ReentrantLock

ReentrantLock与synchronized相比，synchronized在代码块执行完后会自动解锁，异常也会自动解锁，单reentrantLock需要手动加锁何解锁。

public class Study05 {

    public static void main(String[] args) {

        Study05 study =new Study05();

        new Thread(study :: m1).start();

        try {

            TimeUnit.SECONDS.sleep(1);

        }catch (InterruptedException e) {

            e.printStackTrace();

        }

        new Thread(study :: m2).start();

    }

    Locklock =new ReentrantLock();

    void m1(){

        try {

            lock.lock();

            for (int i =0; i <10; i++) {

                TimeUnit.SECONDS.sleep(1);

                System.out.println(i);

            }

        }catch (InterruptedException e) {

            e.printStackTrace();

        } finally {

            lock.unlock();

        }

    }

    void m2(){

        try {

            lock.lock();

            System.out.println("m2...");

        } finally {

            lock.unlock();

        }

    }

}

运行结果：

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8

9

m2...

在m1中reentrantLock没释放之前不会执行m2中的语句。

ReentrantLock还有些功能比synchronized强大。

1.tryLock尝试加锁，synchronized如果没有获取到锁就会一直阻塞，但tryLock可以指定等待时间，到达指定时间后，不管是否获得锁，方法都将得到执行。

public class Study05 {

    public static void main(String[] args) {

        Study05 study =new Study05();

        new Thread(study::m3).start();

        new Thread(study::m4).start();

    }

    Locklock =new ReentrantLock();

    void m3(){

        try{

            lock.lock();

            for (int i=0; i<10; i ++){

                TimeUnit.SECONDS.sleep(1);

                    System.out.println(i);

                }

            }catch (InterruptedException e) {

                e.printStackTrace();

            }finally {

                lock.unlock();

            }

    }

    void m4(){

        try {

            boolean locked =lock.tryLock(5, TimeUnit.SECONDS);

            System.out.println("m4..."+ locked);

        }catch (InterruptedException e) {

            e.printStackTrace();

        }finally {

            lock.unlock();

        }

    }

}

运行结果：

0

1

2

3

m4...false

Exception in thread "Thread-1" java.lang.IllegalMonitorStateException at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151) at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java1261) at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:457) at study.Study05.m4(Study05.java:67) at java.lang.Thread.run(Thread.java:748)

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9

因为m4没获得锁却在finally中释放锁，所以会报错

2.lock.lockInterruptibly()

3.ReentrantLock可以指定为公平锁

Lock lock = new ReentrantLock(true);//参数为true表示公平锁