Why is incrementing a number in 10 Java threads not resulting in a value of 10?

Question

I don't understand the value of 'a' is 0,Why is 'a' not 10,What is the running process of that code,Is it necessary to analyze from Java Memory Model? Here is my test code

package com.study.concurrent.demo;

import lombok.extern.slf4j.Slf4j;
import org.junit.jupiter.api.Test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

//@SpringBootTest
@Slf4j
class DemoApplicationTests {

    int a = 0;
    int b = 0;
    @Test
    void contextLoads() {
        ExecutorService executorService = Executors.newFixedThreadPool(1);
//        final Semaphore semaphore = new Semaphore(3);
        for (int i = 0; i < 10; i++) {
            executorService.execute(() -> {
//                try {
//                    semaphore.acquire();
//                } catch (InterruptedException e) {
//                    e.printStackTrace();
//                }
                add();
                bdd();
//              log.info("a: {},concurrent_id: {}",a,Thread.currentThread().getName());
//                semaphore.release();
            });
        }
        executorService.shutdown();
        log.info("The final value of a：{}",a);
        log.info("The final value of b：{}",b);
    }


    public void add(){
        a++;
    }
    public void bdd(){
        b++;
    }

}

Answer 1

Two reasons:

1. You're not waiting for the threads to finish, you're just shutting down the thread pool (that is: causing the thread pool to reject new tasks but continue to process existing tasks).

2. You're not establishing a happens-before relationship between the writes in the thread pool and the read in the main thread.

   You could do this by (amongst other methods):

   1. Acquiring the semaphore before reading a;
   2. By using submit instead of execute to get a Future<?> for each of the tasks submitted, and invoking the Future.get() method on all of the returned futures. It is documented in the Javadoc of ExecutorService that this establishes a happens-before.

The first point is the "main" reason why a is coming out as zero: if I run it locally, and wait for the the thread pool to terminate, a comes out to 10.

However, just because it comes out as 10 doesn't mean the code works correctly without paying attention to the second point: you need to apply the Java Memory Model to have guarantees of correct functioning.

Answer 2

Issues

1. Visibility - Multiple threads are accessing the same variable and the code does not have any visibility guarantees

2. volatile can help with visibility guarantee

3. Atomicity - Multiple threads are updating through a++ or b++ operations. These are not atomic operations. This is primarily set of operations 1. fetch a. 2. increment a. 3. update a. A context switch can happen in any of these states and result in incorrect value.

4. So volatile visibility alone is not enough for correctness

5. Use AtomicInteger to guarantee atomicity of the increment operation

6. AtomicXXX can guarantee atomicity of a single operation

7. If there was a need to increment both a and b together, then some form of synchronization is needed

8. Communication - This is not communication between the main thread and executor task threads to communicate completion events

9. executorService.shutdown() will not ensure this communication

10. Latch can be used for this communication

11. Or as mentioned by Andy, Future can be used

An example code with AtomicInteger and Latch

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class DemoApplicationTests {
    final AtomicInteger a = new AtomicInteger(0);
    final AtomicInteger b = new AtomicInteger(0);

    void contextLoads() throws Exception {
        CountDownLatch latch = new CountDownLatch(10);
        ExecutorService executorService = Executors.newFixedThreadPool(1);
        for (int i = 0; i < 10; i++) {
            executorService.execute(() -> {
                add();
                bdd();
                latch.countDown();
            });
        }
        latch.await();
        executorService.shutdown();
        System.out.println("The final value of a：" + a);
        System.out.println("The final value of b：" + b);
    }

    public void add() {
        a.incrementAndGet();
    }
    public void bdd() {
        b.incrementAndGet();
    }

    public static void main(String[] args) throws Exception {
        new DemoApplicationTests().contextLoads();
    }
}

An incorrect solution with threadpool size > 1 and CompletableFuture due to race conditions in a++, b++.

