Java多线程死锁

概念

Java中线程死锁的概念：

线程A占用某个锁a的同时并尝试占用锁b，而线程B占用某个锁b的同时并尝试占用锁a，这种情况变会出现死锁。如图所示：

例子

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

 class ThreadTask1 extends Thread{
    @Override
    public void run() {
        try {
            ReentrantLockExample.lock1.lock();
            System.out.println(this.getName() +" run task");
            ReentrantLockExample.lock2.lock();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ReentrantLockExample.lock1.unlock();
            ReentrantLockExample.lock2.unlock();

        }
        System.out.println(this.getName() + "run complete");

    }
}

 class ThreadTask2 extends Thread{
    @Override
    public void run() {
        try {
            ReentrantLockExample.lock2.lock();
            System.out.println(this.getName() +" run task");
            ReentrantLockExample.lock1.lock();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            ReentrantLockExample.lock2.unlock();
            ReentrantLockExample.lock1.unlock();
        }
        System.out.println(this.getName() + "run complete");
    }
}

public class ReentrantLockExample extends Thread {
    public static  Integer count=1;
    public static final Lock lock1 = new ReentrantLock();
    public static final Lock lock2 = new ReentrantLock();

    public static void main(String args[]) throws InterruptedException {
        ThreadTask1 task1 = new ThreadTask1();
        ThreadTask2 task2 = new ThreadTask2();

        task1.start();
        task2.start();

    }
}

运行以上代码，程序不会退出，并发生了死锁，如图所示：

如何避免死锁

在对象的同步方法中避免调用其它对象的同步方法即可。