- Generated on Mon Mar 4 2013 10:25:52 for activemq-cpp-3.6.0 by
1.8.1.2
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activemq-cpp-3.6.0
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A counting semaphore. More...
#include <src/main/decaf/util/concurrent/Semaphore.h>
Public Member Functions | |
| Semaphore (int permits) | |
| Creates a Semaphore with the given number of permits and nonfair fairness setting. | |
| Semaphore (int permits, bool fair) | |
| Creates a Semaphore with the given number of permits and the given fairness setting. | |
| virtual | ~Semaphore () |
| void | acquire () |
| Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted. | |
| void | acquireUninterruptibly () |
| Acquires a permit from this semaphore, blocking until one is available. | |
| bool | tryAcquire () |
| Acquires a permit from this semaphore, only if one is available at the time of invocation. | |
| bool | tryAcquire (long long timeout, const 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. | |
| void | release () |
| Releases a permit, returning it to the semaphore. | |
| void | acquire (int permits) |
| Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is interrupted. | |
| void | acquireUninterruptibly (int permits) |
| Acquires the given number of permits from this semaphore, blocking until all are available. | |
| bool | tryAcquire (int permits) |
| Acquires the given number of permits from this semaphore, only if all are available at the time of invocation. | |
| bool | tryAcquire (int permits, long long timeout, const TimeUnit &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. | |
| void | release (int permits) |
| Releases the given number of permits, returning them to the semaphore. | |
| int | availablePermits () const |
| Returns the current number of permits available in this semaphore. | |
| int | drainPermits () |
| Acquires and returns all permits that are immediately available. | |
| bool | isFair () const |
| std::string | toString () const |
| Returns a string identifying this semaphore, as well as its state. | |
| int | getQueueLength () const |
| Gets an estimated count of the number of threads that are currently waiting to acquire, this value changes dynamically so the result of this method can be invalid immediately after it is called. | |
| bool | hasQueuedThreads () const |
Protected Member Functions | |
| void | reducePermits (int reduceBy) |
| Reduces the number of available permits which can be useful for subclasses. | |
| decaf::util::Collection < decaf::lang::Thread * > * | getQueuedThreads () const |
| Creates and returns a new Collection object that contains a best effort snapshot of the threads that are currently waiting to acquire. | |
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.
class Pool { private:
static const int MAX_AVAILABLE = 100; Semaphore available;
std::vector<std::string> items; std::vector<bool> used;
Mutex lock;
public:
Pool() : available( MAX_AVAILABLE, true ) { used.resize( MAX_AVAILABLE ); items.resize( MAX_AVAILABLE ); }
std::string getItem() throws InterruptedException { available.acquire(); return getNextAvailableItem(); }
void putItem( std::string x ) { if( markAsUnused(x) ) { available.release(); } }
std::string getNextAvailableItem() {
synchronized( &lock ) { for( int i = 0; i < MAX_AVAILABLE; ++i ) { if( !used[i] ) { used[i] = true; return items[i]; } }
return std::string(); // not reached }
bool markAsUnused( const std::string& item ) { synchronized( &lock ) { 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 indefinitely postponing when these methods are used without fairness set true.
| decaf::util::concurrent::Semaphore::Semaphore | ( | int | permits | ) |
Creates a Semaphore with the given number of permits and nonfair 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. |
| decaf::util::concurrent::Semaphore::Semaphore | ( | int | permits, |
| bool | fair | ||
| ) |
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. |
| fair | true if this semaphore will guarantee first-in first-out granting of permits under contention, else false |
|
virtual |
| void decaf::util::concurrent::Semaphore::acquire | ( | ) |
Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted.
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 two things happens:
If the current thread:
then InterruptedException is thrown and the current thread's interrupted status is cleared.
| InterruptedException | - if the current thread is interrupted. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| void decaf::util::concurrent::Semaphore::acquire | ( | int | 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:
If the current thread:
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. |
| InterruptedException | if the current thread is interrupted. |
| IllegalArgumentException | if the permits argument is negative. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| void decaf::util::concurrent::Semaphore::acquireUninterruptibly | ( | ) |
Acquires a permit from this semaphore, blocking until one is available.
