1. ReentrantLock lock = new ReentrantLock();  // 创建锁
    2. lock.lock();  // 加锁
    3. System.out.println("ReentrantLock");
    4. lock.unlock();  // 释放锁
    1. public void lock() {
    2.     sync.lock();  // 默认调用非公平锁加锁
    3. }
    1. final void lock() {
    2.     // 如果当前锁没有被持有，则原子性地更新获取锁的状态为1 
    3.     if (compareAndSetState(0, 1))  
    4.         setExclusiveOwnerThread(Thread.currentThread());
    5.     else
    6.         // 如果当前锁被持有  todo
    7.         acquire(1);
    8. }
    1. protected final boolean compareAndSetState(int expect, int update) {
    2.     // 调用unSafe方法更新，将锁被获取的状态更新为1
    3.     return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    4. }