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Classes javautilconcurrentSemaphore Methods void acquire( ) void release( )
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Beginning with version 5, Java has included expanded support for concurrent programming called the concurrency utilities This is a set of classes and interfaces packaged in javautilconcurrent and two subpackages, javautilconcurrentatomic and javautilconcurrentlocks Commonly referred to as the concurrent API, the concurrency utilities supplement Java s built-in support for multithreaded programming by supplying several well-known, high-level features, such as semaphores and locks These features are of particular value when creating programs that make intensive use of multiple threads whose actions must be coordinated This chapter presents two recipes that use the concurrent API This recipe shows how to use a semaphore, which is an object of type Semaphore The following recipe shows how to use the Callable and Future interfaces to asynchronously compute and return a value Although much of the concurrent API is best suited for specialized situations, these two features are more generally applicable They also show the power available in the concurrent API The semaphore will be familiar to most readers It is the classic device that is used to manage access to a resource shared by multiple threads A semaphore controls access through the use of a counter If the counter is greater than zero, then access is allowed If it is zero, then access is denied What the counter is counting are permits that allow access to the shared resource In general, here is how a semaphore is used When a thread wants access to a shared resource, it tries to acquire a permit from the semaphore that controls access to the resource If the semaphore s count is greater than zero, the permit is granted and the semaphore s count is decremented Otherwise, if the semaphore s count is already zero when the thread tries to acquire the permit, the thread will be blocked until a permit can be granted When the thread no longer needs access to the shared resource, it releases the permit, which causes the semaphore s count to be incremented If there is another thread waiting for a permit, then that thread will acquire a permit at that time This mechanism is implemented by the Semaphore class, which is packaged in javautilconcurrent
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NOTE It is important to emphasize that the concurrency utilities add to, not replace, Java's original
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approach to multithreading and synchronization that has been in place from the start As all Java programmers know, threads are created from classes that implement Runnable, synchronization is available through the use of the synchronized keyword, and interthread communication is handled by wait( ) and notify( ), defined by Object Java's original approach is both elegant and efficient, and it is still the mechanism that should be employed for many Java programs The main purpose of the concurrency utilities is to provide off-the-shelf solutions to some fairly challenging threading situations They also supply implementations of several classic synchronization objects, such as the semaphore described in this recipe, which are familiar to many programmers
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To use a semaphore to control access to a shared resource involves these steps: 1 Create a Semaphore instance, specifying the number of permits available For a mutex semaphore, specify one permit 2 Before a thread accesses the shared resource, it must acquire a permit by calling acquire( ) on the semaphore 3 When the thread is finished with the shared resource, it must release the permit by calling release( ) on the semaphore
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Discussion
Semaphore has two constructors The one used by the recipe is shown here: Semaphore(int num, boolean how) Here, num specifies the initial permit count Thus, num specifies the number of threads that can access a shared resource at any one time If num is one, then only one thread can access the resource at any one time (In this case, a mutex semaphore is created) If how is true, waiting threads are granted a permit in the order in which they requested access This is called fairness, and it is what you will typically want to be the case when synchronizing access to a shared resource Fairness prevents the same thread from immediately reacquiring the semaphore and effectively locking out another thread, for example If how is false, then the first in line rule is not enforced To acquire a permit, call the acquire( ) method The form used by this recipe is shown here: void acquire( ) throws InterruptedException This form acquires one permit If the permit cannot be granted at the time of the call, then the invoking thread suspends until the permit is available To release a permit, call release( ) The form used by this recipe is shown next: void release( ) The first form releases one permit Here is how you use a semaphore to control access to a resource When a thread wants to use the resource, it must first call acquire( ) before accessing the resource If a permit is not available, then the calling thread will block until another thread releases a permit When the thread is done with the resource, it must call release( ) Failure to call release( ) causes the resource to remain unavailable to other threads Therefore, you must not forget this important step
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