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 some other thread invokes the release() method for this semaphore and the current thread is next to be assigned a permit.
If the current thread is interrupted while waiting for a permit then it will continue to wait, but the time at which the thread is assigned a permit may change compared to the time it would have received the permit had no interruption occurred. When the thread does return from this method its interrupt status will be set.
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| void decaf::util::concurrent::Semaphore::acquireUninterruptibly | ( | int | 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. |
| IllegalArgumentException | if the permits argument is negative. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| int decaf::util::concurrent::Semaphore::availablePermits | ( | ) | const |
Returns the current number of permits available in this semaphore.
This method is typically used for debugging and testing purposes.
| int decaf::util::concurrent::Semaphore::drainPermits | ( | ) |
Acquires and returns all permits that are immediately available.
| RuntimeException | if an unexpected error occurs while draining the Semaphore. |
|
protected |
Creates and returns a new Collection object that contains a best effort snapshot of the threads that are currently waiting to acquire.
| int decaf::util::concurrent::Semaphore::getQueueLength | ( | ) | const |
Gets an estimated count of the number of threads that are currently waiting to acquire, this value changes dynamically so the result of this method can be invalid immediately after it is called.
| bool decaf::util::concurrent::Semaphore::hasQueuedThreads | ( | ) | const |
| bool decaf::util::concurrent::Semaphore::isFair | ( | ) | const |
|
protected |
Reduces the number of available permits which can be useful for subclasses.
If the subclass is tracking a resource that is transiently available this method can be used to modify the Semaphore to reflect that resources current state. This method does not block waiting for the number of permits to be available, unlike the acquire method.
| reduceBy | The number of permits to remove from the current available set. |
| IllegalArgumentException | if the param passed in negative. |
| void decaf::util::concurrent::Semaphore::release | ( | ) |
Releases a permit, returning it to the semaphore.
Releases a permit, increasing the number of available permits by one. If any threads are trying to acquire a permit, then one is selected and given the permit that was just released. That thread is (re)enabled for thread scheduling purposes.
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.
| RuntimeException | if an unexpected error occurs while releasing the Semaphore. |
| void decaf::util::concurrent::Semaphore::release | ( | int | 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.
| permits | the number of permits to release |
| IllegalArgumentException | if the permits argument is negative. |
| RuntimeException | if an unexpected error occurs while releasing the Semaphore. |
| std::string decaf::util::concurrent::Semaphore::toString | ( | ) | const |
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.
| bool decaf::util::concurrent::Semaphore::tryAcquire | ( | ) |
Acquires a permit from this semaphore, only if one is available at the time of invocation.
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.
Even when this semaphore has been set to use a fair ordering policy, a call to tryAcquire() will immediately acquire a permit if one is available, whether or not other threads are currently waiting. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting, then use tryAcquire(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption).
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
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:
If a permit is acquired then the value true is returned.
If the current thread:
then InterruptedException is thrown and the current thread's interrupted status is cleared.
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.
| timeout | the maximum time to wait for a permit |
| unit | the time unit of the timeout argument |
| InterruptedException | if the current thread is interrupted. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| bool decaf::util::concurrent::Semaphore::tryAcquire | ( | int | permits | ) |
Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.
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.
Even when this semaphore has been set to use a fair ordering policy, a call to tryAcquire will immediately acquire a permit if one is available, whether or not other threads are currently waiting. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting, then use tryAcquire(permits, 0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption).
| permits | the number of permits to acquire |
| IllegalArgumentException | if the permits argument is negative. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
| bool decaf::util::concurrent::Semaphore::tryAcquire | ( | int | permits, |
| long long | timeout, | ||
| const TimeUnit & | 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:
If the permits are acquired then the value true is returned.
If the current thread:
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 |
| timeout | the maximum amount of time to wait to acquire the permits. |
| unit | the units that the timeout param represents. |
| IllegalArgumentException | if the permits argument is negative. |
| RuntimeException | if an unexpected error occurs while acquiring the Semaphore. |
1.8.1.2