java - 多线程死锁测试

问题描述

package test;import java.text.DateFormat;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.TimeUnit;/** * Created by rhwayfun on 16-4-3. */public class ThreadTest { private static DateFormat format = new SimpleDateFormat('HH:mm:ss'); public synchronized void tryOther(ThreadTest other) throws InterruptedException {System.out.println(Thread.currentThread().getName() + ' enter tryOther method at ' + format.format(new Date())); System.out.println(Thread.currentThread().getName() + ' tryOther method is about to invoke other method at ' + format.format(new Date()));other.other(); } public synchronized void other() throws InterruptedException {System.out.println(Thread.currentThread().getName() + ' enter other method atatatatat ' + format.format(new Date())); } public static void main(String[] args) throws InterruptedException {final ThreadTest d1 = new ThreadTest();final ThreadTest d2 = new ThreadTest();Thread t1 = new Thread(new Runnable() { public void run() {try { d1.tryOther(d2);} catch (InterruptedException e) { e.printStackTrace();} }}, 'threadA');Thread t2 = new Thread(new Runnable() { public void run() {try { d2.tryOther(d1);} catch (InterruptedException e) { e.printStackTrace();} }}, 'threadB');t1.start();//让threadA先运行一秒TimeUnit.SECONDS.sleep(1);t2.start(); }}

如上，随便找的产生死锁的代码，问题：TimeUnit.SECONDS.sleep(1);加上这行后，不存在死锁问题。sleep并不释放锁，为何这边死锁情况会消失。输出结果为：threadA enter tryOther method at 15:37:39threadA tryOther method is about to invoke other method at 15:37:39threadA enter other method atatatatat 15:37:39threadB enter tryOther method at 15:37:40threadB tryOther method is about to invoke other method at 15:37:40threadB enter other method atatatatat 15:37:40

注掉这行，正常死锁。输出结果为：threadB enter tryOther method at 15:37:10threadA enter tryOther method at 15:37:10threadB tryOther method is about to invoke other method at 15:37:10threadA tryOther method is about to invoke other method at 15:37:10

问题解答

线程A拿到tryOther锁但是他还要得到other的锁线程B拿到tryOther的锁但是他还要拿到other的锁 有可能A刚刚释放锁B也刚刚释放tryOther的锁.此时但是他们同时都想要获取other的锁 此时谁也不让谁 发生死锁解决方法让俩个线程不要同时去抢第二把锁.让A停一会但是如果你把时间调成纳秒级别 多次尝试也会发生死锁不建议这样预防死锁.如果并发量高的情况下.

双方在争同一把锁，不会死锁啊

自己突然知道为什么了，懒得删帖子了。写下我的看法，如果有错，欢迎指正，轻喷在没有sleep时，a线程启动，完成tryOther方法，释放锁并去执行other方法，此时b获得锁执行tryOther方法，此时a在other方法中所需资源被b线程锁住，b在执行完tryOther后需要获得a资源，由此产生死锁。

加上sleep后。在a执行tryOther方法释放锁，此时b线程并没有执行，此时顺利获得other锁。2s后b线程执行，无死锁环境。