Java多线程错误共享问题

Question

class PersonValue {
volatile public int valueA;
@Contended
volatile public int valueB;
}
class Person extends Thread {
public final static long ITERATIONS = 500L * 1000L * 1000L ;
public static volatile PersonValue personValue = new PersonValue();
int index = 0;
public Person(int index) {
this.index = index;
}
@Override
public void run() {
long i = ITERATIONS;
while (--i > 0) {
if (index == 0) {
personValue.valueA = 3;
} else {
personValue.valueB = 3;
}
}
}
}
public class TestContend {
public static void main(String[] args) throws InterruptedException {
long start = System.currentTimeMillis();
Person person = new Person(0);
Person person1 = new Person(1);

person.start();
person1.start();
person.join();
person1.join();
long end = System.currentTimeMillis();
System.out.println("Duration " + TimeUnit.MILLISECONDS.toSeconds(end - start));
}
}

输出：

Duration 3

当我在PersonValue中注释@Contended代码并输出时：Duration 11

上面的输出满足了我的期望，但我修改了代码如下：

class Person extends Thread {
public final static long ITERATIONS = 500L * 1000L * 1000L ;
public volatile static int personValuesA = 3;
public volatile static int personValuesB = 1;
int index = 0;
public Person(int index) {
this.index = index;
}
@Override
public void run() {
long i = ITERATIONS;
while (--i > 0) {
if (index == 0) {
personValuesA = 3;
} else {
personValuesB = 3;
}
}
}
}
public class TestContend {
public static void main(String[] args) throws InterruptedException {
long start = System.currentTimeMillis();
Person person = new Person(0);
Person person1 = new Person(1);

person.start();
person1.start();
person.join();
person1.join();
long end = System.currentTimeMillis();
System.out.println("Duration" + TimeUnit.MILLISECONDS.toSeconds(end - start));
}
}

输出：

Duration 12

然后我在personValuesB:中添加了注释@Contended

class Person extends Thread {
//...
public volatile static int personValuesA = 3;
@Contended
public volatile static int personValuesB = 1;
}

和输出：Duration 12

这两个程序同时运行。为什么？

我的问题：

1. 为什么添加@Contended注释后运行时间相同

Answer 1

@Contended不适用于静态字段，仅适用于实例字段。