为什么我在此Java代码中进行读书时没有比赛条件？ [重复]

Question

，经过多次试验，我看到这些值总是250m，因此不会丢失更新。 FWIW，我正在编写此玩具应用程序，以将其与使用Collections.SynchronizedMap进行比较，但是，我需要此应用程序失败（显示竞赛条件），但是它似乎总是会产生一个不及格的结果。

Work进行了：39.496

键：0 val：250000000

键：1 val：250000000

键：2 Val：250000000

键：3 Val：250000000

package threadsafecollections;

import java.util.HashMap;
import java.util.Map;

public class SynchronizedCollectionTask {
    
    public static int NUM_THREADS = 1000;
    
    public static class ReadUpdateWrite implements Runnable {
        
        Map<Integer, Integer> map;
        int threadId;
        public static int NUM_ITERATIONS = 1000000;
        
        ReadUpdateWrite(Map<Integer, Integer> m, int threadId) {
            this.map = m;
            this.threadId = threadId;
        }

        @Override
        public void run() {
            for (int i = 0; i < NUM_ITERATIONS; i++) {
                int key = threadId % 4;
                Integer val = this.map.get(key);
                map.put(key, val == null ? 1 : val + 1);
            }
            
            // At this point I expect each key in map to have value of CLOSE TO
            // numThreads * NUM_ITERATIONS / 4 = 250M.  I expect some threads to have
            // both read the same value, and therefore "miss" a write.  For example,
            // thread 4 and 8 both read key = 0 and see value = 3.  Now both thread 
            // increment value to 4, instead of one thread incrementing to 4 and the
            // other incrementing to 5.
        }
        
    }
    public static void main(String[] args) throws InterruptedException {
        Map<Integer, Integer> sharedMap = new HashMap<Integer, Integer>();
        
        
        
        // Initialize threads
        Thread[] readers = new Thread[NUM_THREADS];
        for (int i = 0; i < NUM_THREADS; i++) {
            readers[i] = new Thread(new ReadUpdateWrite(sharedMap, i));
        }
        
        long start = System.currentTimeMillis();
        
        // Start threads
        for (int i = 0; i < NUM_THREADS; i++) {
            readers[i].run();
        }
        
        // Join threads
        for (int i = 0; i < NUM_THREADS; i++) {
            readers[i].join();
        }
        
        long end = System.currentTimeMillis();
        
        System.out.println("Work took: " + (end - start) / 1000D);
        
        for (int key : sharedMap.keySet()) {
            System.out.println ("Key: " + key + " val: " + sharedMap.get(key));
        }
    }
}

我的恭喜：您已经编写了线程安全代码，因为

Thread.run()

只是启动内部

Runnable.run

，因此您依次调用所有操作。使用

Answer 1

INSTEAD：

this this是：

：

Thread.run()