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ReentrantLock 能做的事

ReentrantLock 能做的事

作者: 王小杰at2019 | 来源:发表于2019-02-01 08:14 被阅读33次

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简介

jdk中独占锁的实现除了使用关键字synchronized外,还可以使用ReentrantLock。虽然在性能上ReentrantLock和synchronized没有什么区别，但ReentrantLock相比synchronized而言功能更加丰富，使用起来更为灵活，也更适合复杂的并发场景。

1. 加锁解锁

synchronized加锁解锁的过程是隐式的,用户不用手动操作,优点是操作简单，但显得不够灵活。一般并发场景使用synchronized的就够了；ReentrantLock 需要手动加锁和解锁,**且解锁的操作尽量要放在finally代码块中**,保证线程正确释放锁。

ReentrantLock lock = new ReentrantLock();
try {
    lock.lock();
} catch (Exception e) {
    log.info("业务异常", e);
} finally {
    lock.unlock();
}

2. 公平锁 & 非公平锁

非公平锁

由 cpu 自动分配线程的执行权限, 不受程序控制, 可能一个线程独占锁很长时间另一个线程获取不到锁

public static void main(String[] args) throws InterruptedException {
    ReentrantLock lock = new ReentrantLock();

    ExecutorService executorService = Executors.newCachedThreadPool();
    executorService.submit(() -> {
        for (int i = 0; i < 10; i++) {
            lock.lock();
            log.info("ThreadA :{}", DateFormatUtils.format(new Date(), "yyyyMMdd HH:mm:ss"));
            lock.unlock();
        }
    });
    executorService.submit(() -> {
        for (int i = 0; i < 10; i++) {
            lock.lock();
            log.info("ThreadB :{}", DateFormatUtils.format(new Date(), "yyyyMMdd HH:mm:ss"));
            lock.unlock();
        }
    });
    TimeUnit.SECONDS.sleep(20);
    executorService.shutdown();
}


12:21:37.535 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.545 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.545 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.548 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.548 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37

公平锁

公平锁可以照顾到每个线程,根据日志我们可以看到两个线程交替执行

ReentrantLock lock = new ReentrantLock(true);

12:24:19.476 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.485 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.486 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.487 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.487 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.487 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.487 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.488 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.488 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.488 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.488 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.489 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.489 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.489 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19

3. 可以中断

synchronized 不能响应中断,该代码执行后,控制台不能够输出"线程B"


@Slf4j
public class ReentrantLockExample4 {

    public static void main(String[] args) throws InterruptedException {
        Object lock = new Object();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.submit(() -> {
            synchronized (lock) {
                while (true){
                    
                }
            }

        });
        TimeUnit.SECONDS.sleep(1);
        ExecutorService executorService2 = Executors.newCachedThreadPool();
        executorService2.submit(() -> {
            synchronized (lock) {
                log.info("线程Ｂ");
            }
        });
        //发送中断信号
        executorService.shutdownNow();
    }
}

ReentrantLock

根据输出结果我们可以看到,ReentrantLock响应了中断事件




@Slf4j
public class ReentrantLockExample3 {

    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lock = new ReentrantLock(true);

        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.submit(() -> {
            try {
                lock.lockInterruptibly();
                while (true) {
                }
            } catch (InterruptedException e) {
                log.info("线程中断", e);
            } finally {
                lock.unlock();
            }
        });
        //发送中断信号
        executorService.shutdownNow();
    }
}

输出结果

12:41:39.523 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample3 - 线程中断
java.lang.InterruptedException: null
    at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1261)
    at java.base/java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:317)
    at cn.wyj.learn.juc.ReentrantLockExample3.lambda$main$0(ReentrantLockExample3.java:18)
    at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)

4. 获取锁时限时等待

基于这个特性我可以使用 ReentrantLock 编程避免死锁,但是要注意活锁的问题


@Slf4j
public class ReentrantLockExample5 {

    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lock = new ReentrantLock();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.submit(() -> {
            lock.lock();
            try {
                TimeUnit.SECONDS.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            log.info("线程A执行完成............");
            lock.unlock();
        });


        executorService.submit(() -> {
            try {
                while (true) {
                    boolean b = lock.tryLock(2, TimeUnit.SECONDS);
                    if (!b) {
                        log.info("还没获取到锁过会再来吧............");
                        continue;
                    }
                    log.info("线程B执行任务完成");
                    lock.unlock();
                    break;
                }

            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        });
    }
}


:48:50.511 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:52.515 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:54.517 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:56.518 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:58.508 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 线程A执行完成............
12:48:58.508 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 线程B执行任务完成

5. Condition实现等待通知机制

基于 ReentrantLock Condition 可以实现阻塞队列等功能

@Slf4j
public class ReentrantLockExample6 {

    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.submit(() -> {
            lock.lock();
            log.info("等待线程B执行完成");
            try {
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            log.info("线程A执行完成............");
            lock.unlock();
        });


        executorService.submit(() -> {
            lock.lock();
            condition.signal();
            log.info("线程B执行任务完成");
            lock.unlock();

        });
    }
}


12:52:47.556 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 等待线程B执行完成
12:52:47.565 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 线程B执行任务完成
12:52:47.565 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 线程A执行完成............

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