Semaphore源码分析

Posted 2022-07-12 yaowen

public class SemaphoreExample1 
    private final static int threadCount = 20;
    public static void main(String[] args) throws Exception 
        ExecutorService exec = Executors1.newCachedThreadPool();
        final Semaphore1 semaphore = new Semaphore1(1);  //并发数是3，每3个3个打印
        for (int i = 0; i < 4; i++) 
            exec.execute(() -> 
                try 
                    semaphore.acquire(); // 获取一个许可，多线程访问
                    System.out.println("咔咔咔咔咔咔扩扩扩扩扩扩扩");
                    semaphore.release(); // 释放一个许可，多线程访问
                 catch (Exception e) 
                
            );
        
        exec.shutdown();
    

//共享锁，跟读写锁的读锁是一样的逻辑。
public class Semaphore1 implements java.io.Serializable 
    private static final long serialVersionUID = -3222578661600680210L;
    private final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer1 
        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) 
            setState(permits);//state
        

        final int getPermits() 
            return getState();
        

        final int nonfairTryAcquireShared(int acquires) //获取许可，多线程
            for (;;) //死循环，直到获取一个许可。只有在许可没了才去排队
                int available = getState();
                int remaining = available - acquires;//state减1
                if (remaining < 0 || compareAndSetState(available, remaining))//，多线程
                    return remaining;
            
        

        protected final boolean tryReleaseShared(int releases) //释放许可，多线程访问
            for (;;) 
                int current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))//减一成功，别的线程就可以获取了。
                    return true;
            
        

        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;
            
        

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

    static final class NonfairSync extends Sync 
        private static final long serialVersionUID = -2694183684443567898L;

        NonfairSync(int permits) 
            super(permits);
        

        protected int tryAcquireShared(int acquires) //获取许可，多线程
            return nonfairTryAcquireShared(acquires);
        
    

    static final class FairSync extends Sync 
        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) 
            super(permits);
        

        protected int tryAcquireShared(int acquires) //获取许可
            for (;;) //死循环，直到获取一个许可。只有在许可没了才去排队
                if (hasQueuedPredecessors())//要不要去排队
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 || compareAndSetState(available, remaining))
                    return remaining;
            
        
    

    public Semaphore1(int permits) 
        sync = new NonfairSync(permits);
    

    public Semaphore1(int permits, boolean fair) 
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    

    public void acquire() throws InterruptedException //多线程访问
        sync.acquireSharedInterruptibly(1);//获取共享锁
    

    public void acquireUninterruptibly() throws InterruptedException 
        sync.acquireShared(1);
    

    public boolean tryAcquire() 
        return sync.nonfairTryAcquireShared(1) >= 0;
    

    public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException 
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    

    public void release() //多线程访问
        sync.releaseShared(1);
    

    public void acquire(int permits) throws InterruptedException 
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    

    public void acquireUninterruptibly(int permits) throws InterruptedException 
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireShared(permits);
    

    public boolean tryAcquire(int permits) 
        if (permits < 0) throw new IllegalArgumentException();
        return sync.nonfairTryAcquireShared(permits) >= 0;
    

    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
        throws InterruptedException 
        if (permits < 0) throw new IllegalArgumentException();
        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
    

    public void release(int permits) 
        if (permits < 0) throw new IllegalArgumentException();
        sync.releaseShared(permits);
    

    public int availablePermits() 
        return sync.getPermits();
    

    public int drainPermits() 
        return sync.drainPermits();
    

    protected void reducePermits(int reduction) 
        if (reduction < 0) throw new IllegalArgumentException();
        sync.reducePermits(reduction);
    

    public boolean isFair() 
        return sync instanceof FairSync;
    

    public final boolean hasQueuedThreads() 
        return sync.hasQueuedThreads();
    

    public final int getQueueLength() 
        return sync.getQueueLength();
    

    protected Collection<Thread> getQueuedThreads() 
        return sync.getQueuedThreads();
    

    public String toString() 
        return super.toString() + "[Permits = " + sync.getPermits() + "]";