JAVA多线程编程核心技术--synchronized--解决死锁

死锁

什么是死锁

• 死锁是指两个或两个以上的进程在执行过程中，由于竞争资源或者由于彼此通信而造成的一种阻塞的现象，若无外力作用，它们都将无法推进下去。

死锁代码

Service.java

package com.service;

/**
 * 死锁现象
 * 本质是资源竞争引起的
 * 1、A线程持有者B线程需要的锁
 * 2、B线程持有者A线程需要的锁
 * 3、两个synchronized嵌套非常容易引起死锁
 */
public class Service {

    private final String lock = "1";
    private final String lock2 = "2";

    public void lock1() throws InterruptedException {
        synchronized (lock) {
            Thread.sleep(500);
            synchronized (lock2) {
                System.out.println("lock1");
            }
        }
    }

    public void lock2() throws InterruptedException {
        synchronized (lock2) {
            Thread.sleep(500);
            synchronized (lock) {
                System.out.println("lock2");
            }
        }

    }
}

ThreadA.java

public class ThreadA extends Thread {
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {

        try {
            service.lock1();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

ThreadB.java

package com.service;

public class ThreadB extends Thread {

    private Service service;

    public ThreadB(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        try {
            service.lock2();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

Run.java

package com.service;

public class Run {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA a = new ThreadA(service);
        a.setName("A");
        a.start();

        ThreadB b = new ThreadB(service);
        b.setName("B");
        b.start();
    }
}

运行结果

image.png

总结

• 嵌套synchronized少用