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活锁

基本介绍

两个线程互相改变对方的结束条件，导致最后谁也没办法结束

代码示例

两个线程t1 t2 一个成员变量count=10，t1线程的结束条件是count<=0，t2线程的结束条件是count>=20，但是t1线程是在不断减少count，t2线程是在不断增加count，最终导致两个线程动达不到结束条件导致活锁

public class TestLiveLock {
    static volatile int count = 10;
    static final Object lock = new Object();

    public static void main(String[] args) {
        new Thread(() -> {
            // 期望减到 0 退出循环
            while (count > 0) {
                sleep(0.2);
                count--;
                log.debug("count: {}", count);
            }
        }, "t1").start();
        new Thread(() -> {
            // 期望超过 20 退出循环
            while (count < 20) {
                sleep(0.2);
                count++;
                log.debug("count: {}", count);
            }
        }, "t2").start();
    }
}

结果：

一直运行，数值保持在0~20期间

解决方法：

把两个线程指令执行的时间进行交错就行了，具体就是暂停时间改为随机

饥饿

基本介绍

多个线程中有一个线程t，由于线程之间的冲突导致t线程分的时间片低，导致t线程基本不运行，这种情况下t线程就处于饥饿

代码示例

我们前面遇见了死锁-哲学家问题，这个问题可以通过顺序加锁的方法解决，但是这个方法会导致饥饿

public class TestDeadLock {
    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("苏格拉底", c1, c2).start();
        new Philosopher("柏拉图", c2, c3).start();
        new Philosopher("亚里士多德", c3, c4).start();
        new Philosopher("赫拉克利特", c4, c5).start();
        new Philosopher("阿基米德", c1, c5).start();
    }
}

@Slf4j(topic = "c.Philosopher")
class Philosopher extends Thread {
    Chopstick left;
    Chopstick right;

    public Philosopher(String name, Chopstick left, Chopstick right) {
        super(name);
        this.left = left;
        this.right = right;
    }

    @Override
    public void run() {
        while (true) {
            //　尝试获得左手筷子
            synchronized (left) {
                // 尝试获得右手筷子
                synchronized (right) {
                    eat();
                }
            }
        }
    }

    Random random = new Random();
    private void eat() {
        log.debug("eating...");
        Sleeper.sleep(0.5);
    }
}

class Chopstick {
    String name;

    public Chopstick(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "筷子{" + name + '}';
    }
}

结果：

02:32:24.639 c.Philosopher [阿基米德] - eating…
02:32:24.639 c.Philosopher [亚里士多德] - eating…
02:32:25.149 c.Philosopher [赫拉克利特] - eating…
02:32:25.658 c.Philosopher [赫拉克利特] - eating…
02:32:26.168 c.Philosopher [亚里士多德] - eating…
02:32:26.168 c.Philosopher [阿基米德] - eating…
02:32:26.678 c.Philosopher [亚里士多德] - eating…
02:32:26.678 c.Philosopher [阿基米德] - eating…
02:32:27.188 c.Philosopher [赫拉克利特] - eating…
02:32:27.697 c.Philosopher [阿基米德] - eating…
02:32:27.697 c.Philosopher [亚里士多德] - eating…
02:32:28.207 c.Philosopher [赫拉克利特] - eating…
02:32:28.717 c.Philosopher [赫拉克利特] - eating…
02:32:29.226 c.Philosopher [赫拉克利特] - eating…
02:32:29.736 c.Philosopher [阿基米德] - eating…
02:32:29.736 c.Philosopher [亚里士多德] - eating…
02:32:30.246 c.Philosopher [阿基米德] - eating…
02:32:30.246 c.Philosopher [亚里士多德] - eating…
02:32:30.756 c.Philosopher [赫拉克利特] - eating…
02:32:31.265 c.Philosopher [赫拉克利特] - eating…
02:32:31.775 c.Philosopher [亚里士多德] - eating…
02:32:31.775 c.Philosopher [阿基米德] - eating…
02:32:32.285 c.Philosopher [亚里士多德] - eating…
02:32:32.795 c.Philosopher [亚里士多德] - eating…
02:32:33.304 c.Philosopher [赫拉克利特] - eating…
02:32:33.814 c.Philosopher [亚里士多德] - eating…
02:32:34.324 c.Philosopher [赫拉克利特] - eating…
02:32:34.833 c.Philosopher [赫拉克利特] - eating…
02:32:35.343 c.Philosopher [赫拉克利特] - eating…
02:32:35.853 c.Philosopher [赫拉克利特] - eating…
02:32:36.363 c.Philosopher [赫拉克利特] - eating…
02:32:36.872 c.Philosopher [赫拉克利特] - eating…

解释：

观察下面两张图

我们通过顺序加锁解决了死锁问题，但也同时使得阿基米德竞争加大获得两个筷子的概率大大下降使得赫拉克利特竞争减小获得两个筷子的概率大大增强，也就是最终结果显示的那样。
阿基米德现在的这种情况就被成为饥饿。

【深入操作系统】线程相关常见现象解释（二）

活锁

基本介绍

代码示例

饥饿

基本介绍

代码示例