Java.util.concurrent.Semaphore Java中的类

信号量通过使用计数器来控制对共享资源的访问。如果计数器大于零，则允许访问。如果为零，则拒绝访问。计数器计数的是允许访问共享资源的许可。因此，要访问资源，线程必须从信号量中获得许可。

句法：

public class Semaphore
extends Object
implements Serializable

从概念上讲，信号量维护一组许可。如有必要，每个 acquire() 都会阻塞，直到获得许可，然后再接受它。每个 release() 添加一个许可，可能会释放一个阻塞的获取者。但是，没有使用实际的许可对象； Semaphore 只是对可用数量进行计数并采取相应措施。

信号量类的方法

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.

示例：每次执行的输出都不相同。

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();
    }
}

输出：

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.