Several threads showing a value other than System.out.println are presented in the critical section.

Question

I have the following codes:

public class MyApp {
    public static void main(String[] args) throws InterruptedException {
        SharedResource sharedResource = new SharedResource();

        Runnable first = () -> {
            sharedResource.increment(10000);
        };

        Runnable second = () -> {
            sharedResource.increment(10000);
        };

        Thread thread = new Thread(first, "FirstThread");
        Thread thread2 = new Thread(second, "SecondThread");
        thread.start();
        thread2.start();
        thread.join();
        thread2.join();
        System.out.println("The value of counter is " + sharedResource.getCounter());
    }
}

Using this class:

public class SharedResource {
    private int counter;

    public void increment(int times) {
        for (int x=1; x<=times;x++) {
            counter++;
            // System.out.println(counter);
        }
    }

    public void decrement() {
        counter--;
    }

    public int getCounter() {
        return counter;
    }
}

I am curious why this happens all the time.

When System.out.println()removed from the increment method, the total value

System.out.println("The value of counter is " + sharedResource.getCounter());

is random - this is eliminated because multiple threads use the same one counter.

However, when System.out.println(counter);presented in the increment method, the code no longer has a multithreading problem.

The end result of the counter is always 20,000, except that the code processes 10,000 times from each thread. Can someone explain to me why this is happening?

Answer 1

- .

- PrintStream, :

public void println(int x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}

, :

• (~ ns)
• , , (~ , 1000 ).
• 1

, 1000 , , , .

:

• PrintStream:

  public class NullPrintStream extends PrintStream {
    public NullPrintStream() {
        // Utility constant from apache-commons
        super(NullOutputStream.NULL_OUTPUT_STREAM);
    }
  
    @Override
    public void println(int x) {
       // No synchronization here
    }
  }
  

System.setOut(new NullPrintStream()), .

2. , runnables , :

   CountDownLatch latch = new CountDownLatch(1);
   Runnable first = () -> {
     try {
       latch.await();
       sharedResource.increment(10000);
     }
     catch (Exception e) {
     }
   };
   // ... start your threads here
   latch.countDown();
   

, , - ( , System.err, System.out)

20000

- 19996

- 19994

- 19999

20000