Read Semaphore source code

// A semaphore that allows the thread to continue execution only if the pool still has permission.
// first look at the constructor
//Default is unfair mode
public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }


NonfairSync(int permits) {
            super(permits);
        }

//set state
 Sync(int permits) {
            setState(permits);
        }
 public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

// get a license
public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
	    //The method in AQS is blocked without acquiring the lock
            doAcquireSharedInterruptibly (arg);
    }

 final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
		//If remaining<0, there is no permission. Or have permission to directly CAS try to set the value of state
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

 private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter (Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt ())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

// Get the specified number of licenses
public void acquire(int permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    }

// release permission
 public void release() {
        sync.releaseShared(1);
    }

public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
 
 protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
		//CAS sets the current number of licenses
                if (compareAndSetState(current, next))
                    return true;
            }
        }

// release the specified number of licenses
  public void release(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.releaseShared(permits);
    }

// get permission
public void acquireUninterruptibly() {
        sync.acquireShared(1);
    }

//Do not block the direct access to the license, if the acquisition fails, return false directly
public boolean tryAcquire() {
        return sync.nonfairTryAcquireShared(1) >= 0;
    }
// try to acquire the lock within a certain period of time and return false
public boolean tryAcquire(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

//Return the number of available licenses for the secondary semaphore
 public int availablePermits() {
        return sync.getPermits();
    }

// Get all available licenses and clear the licenses
public int drainPermits() {
        return sync.drainPermits();
    }

final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }

// Reduce the current amount of available licenses
 protected void reducePermits(int reduction) {
        if (reduction < 0) throw new IllegalArgumentException();
        sync.reducePermits(reduction);
    }


 final void reducePermits(int reductions) {
            for (;;) {
                int current = getState();
                int next = current - reductions;
                if (next > current) // underflow
                    throw new Error("Permit count underflow");
                if (compareAndSetState(current, next))
                    return;
            }
        }
// Is it fair mode
 public boolean isFair() {
        return sync instanceof FairSync;
    }

//Check if there are threads waiting to get
 public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

  public final boolean hasQueuedThreads() {
        return head != tail;
    }

//Get the number of threads waiting for permission
public final int getQueueLength() {
        return sync.getQueueLength();
    }

public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }

// Encapsulate possibly waiting threads into a collection
protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }