简单的Java多线程测试不起作用(Simple Java multihreading test not working)

 我写了一些java代码，使用while循环可以在1秒内增加一个值。 
 
 然后我尝试使用4个线程在1秒内尽可能多地增加四个值。 我期待有四个线程，与简单循环相比，我将获得四倍的值。 
 
 代码如下： 
 
package threadTest;

public class ThreadTestMain {

public static void main(String[] args) {

    long end_time = 0;
    long value = 0;
    long runtime = 0;

    long start_time = System.currentTimeMillis();
    while (runtime<=1000) {
        value++;
        runtime = System.currentTimeMillis() - start_time;
    }

    System.out.println(value);
    end_time = System.currentTimeMillis();
    System.out.println("DEBUG-> Time should be 1sec : " + (end_time - start_time));

    SeperateExecution myObject = new SeperateExecution(0);
    SeperateExecution myObject2 = new SeperateExecution(0);
    SeperateExecution myObject3 = new SeperateExecution(0);
    SeperateExecution myObject4 = new SeperateExecution(0);

    Thread worker1 = new Thread(myObject,"worker1");
    Thread worker2 = new Thread(myObject2,"worker2");
    Thread worker3 = new Thread(myObject3,"worker3");
    Thread worker4 = new Thread(myObject4,"worker4");

    worker1.start();worker2.start();worker3.start();worker4.start();
}

}
 
 和 
 
package threadTest;

//import java.util.concurrent.atomic.AtomicLong;

public class SeperateExecution implements Runnable {
private long value = 0;
//private AtomicLong value = null;
private long start_time = 0;

public SeperateExecution (long p_val)
{
    this.value = p_val;
    //this.value = new AtomicLong(p_val);
    //this.start_time = p_st;
}

@Override
public void run(){
    long runtime = 0;
    this.start_time = System.currentTimeMillis();
    while (runtime<=1000) {
        //value.incrementAndGet();
        value++;
        runtime = System.currentTimeMillis() - start_time;
    }
    System.out.println(Thread.currentThread().getName() + " DONE " + value + 
            " Thread Exection Time " + (System.currentTimeMillis()- start_time));
}
}
 
 代码输出如下： 
 
52266551
DEBUG-> Time should be 1sec : 1001
worker1 DONE 13364059 Thread Exection Time 1001
worker2 DONE 13744972 Thread Exection Time 1001
worker4 DONE 11652084 Thread Exection Time 1001
worker3 DONE 13605645 Thread Exection Time 1001
 
 有人可以帮助我理解为什么多线程值总和一个小于正常while循环的值？ 
 
 一个类比，以帮助理解我的问题：如果一个工人一天可以挖一个洞，那么四个工人可以一天挖四个洞。 
 
 谢谢 

I wrote some java code that would increase a value as much it could in 1 second using a while loop.
 
I then tried increasing four values as much as it could in 1 second using 4 threads. I was expecting that with four threads I would get to four times the value as compared with the simple loop.
 
Code is as follows:
 
package threadTest;

public class ThreadTestMain {

public static void main(String[] args) {

    long end_time = 0;
    long value = 0;
    long runtime = 0;

    long start_time = System.currentTimeMillis();
    while (runtime<=1000) {
        value++;
        runtime = System.currentTimeMillis() - start_time;
    }

    System.out.println(value);
    end_time = System.currentTimeMillis();
    System.out.println("DEBUG-> Time should be 1sec : " + (end_time - start_time));

    SeperateExecution myObject = new SeperateExecution(0);
    SeperateExecution myObject2 = new SeperateExecution(0);
    SeperateExecution myObject3 = new SeperateExecution(0);
    SeperateExecution myObject4 = new SeperateExecution(0);

    Thread worker1 = new Thread(myObject,"worker1");
    Thread worker2 = new Thread(myObject2,"worker2");
    Thread worker3 = new Thread(myObject3,"worker3");
    Thread worker4 = new Thread(myObject4,"worker4");

    worker1.start();worker2.start();worker3.start();worker4.start();
}

}
 
and 
 
package threadTest;

//import java.util.concurrent.atomic.AtomicLong;

public class SeperateExecution implements Runnable {
private long value = 0;
//private AtomicLong value = null;
private long start_time = 0;

public SeperateExecution (long p_val)
{
    this.value = p_val;
    //this.value = new AtomicLong(p_val);
    //this.start_time = p_st;
}

@Override
public void run(){
    long runtime = 0;
    this.start_time = System.currentTimeMillis();
    while (runtime<=1000) {
        //value.incrementAndGet();
        value++;
        runtime = System.currentTimeMillis() - start_time;
    }
    System.out.println(Thread.currentThread().getName() + " DONE " + value + 
            " Thread Exection Time " + (System.currentTimeMillis()- start_time));
}
}
 
The code output is as follows:
 
52266551
DEBUG-> Time should be 1sec : 1001
worker1 DONE 13364059 Thread Exection Time 1001
worker2 DONE 13744972 Thread Exection Time 1001
worker4 DONE 11652084 Thread Exection Time 1001
worker3 DONE 13605645 Thread Exection Time 1001
 
Can someone please help me understand why the multithreaded values sum up to a value less that the normal while loop?
 
An analogy to help understand my question: If one worker can dig one hole in a day then surly four workers can dig four holes in a day.
 
Thanks

原文：https://stackoverflow.com/questions/24069210