双非本科准备秋招（20.2）—— 线程活跃性：死锁、活锁、饥饿

一、死锁

一个线程需要获得多把锁，就容易出现死锁。

比如此时有两把锁，分别是A和B。线程1首先需要获得A，然后获得B；线程2首先需要获得B，然后获得A。于是两个线程就一直等待对方释放锁。

二、死锁之哲学家就餐

一个圆桌，五个人，五只筷子，每个人吃饭需要拿起左右两边的筷子吃，设计代码：

设计筷子类

class Chopstick{String name;public Chopstick(String name) {this.name = name;}@Overridepublic String toString() {return "筷子{" +"name='" + name + '\'' +'}';}
}

设计哲学家类

@Slf4j(topic = "c.phi")
class philosopher extends Thread{Chopstick left;Chopstick right;public philosopher(String name, Chopstick left, Chopstick right) {super(name);this.left = left;this.right = right;}private void eat(){log.debug("吃饭吃饭");try {Thread.sleep(400);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic void run(){while (true){synchronized (left){synchronized (right){eat();}}}}
}

main中测试代码如下：

public class testPhilosopher {public static void main(String[] args) {Chopstick c1 = new Chopstick("1");Chopstick c2 = new Chopstick("2");Chopstick c3 = new Chopstick("3");Chopstick c4 = new Chopstick("4");Chopstick c5 = new Chopstick("5");new philosopher("柏拉图", c1, c2).start();new philosopher("康德", c2, c3).start();new philosopher("尼采", c3, c4).start();new philosopher("庄子", c4, c5).start();new philosopher("马克思", c5, c1).start();}
}

可以看到，执行不下去了：

死锁定位

我们先用jps命令定位进程id，可以看到id为6620

再使用jstack 6620查看：这样就能看到死锁的准确定位了

Found one Java-level deadlock:
=============================
"马克思":waiting to lock monitor 0x0000000020521d58 (object 0x000000076bf84248, a com.smy.day4.Chopstick),which is held by "柏拉图"
"柏拉图":waiting to lock monitor 0x000000001cdd2868 (object 0x000000076bf84288, a com.smy.day4.Chopstick),which is held by "康德"
"康德":waiting to lock monitor 0x000000001cdd27b8 (object 0x000000076bf842c8, a com.smy.day4.Chopstick),which is held by "尼采"
"尼采":waiting to lock monitor 0x000000001cdcff28 (object 0x000000076bf84308, a com.smy.day4.Chopstick),which is held by "庄子"
"庄子":waiting to lock monitor 0x000000001cdcffd8 (object 0x000000076bf84348, a com.smy.day4.Chopstick),which is held by "马克思"at com.smy.day4.philosopher.run(testPhilosopher.java:62)- waiting to lock <0x000000076bf84288> (a com.smy.day4.Chopstick)- locked <0x000000076bf84248> (a com.smy.day4.Chopstick)
"康德":at com.smy.day4.philosopher.run(testPhilosopher.java:62)- waiting to lock <0x000000076bf842c8> (a com.smy.day4.Chopstick)- locked <0x000000076bf84288> (a com.smy.day4.Chopstick)
"尼采":at com.smy.day4.philosopher.run(testPhilosopher.java:62)- waiting to lock <0x000000076bf84308> (a com.smy.day4.Chopstick)- locked <0x000000076bf842c8> (a com.smy.day4.Chopstick)
"庄子":at com.smy.day4.philosopher.run(testPhilosopher.java:62)- waiting to lock <0x000000076bf84348> (a com.smy.day4.Chopstick)- locked <0x000000076bf84308> (a com.smy.day4.Chopstick)Found 1 deadlock.