为什么是 1？这行代码的结果

This topic created in 1259 days ago, the information mentioned may be changed or developed.

简单描述下代码：

static 变量 i

线程 1：尝试把 i 设为 1

线程 2：尝试把 i 设为 2 ，如果 i 为 1 ，print i

不断运行线程 1 和 2

public class Main {

    // static 变量 i
    static volatile Integer i = 0;

    public static void main(String[] args) throws Exception {

        // 线程 1
        Thread thread1 = new Thread(()->{
            synchronized(i) {
                try {

                    i = 1;
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        });

        // 线程 2
        Thread thread2 = new Thread(()->{
            synchronized (i) {
                try {

                    i = 2;
                    Thread.sleep(100);
                    if (i == 1) {
                        System.out.println("1"); // 实际运行结果为 1
                    }
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        });

        // 不断运行线程 1 和 2
        while (true) {
            new Thread(thread1).start();
            new Thread(thread2).start();
        }
    }
}

为什么运行的结果是 1

还请各位大佬不吝赐教（抱拳），

谢谢各位大哥

7 replies • 2023-02-03 10:47:01 +08:00

RedBeanIce

Feb 2, 2023

import java.util.concurrent.atomic.AtomicLong;

public class VolatileTest1 {

private static final AtomicLong THREAD1_COUNT = new AtomicLong(0);
private static final AtomicLong THREAD2_COUNT = new AtomicLong(0);

// static 变量 i
static volatile Integer i = 0;

public static void main(String[] args) throws Exception {

// 线程 1
Thread thread1 = new Thread(() -> {
synchronized (i) {
try {

i = 1;
Thread.sleep(100);
THREAD1_COUNT.incrementAndGet();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
});

// 线程 2
Thread thread2 = new Thread(() -> {
synchronized (i) {
try {

i = 2;
Thread.sleep(100);
if (i == 1) {
System.out.println("1"); // 实际运行结果为 1
System.out.println("thread1 运行次数" + THREAD1_COUNT.get());
System.out.println("thread2 运行次数" + THREAD2_COUNT.get());
}
THREAD2_COUNT.incrementAndGet();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
});

// 不断运行线程 1 和 2
while (true) {
new Thread(thread1).start();
new Thread(thread2).start();
}
}
}

要不你试试这个，，看看