四.AQS的实现

一.ReentrantLock

• 层次图
  • ReentrantLock是独占模式，其中NonfairSync 是非公平锁，FairSync是公平锁，构造器就是构造它们，Sync定义抽象Lock方法供子类实现，unlock方法就是state减一。

public class ReentrantLock implements Lock, java.io.Serializable {
    public ReentrantLock() {  sync = new NonfairSync(); }
    public ReentrantLock(boolean fair) { sync = fair ? new FairSync() : new NonfairSync(); }
    private final Sync sync;
    abstract static class Sync extends AbstractQueuedSynchronizer { abstract void lock();}
    static final class NonfairSync extends Sync { }
    static final class FairSync extends Sync { }
   public void unlock() {sync.release(1);  }
}

• 公平锁的实现
  • 回想一下上节获取锁流程（Acquire）：试图获取锁（tryAcquire）若失败，则进入acquireQueued方法获取阻塞队列
  • ReentrantLock获取锁方式:当state=1，若当前节点不是头结点且是阻塞队列第一个节点，并通过CAS将state值+1，阻止其他线程获取；若state！=0且为当前线程是获取锁的线程，则继续讲state+1；其他情况获取锁失败

 static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
        //AQS方法
        final void lock() {
            acquire(1);
        } 
       //AQS tryAcquire方法子类的实现
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

• 非公平锁的实现
  • 回想一下上节获取锁流程（Acquire）：试图获取锁（tryAcquire）若失败，则进入acquireQueued方法获取阻塞队列
  • ReentrantLock获取锁方式:若state=0，当前线程尝试通过CAS看看能不能把state从0变为1（即获取锁），如果可以的话，直接获取锁而不需要排队(按队列顺序)

 static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
  final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

二.信号量Semaphore实现原理

• 信号量允许多条线程获取锁，它的锁是一种共享锁，信号量也有公平模式与非公平模式，与ReentrantLock类似
• 实现方式
• 通过设置permits（允许几个线程）作为初始state，没获取一次state-1，，如果返回的是一个<0的数字，那么构建FIFO队列，线程阻塞，直到前面的执行完才能唤醒后面的。

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

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

三.CountDownLatch实现原理

• 传入一个count作为初始state，await的时候判断是不是0，是0返回1表示成功,返回-1表示失败，构建FIFO队列，head头只连接一个Node，Node中的线程就是调用CountDownLatch的await()方法的线程,每次countDown的时候对state-1，直到state减到0的时候才算tryReleaseShared成功，tryReleaseShared成功，唤醒被挂起的线程

   private static final class Sync extends AbstractQueuedSynchronizer {
        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

四.ThreadPoolExecutor的实现