interrupted

IllegalStateException - if hazelcast instance is shutdown while waiting

tryAcquireAttach

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

Acquires the given number of permits from this semaphore and attaches them to the calling member, only if all become available within the given waiting time and the current thread has not been interrupted or the instance destroyed.

See tryAcquire(int permits, long timeout, TimeUnit unit) and attach(int).

Parameters:

permits - the number of permits to try to acquire and attach

timeout - the maximum time to wait for the permits

unit - unit the time unit of the timeout argument

Returns:

true if permit(s) were acquired and attached to the caller's address and false if the waiting time elapsed before the permits could be acquired

Throws:

InstanceDestroyedException - if the instance is destroyed while waiting

InterruptedException - if the current thread is interrupted

IllegalArgumentException - if permits is negative

IllegalStateException - if hazelcast instance is shutdown while waiting

getLocalSemaphoreStats

LocalSemaphoreStats getLocalSemaphoreStats()