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

Question

My code has many threads doing read-update-writes to a shared hash-map without any synchronization. I ran this many times with 1000 threads, 1M iterations, so I expect the final values for 4 keys will be around 250M, but not exactly, since the updates/writes may be lost, due to several threads reading the same value due to reading at the same time.

However, after many trials, I see that the values are always 250M, so no updates are being lost. FWIW, I am writing this toy application to compare it to when I use Collections.synchronizedMap, however, I need this app to fail (to show a race condition) but it always seems to produce a no-update-lost result.

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

My congratulations: you've written thread-safe code, as Thread.run() just starts internal Runnable.run, so you invoke all operations sequentially. Use Thread.start()instead:

This is how Thread.run() looks like:

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