各种同步工具使用

1. 如何使用 ReentrantLock 中的 可中断锁防止死锁？

答: 在 执行语句前 加 可中断锁，此时，当被打断时，会抛出 被中断异常，这样就可以解锁。

注意：要外部中断，死锁中的线程不会自己中断

public class ReentrantLockTest {
    static Lock lock1 = new ReentrantLock();
    static Lock lock2 = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {

        Thread thread = new Thread(new ThreadDemo(lock1, lock2));//该线程先获取锁1,再获取锁2
        Thread thread1 = new Thread(new ThreadDemo(lock2, lock1));//该线程先获取锁2,再获取锁1
        thread.start();
        thread1.start();
        thread.interrupt();//是第一个线程中断，这个地方也可以弄一个线程 专门检查死锁，然后中断其中一个线程
    }

    static class ThreadDemo implements Runnable {
        Lock firstLock;
        Lock secondLock;
        public ThreadDemo(Lock firstLock, Lock secondLock) {
            this.firstLock = firstLock;
            this.secondLock = secondLock;
        }
        @Override
        public void run() {
            try {
                firstLock.lockInterruptibly();
                TimeUnit.MILLISECONDS.sleep(10);//更好的触发死锁
                secondLock.lockInterruptibly();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                firstLock.unlock();
                secondLock.unlock();
                System.out.println(Thread.currentThread().getName()+"正常结束!");
            }
        }
    }
}

2.   ReentrantLock还给我们提供了获取锁限时等待的方法tryLock(),可以选择传入时间参数,表示等待指定的时间,无参则表示立即返回锁申请的结果:true表示获取锁成功,false表示获取锁失败。我们可以使用该方法配合失败重试机制来更好的解决死锁问题。

例子：

package com.xuecheng.manage_cms;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

/**
 * 同步控制工具 ReentrantLock
 */
public class ReentrantLockTest implements Runnable{
    private  ReentrantLock lock = new ReentrantLock();

    @Override
    public void run() {
        try {
            if(lock.tryLock(5, TimeUnit.SECONDS)){
                Thread.sleep(6000);
            }else{
                System.out.println("get lock fail");
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            lock.unlock();
            System.out.println("lock unlocked");
        }
    }

    public static void main(String[] args) {
        ReentrantLockTest task = new ReentrantLockTest();
        Thread thread1 = new Thread(task);
        Thread thread2 = new Thread(task);
        thread1.start();
        thread2.start();
    }
}

执行结果为：

get lock fail
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.java:1261)
    at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:457)
    at com.xuecheng.manage_cms.ReentrantLockTest.run(ReentrantLockTest.java:24)
    at java.lang.Thread.run(Thread.java:745)
lock unlocked

抛出异常原因为: 获取锁失败的线程 并没有持有监视器的锁，却释放了监视器的锁

原文地址：https://www.cnblogs.com/yk414/p/12019353.html