为什么当我在此 java 代码中执行读取-更新-写入时不存在竞争条件？

Question

我的代码有很多线程在没有任何同步的情况下对共享哈希映射执行读取-更新-写入。我运行多次使用 1000 个线程，1M 次迭代，所以我预计 4 个键的最终值将在 250M 左右，但不完全是，因为 updates/writes 可能会丢失，因为多个线程读取由于同时读取相同的值。但是，经过多次试验，我发现值始终为 250M，因此没有更新丢失。有人请帮帮我。 FWIW，我正在编写这个玩具应用程序来将它与我使用 Collections.syncronizedMap 时进行比较，但是，我需要这个应用程序失败（以显示竞争条件）但它似乎总是产生一个没有更新丢失的结果。

Work took: 39.496

Key: 0 val: 250000000

Key: 1 val: 250000000

Key: 2 val: 250000000

Key: 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));
        }
    }
}

Answer 1

恭喜：您已经编写了线程安全代码，因为 Thread.run() 刚刚开始内部 Runnable.run，因此您可以按顺序调用所有操作。使用 Thread.start() 代替：

这是 Thread.run() 的样子：

@Override
public void run() {
    if (target != null) {
        target.run();
    }
}

为什么当我在此 java 代码中执行读取-更新-写入时不存在竞争条件？

Why ISN'T there a race condition when I do a read-update-write in this java code?

java

multithreading

race-condition

runnable