ReentrantLock是一个排它重入锁，与synchronized关键字语意类似，但比其功能更为强大。该类位于java.util.concurrent.locks包下，是Lock接口的实现类。基本用法如下：
class X { private final ReentrantLock lock = new ReentrantLock(); // ... public void m() { lock.lock(); // block until condition holds try { // ... method body } finally { lock.unlock() } } }