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Java.util.concurrent.Semaphore Class in Java

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  • Difficulty Level : Hard
  • Last Updated : 24 Apr, 2022

A semaphore controls access to a shared resource 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. Thus, to access the resource, a thread must be granted a permit from the semaphore.

Syntax:

public class Semaphore
extends Object
implements Serializable

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. 

Methods of Semaphore Class 

Method Action performed
acquire() Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one.If the current thread is interrupted while waiting for a permit then InterruptedException is thrown
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. If the current thread is interrupted while waiting for a permit then InterruptedException is thrown.
acquireUninterruptibly() Acquires a permit if one is available and returns immediately, reducing the number of available permits by one. If the current thread is interrupted while waiting for a permit then it will continue to wait. 
acquireUninterruptibly(int permits) Given a number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount. If the current thread is interrupted while waiting for a permit then it will continue to wait.
availablePermits() Returns the current number of permits available in this semaphore. This method is typically used for debugging and testing purposes. 
drainPermits() Acquires and returns all permits that are immediately available. 
getQueueLength() 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. 
getQueuedThreads() Returns a collection containing threads that may be waiting to acquire. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order
hasQueuedThreads() 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. 
isFair() Returns true if this semaphore has fairness set true. 
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.
 reducePermits(int reduction) Shrinks the number of available permits by the indicated reduction. This method can be useful in subclasses that use semaphores to track resources that become unavailable. This method differs from acquiring in that it does not block waiting for permits to become available. 
release() 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. 
release(int permits) 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. 
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.
tryAcquire(long timeout, TimeUnit unit) Acquires a permit if one is available and returns immediately, with the value true, reducing the number of available permits by one. 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. 
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. 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. 
toString() 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.

Example: Output will not be same at every execution.

Java




// Java program to demonstrate
// methods of Semaphore class
import java.util.concurrent.*;
  
class MyThread extends Thread {
    Semaphore sem;
    String threadName;
    public MyThread(Semaphore sem, String threadName)
    {
        super(threadName);
        this.sem = sem;
        this.threadName = threadName;
    }
  
    @Override public void run()
    {
  
        // First, get a permit.
        System.out.println(threadName
                           + " is waiting for a permit.");
  
        try {
            // acquire method
            sem.acquire();
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
  
        System.out.println(threadName + " gets a permit");
  
        // Now, critical section
        // other waiting threads will wait, until this
        // thread release the lock
        for (int i = 0; i < 2; i++) {
            // hasQueuedThreads() methods
            boolean b = sem.hasQueuedThreads();
            if (b)
                // getQueuedLength() methods
                System.out.println("Length of Queue : "
                                   + sem.getQueueLength());
  
            // Now, allowing a context switch -- if
            // possible.
            try {
                Thread.sleep(10);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
  
        // Release the permit.
        System.out.println(threadName
                           + " releases the permit.");
  
        // release() method
        sem.release();
    }
}
  
// Driver class
public class SemaphoreDemo {
  
    public static void main(String args[])
        throws InterruptedException
    {
        // creating a Semaphore object
        // with number of permits 3 and fairness true
        Semaphore sem = new Semaphore(3, true);
  
        // isFair() method
        System.out.println("is Fairness enabled : "
                           + sem.isFair());
  
        // Main thread try to acquire 2 permits
        // tryAcquire(int permits) method
        sem.tryAcquire(2);
  
        // availablePermits() method
        System.out.println("Available permits : "
                           + sem.availablePermits());
  
        // drainPermits() method
        System.out.println(
            "number of permits drain by Main thread : "
            + sem.drainPermits());
  
        // permit released by Main thread
        sem.release(1);
  
        // creating two threads with name A and B
        MyThread mt1 = new MyThread(sem, "A");
        MyThread mt2 = new MyThread(sem, "B");
  
        // starting threads A
        mt1.start();
  
        // starting threads B
        mt2.start();
  
        // toString method
        System.out.println(sem.toString());
  
        // waiting for threads A and B
        mt1.join();
        mt2.join();
    }
}


Output:

is Fairness enabled : true
Available permits : 1
number of permits drain by Main thread : 1
java.util.concurrent.Semaphore@5b6f7412[Permits = 1]
A is waiting for a permit.
A gets a permit
B is waiting for a permit.
Length of Queue : 1
A releases the permit.
B gets a permit
B releases the permit.

This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


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