多线程读写int值不需要同步？

Question

很多地方讲多线程时使用了下面UnsafeSequence类的getNext方法，说明如果不同步会得到错误的结果，但我的测试却从未出现过错误，请高手看看是怎么回事，谢谢！
多个线程同时持有对象us，它的getNext方法没有进行同步，按照书上的说法，由于有几个线程会同时拿到value，然后加1，10个线程分别加1后最终应该是一个小于9的值，但测试结果总是9。
[code="java"]
public class Main2 {
public static void main(String[] args) {
UnsafeSequence us = new UnsafeSequence();
for (int i = 0; i < 10; i++) {
Runnable myRunnable = new MyRunnable(us);
new Thread(myRunnable).start();
}
}
}
public class MyRunnable implements Runnable {
private final UnsafeSequence us;
public MyRunnable(UnsafeSequence _us) {
us = _us;
}
@Override
public void run() {
System.out.println("next in " + Thread.currentThread().getName() + ": "
+ +us.getNext());
}
}
public class UnsafeSequence {
public int value = 0;

public int getNext() {
    return value++;
}

}[/code]
下面是运行结果：

next in Thread-0: 0
next in Thread-9: 5
next in Thread-7: 4
next in Thread-5: 3
next in Thread-3: 2
next in Thread-1: 1
next in Thread-2: 6
next in Thread-4: 7
next in Thread-6: 8
next in Thread-8: 9

Answer 1

多线程同步问题就是这点让人头疼：不是每次必然发生，有时候依赖数据或运行环境。

你的测试中没有发生的原因可能有很多，比如：

• 机器是单核的话，基本不会发生。多核的更加容易发生。
• 机器是多核的，但测试程序没能被系统分配成多线程。
• 运气问题。

我修改了下你的程序，在红线部分加入1秒睡眠，在我的环境下，是第3次发生的。

[code="java"]

public class Test2 {
public static void main(String[] args) {

    UnsafeSequence us = new UnsafeSequence();
    int count = 10;

    Thread[] ts = new Thread[count];
    for (int i = 0; i < count; i++) {
        Thread t = new Thread(new MyRunnable(us));
        ts[i] = t;
        t.start();
    }
}

}

class MyRunnable implements Runnable {
private final UnsafeSequence us;

public MyRunnable(UnsafeSequence _us) {
    us = _us;
}

@Override
public void run() {
    try {
        [color=red]Thread.sleep(1000);[/color]
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println("next in " + Thread.currentThread().getName() + ": "
            + +us.getNext());
}

}

class UnsafeSequence {
public int value = 0;

public int getNext() {
    return value++;
}

}
[/code]

出错的结果如下

[code="java"]

next in Thread-1: 0
next in Thread-0: 0
next in Thread-2: 1
next in Thread-5: 2
next in Thread-3: 3
next in Thread-4: 4
next in Thread-7: 5
next in Thread-8: 6
next in Thread-6: 7
next in Thread-9: 8
[/code]