为什么我的同步方法不起作用？

Question

Simply put, i'm trying to see the difference when using sychronized keyword over just running a function over threads without locks at all.

In this code:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.stream.IntStream;


public class mainClass {
static int count=0;

public static void main(String[] args) {
    ExecutorService executor = Executors.newFixedThreadPool(2);
    Runnable r =new Runnable() {

        public synchronized void run() {
            count = count + 1;
        }
    };
    IntStream.range(0, 10000)
            .forEach(i -> executor.submit(r::run));

    executor.shutdown();

    System.out.println(count);  // 10000

}
}

It doesn't work as i predicated it to work, it returns 10000 in like 40% of the runs. Why is that? Where is the problem? I thought that the function run is being run by only 1 Thread at a time, so there shouldn't be problem, but clearly i'm wrong.

Answer 1

ExecutorService#shutdown does not wait for tasks to complete. You should use awaitTermination for that.

See the documentation for ExecutorService#shutdown .

IntStream.range(0, 10000)
        .forEach(i -> executor.submit(r::run));

executor.shutdown();
executor.awaitTermination(1, TimeUnit.MINUTE); // <!-- HERE