Java多线程_ReentrantLock

Posted 2021-02-04 橘子洲头。

ReentrantLock是重入锁，它与synchronized很像，它是synchronized的加强版，因为它具有一些synchronized没有的功能。
下面我们看看两者的区别：
synchronized具有一定的局限性：

• 当线程尝试获取锁的时候，如果获取不到锁会一直阻塞；
• 如果获取锁的线程进入休眠或者阻塞，除非当前线程异常，否则其他线程尝试获取锁必须一直等待；
• 是非公平的。

而ReentrantLock实现了AQS，可以完成下列功能：

• 可中断响应；
• 锁申请等待限时；
• 公平锁；
• 与Condition一起使用，实现synchronized与wait/notify的功能。

引入几个概念：
提到ReentrantLock，我们不得不明白几个概念：

• 可重入锁。可重入锁是指同一个线程可以多次获取同一把锁。ReentrantLock和synchronized都是可重入锁。
• 可中断锁。可中断锁是指线程尝试获取锁的过程中，是否可以响应中断。synchronized是不可中断锁，ReentrantLock则提供了中断功能。
• 公平锁与非公平锁。公平锁是指多个线程必须按顺序，不许插队。非公平锁允许插队。synchronized是非公平锁，而ReentrantLock的默认实现是非公平锁，但是也可以设置为公平锁。
• CAS，在前面已经提到。

使用示例
具体用法通过简单代码通过代码演示：

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockDemo implements Runnable {
    static ReentrantLock lock = new ReentrantLock();
    static int count = 0;

    @Override
    public void run() {
        for (int i = 0; i < 10000; i++) {
            lock.lock();
            try {
                count++;
            } finally {
                lock.unlock();
            }
        }

    }

    public static void main(String[] args) {
        ReentrantLockDemo r = new ReentrantLockDemo();
        Thread t1 = new Thread(r);
        Thread t2 = new Thread(r);
        t1.start();
        t2.start();
        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(count);
        //结果输出：20000
    }
}

 

通过ReentrantLock解决死锁问题：

 

import java.util.concurrent.locks.ReentrantLock;

public class KillDeadlockDemo implements Runnable {
    static ReentrantLock lock1 = new ReentrantLock();
    static ReentrantLock lock2 = new ReentrantLock();
    int lock;

    public KillDeadlockDemo(int lock) {
        super();
        this.lock = lock;
    }

    @Override
    public void run() {
        try {
            if (lock == 1) {
                lock1.lockInterruptibly();
                Thread.sleep(500);
                lock2.lockInterruptibly();
            } else {
                lock2.lockInterruptibly();
                Thread.sleep(500);
                lock1.lockInterruptibly();
            }
        } catch (InterruptedException e) {
        } finally {
            if (lock1.isHeldByCurrentThread()) {
                lock1.unlock();
            }
            if (lock2.isHeldByCurrentThread()) {
                lock2.unlock();
            }
            System.out.println(Thread.currentThread().getName() + "退出！");
        }

    }

    public static void main(String[] args) {
        KillDeadlockDemo kdd1 = new KillDeadlockDemo(1);
        KillDeadlockDemo kdd2 = new KillDeadlockDemo(2);
        Thread t1 = new Thread(kdd1);
        Thread t2 = new Thread(kdd2);
        t1.start();
        t2.start();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        t2.interrupt();
      /*
       *结果输出：
       *Thread-1退出！
       *Thread-0退出！
       */

    }

}

 

使用 tryLock()或者tryLock(long timeout, TimeUtil unit) 方法进行一次限时的锁等待，也可以解决死锁问题：

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

public class TryLockDemo implements Runnable {
    static ReentrantLock lock = new ReentrantLock();

    @Override
    public void run() {
        try {
            if (lock.tryLock(1, TimeUnit.SECONDS)) {
                Thread.sleep(1100);
            } else {
                System.out.println(Thread.currentThread().getName() + "获取锁失败！释放");
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            if (lock.isHeldByCurrentThread()) {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        TryLockDemo td = new TryLockDemo();
        Thread thread1 = new Thread(td);
        Thread thread2 = new Thread(td);
        thread1.start();
        thread2.start();
        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
              //结果输出：Thread-1获取锁失败！释放
    }
}

 

公平锁演示：

import java.util.concurrent.locks.ReentrantLock;

public class FairLockDemo implements Runnable {
    static ReentrantLock lock = new ReentrantLock(true);

    @Override
    public void run() {
        while (true) {
            try {
                lock.lock();
                System.out.println(Thread.currentThread().getName() + " get lock");
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            } finally {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        FairLockDemo fld = new FairLockDemo();
        Thread t1 = new Thread(fld);
        Thread t2 = new Thread(fld);
        Thread t3 = new Thread(fld);
        t1.start();
        t2.start();
        t3.start();
/*
*Thread-0 get lock
*Thread-1 get lock
*Thread-2 get lock
*Thread-0 get lock
*Thread-1 get lock
*Thread-2 get lock
*..........
*/
    }
}

浅谈原理

 请转到：https://www.cnblogs.com/ericz2j/p/13445822.html