ReentrantLock源码之公平锁的实现

• 代码案例

public class reentrantlockDemo {

    public static void main(String[] args) {
        reentrantlock reentrantlock = new reentrantlock(true);
        reentrantlock.lock();
        reentrantlock.unlock();
    }

}

• 查看无参构造方法

    public reentrantlock() {
        sync = new Nonfairsync();    // new了1个非公平实现
    }

• 查看sync
  

  

• 查看有参构造

    public reentrantlock(boolean fair) {
        sync = fair ? new Fairsync() : new Nonfairsync();    // 为true时返回公平实现，否则返回非公平实现
    }

• reentrantlock实现了Lock接口
  

  

• 查看方法
  

  

  
  

  

• 查看内部类
  

  

• 查看lock方法

public class reentrantlockDemo {

    public static void main(String[] args) {
        reentrantlock reentrantlock = new reentrantlock(true);
        reentrantlock.lock();
        reentrantlock.unlock();
    }

}

public void lock() {
    sync.acquire(1);
}

public final void acquire(int arg) {
    // 尝试获取锁
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))    // 同时获取队列失败
        selfInterrupt();      // 则线程终止
}

• 查看尝试获取锁的方法

    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

• 查看非公平实现
  

  

static final class Nonfairsync extends Sync {
    private static final long serialVersionUID = 7316153563782823691L;
    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

@ReservedStackAccess
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();    // 获取当前线程
    int c = getState();    // 获取状态
    if (c == 0) {    // 当前没有线程获取到这个锁
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;    // 获得锁
        }
    }
    else if (current == getExclusiveOwnerThread()) {    // 是否是持有锁的线程，即重入状态
        int nextc = c + acquires;    // 重入状态加1
        if (nextc < 0) // overflow      // 超过最大重入状态，抛出异常
            throw new Error("Maximum lock count exceeded");
        setState(nextc);    // 更新
        return true;    // 获得锁
    }
    return false;
}

• 查看addWaiter方法

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

• 查看EXCLUSIVE
  

  

• 查看addWaiter方法

    private Node addWaiter(Node mode) {
        Node node = new Node(mode);    // 传入参数，当前线程，new1个Node

        for (;;) {
            Node oldTail = tail;    // 前置节点指向尾节点
            if (oldTail != null) {
                node.setPrevRelaxed(oldTail);
                if (compareAndSetTail(oldTail, node)) {    // node设置为尾节点
                    oldTail.next = node;
                    return node;
                }
            } else {
                initializeSyncQueue();    // 若整条队列为null，执行入队
            }
        }
    }

• 查看Node

Node(Node nextWaiter) {
    this.nextWaiter = nextWaiter;
    THREAD.set(this, Thread.currentThread());
}