ReentrantLock

ReentrantLock的底层是aqs,aqs的加锁是通过CAS操作的， 因为aqs大量使用CAS所以性能比syncionzed的性能好很多

AQS图解

ReentrantLock的底层是Sync这个类 Sync是AQS的子类

    public ReentrantLock() {
        sync = new NonfairSync();
    }

lock()的底层还是CAS

cas可以无锁化的保证原子性

  final void lock() {
              //如果加锁成功  cas操作使得state变为1.
            if (compareAndSetState(0, 1))
                //独占锁
                setExclusiveOwnerThread(Thread.currentThread());
            else
                //如果state!=1 健壮性考虑，防止其他线程释放锁
                acquire(1);
        }
//如果state!=1
 public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
   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;
                }
            }
    //代表了线程在可重入的加锁        
    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;
   }

底层还是基于unsafe