Java多线程意外结果

Question

这个问题已经有答案了:

Java Thread priority has no effect (12 个回答)

已关闭 9 年前。

我正在学习 Java 中的多线程，并在下面的代码中测量两个线程 hi 和 lo 的相对 CPU 使用率。

class Clicker implements Runnable{
long click=0;
Thread t;
private volatile boolean running =true;

public Clicker(int p)
{
    t=new Thread(this);
    t.setPriority(p);
}
public void run(){
    while(running){
        click++;
        }
}

public void stop(){
    running =false;
}

public void start(){
    t.start();
}
  }


public class HiLoPri {

  public static void main(String[] args) {
    Thread.currentThread().setPriority(Thread.MAX_PRIORITY);

    Clicker hi = new Clicker(Thread.NORM_PRIORITY +2);
    Clicker lo = new Clicker(Thread.NORM_PRIORITY -2);

    lo.start();
    hi.start();

    try{
        Thread.sleep(10000);
    }catch(InterruptedException e){
        System.out.println("Main Thread interrupted. ");
        }

    lo.stop();
    hi.stop();

    try{
        hi.t.join();
        lo.t.join();
    }catch(InterruptedException e){
        System.out.println("Interrupted Exception Caught");
    }

    System.out.println("Low priority : " + lo.click);
    System.out.println("High priority : " + hi.click);
         }

}

以下是不同优先级的输出:

1. lo = NORM_PRIORITY -2 且 hi = NORM_PRIORITY +2:低优先级:1725664879，高优先级:1774465713 ||高/低 = 1.02827
2. lo = NORM_PRIORITY -4 且 hi = NORM_PRIORITY +4:低优先级:2142378792，高优先级:2180156175 ||高/低 = 1.01763
3. lo = NORM_PRIORITY 且 hi = NORM_PRIORITY:低优先级:2582216343，高优先级:2581415280 ||高/低 = 0.99968

从输出 3 中，我了解到在两个具有相同优先级的线程中，第一个线程的优先级稍高。

对于输出 1 和输出 2，请参阅优先级值。当优先级差异变高时，计数就会增加。但是，当我将差值设置为 0(在输出 3 中)时，与上述观察结果相反，计数显示增加而不是减少。

你能解释一下为什么吗？

(规范:Java SE 7、AMD A10 四核 2.3GHz 和 Window 8)

Answer 1

Java 规范不保证在为线程分配配额时考虑优先级。

Every thread has a priority. When there is competition for processing resources, threads with higher priority are generally executed in preference to threads with lower priority. Such preference is not, however, a guarantee that the highest priority thread will always be running, and thread priorities cannot be used to reliably implement mutual exclusion.