java - 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

我的代码有许多线程对共享 HashMap 进行读取-更新-写入操作，而无需任何同步。我用 1000 个线程、1M 迭代运行了很多次，所以我预计 4 个键的最终值将在 250M 左右，但不完全是这样，因为更新/写入可能会丢失，因为多个线程由于读取而读取相同的值同时。然而，经过多次试验，我发现这些值始终是 250M，因此没有丢失更新。有人请帮帮我。 FWIW，我正在编写这个玩具应用程序，以将其与使用 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)，但它似乎总是产生无更新丢失的结果。

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() 只是启动内部 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();
    }
}