Java并发编程之ReentrantLock解析

public class ReentrantLock implements Lock, java.io.Serializable {
    private final Sync sync;
    abstract static class Sync extends AbstractQueuedSynchronizer {
        ...
    }
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
        final void lock() {
            acquire(1);
        }
        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        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;
        }
    }
    public ReentrantLock() {
        sync = new NonfairSync();
    }
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }
    public void lock() {
        sync.lock();
    }
    public void unlock() {
        sync.release(1);
    }
}

final void lock() {
    //1、CAS将state置为1
    if (compareAndSetState(0, 1))
        //2、设置自己为独占线程
        setExclusiveOwnerThread(Thread.currentThread());
    else
        //3、否则尝试获取资源
        acquire(1);
}
protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}
final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            //1、如果state为0，说明当前锁没有被占用
            if (c == 0) {
                //2、CAS尝试将state设为1
                if (compareAndSetState(0, acquires)) {
                    //3、获取锁成功，设置自己为独占线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //4、如果持有当前锁的线程就是自己
            else if (current == getExclusiveOwnerThread()) {
                //5、那么将state增加acquires
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                //6、更新state，因为锁已经被自己持有了，所以这里不需要CAS设置
                setState(nextc);
                return true;
            }
            return false;
}
//AQS
public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
}

protected final boolean tryRelease(int releases) {
            //1、每次释放，将state减去释放数
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            //2、如果state为0，说明所有资源都已经释放
            if (c == 0) {
                free = true;
                //3、将独占线程置空
                setExclusiveOwnerThread(null);
            }
            //4、更新state
            setState(c);
            return free;
        }

protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                //1、等待队列中是否已经有节点在等待获取锁了
                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;
        }
public final boolean hasQueuedPredecessors() {
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        //2、等待队列以后已经初始化，并且有别的线程正在入队(enq)或者已经入队
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }