ReentrantLock特征之可重入和可打断

可重入

        可重入是指同一个线程如果首次获得了这把锁，那么因为它是这把锁的拥有者，因此有权利再次获取这把锁
        如果是不可重入锁，那么第二次获得锁时，自己也会被锁挡住

synchronized可重入举例

static final Object obj = new Object();
public static void method1() {
    synchronized( obj ) {
        // 同步块 A
        method2();
    }
}
public static void method2() {
    synchronized( obj ) {
        // 同步块 B
    }
}

 ReentrantLock可重入举例

static ReentrantLock lock = new ReentrantLock();
    public static void main(String[] args) {
        method1();
    }
    public static void method1() {
        lock.lock();
        try {
            log.debug("execute method1");
            method2();
        } finally {
            lock.unlock();
        }
    }
    public static void method2() {
        lock.lock();
        try {
            log.debug("execute method2");
            method3();
        } finally {
            lock.unlock();
        }
    }
    public static void method3() {
        lock.lock();
        try {
            log.debug("execute method3");
        } finally {
            lock.unlock();
        }
    }

运行结果，输出

17:59:11.862 [main] c.TestReentrant - execute method1
17:59:11.865 [main] c.TestReentrant - execute method2
17:59:11.865 [main] c.TestReentrant - execute method3

可打断

        synchronized锁不可打断，ReentrantLock的lockInterruptibly()方法可以在等锁的过程中被interrupt打断，结束等待

示例

ReentrantLock lock = new ReentrantLock(); Thread t1 = new Thread(() -> { log.debug("启动..."); try { lock.lockInterruptibly(); } catch (InterruptedException e) { e.printStackTrace(); log.debug("等锁的过程中被打断"); return; } try { log.debug("获得了锁"); } finally { lock.unlock(); } }, "t1"); lock.lock(); log.debug("获得了锁"); t1.start(); try { sleep(1); t1.interrupt(); log.debug("执行打断"); } finally { lock.unlock(); }

输出

18:02:40.520 [main] c.TestInterrupt - 获得了锁
18:02:40.524 [t1] c.TestInterrupt - 启动...
18:02:41.530 [main] c.TestInterrupt - 执行打断
java.lang.InterruptedException
        at
java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchr
onizer.java:898)
        at
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchron
izer.java:1222)
        at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
        at cn.itcast.n4.reentrant.TestInterrupt.lambda$main$0(TestInterrupt.java:17)
        at java.lang.Thread.run(Thread.java:748)
18:02:41.532 [t1] c.TestInterrupt - 等锁的过程中被打断

         通过ReentrantLock创建的条件变量（简单理解为休息室），可以调用signal方法，唤醒全部等待的线程。