The following can(my knowledge is limited and can't confirm either way) be a perfectly legal code for a thread pool size of 1 (copied from Eugene's answer)

But when the same code was executed with thread pool size > 1, it will result in race conditions. (again the intention is to discuss about multiple threads and data visibility issues as is and not to project Eugene's answer as incorrect. Eugene's answer is in the context of single thread in threadpool and might be perfectly valid for single threaded threadpool scenario)

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DemoApplicationTests {
    int a = 0;
    int b = 0;

    void contextLoads() throws Exception {
        final int count = 10000;
        ExecutorService executorService = Executors.newFixedThreadPool(100);
        List<Runnable> list = new ArrayList<>();
        for (int i = 0; i < count; i++) {
            Runnable r = () -> {
                add();
                bdd();
            };
            list.add(r);
        }

        CompletableFuture<?>[] futures = list.stream()
            .map(task -> CompletableFuture.runAsync(task, executorService))
            .toArray(CompletableFuture[]::new);

        CompletableFuture.allOf(futures).join();

        executorService.shutdown();
        System.out.println("The final value of a: " + a);
        System.out.println("The final value of b：" + b);
    }

    public void add() {
        a++;
    }
    public void bdd() {
        b++;
    }

    public static void main(String[] args) throws Exception {
        new DemoApplicationTests().contextLoads();
    }
}

Thank you @Basil Bourque for fixing the grammatical errors

Answer 3

Your pool has 1 thread, and you submit 10 Runnables to it. They will all pile-up in a queue, until it's their turn to execute. Instead of waiting for all of them to finish, you call shutDown, effectively saying : "no more tasks will this pool take". When exactly is that going to happen and how many tasks have already been processed before the call to shutDown happened, is impossible to tell. As such, you get a very non-deterministic result. You could even see 10 as the output (sometimes), but that does not mean this is correct.

Instead, you can wait for the pool to finish executing all of its tasks:

executorService.awaitTermination(2, TimeUnit.SECONDS);
executorService.shutdown();

What slightly "sucks" is that awaitTermination does not explicitly mentions that if it returns true, it would establish a happens-before relationship. So to be pedantic with the JLS, you would need to work with that Semaphore for example, to establish the needed guarantees.

---

You have a race in your code, by updating a shared a and b from multiple threads (even if you currently use Executors.newFixedThreadPool(1)), without any synchronization. So that needs correction also. And a Semaphore semaphore = new Semaphore(3); is not going to help, since you still will allow 3 concurrent threads to work on those variables; you would need only a single permit. But then, this acts as Lock more then a Semaphore.

Answer 4

The other Answers are correct, with important points. In addition, let me show how future technology being developed in Project Loom will simplify such code.

Project Loom

Project Loom will be bringing some changes to Java. Experimental builds of Loom technology, based on early-access Java 17, are available now. The Loom team is soliciting feedback.

AutoCloseable and try-with-resources

One change is that ExecutorService extends AutoCloseable. This means we can use try-with-resources syntax to conveniently and automatically close the service after the try-block completes.

Block until submitted tasks are done

Furthermore, the flow-of-control blocks on that try-block until all the submitted tasks are done/failed/canceled. No need to track progress of the individual tasks, unless you care to.

try (
        ExecutorService executorService = Executors.newVirtualThreadExecutor() ;
)
{
    … submit tasks to the executor service, to be run on background threads.
}
// At this point, all submitted are done/failed/canceled. 
// At this point, the executor service is automatically being shut down.

AtomicInteger

As others said, your use of int primitives for a & b variables across threads may fail because of visibility issues in the Java Memory Model. One option is to mark them as volatile.

I prefer the alternative, using the Atomic… classes. Replace those int vars with AtomicInteger objects to wrap the incrementing count number.

Mark those member fields final so each instance is never replaced.

// Member fields
final AtomicInteger a, b;

// Constructor
public Incrementor ( )
{
    this.a = new AtomicInteger();
    this.b = new AtomicInteger();
}

To increment by one the value within the AtomicInteger, we call incrementAndGet. This call returns the new incremented number. So I altered the signature of your add methods to show that we can return the new value, if ever needed.

// Logic
public int addA ( )
{
    return this.a.incrementAndGet();
}

public int addB ( )
{
    return this.b.incrementAndGet();
}

Virtual threads

Another feature coming in Project Loom is virtual threads, also known as fibers. Many of these lightweight threads are mapped to run on platform/kernel threads. If your code often blocks, then using virtual threads will dramatically speed up performance of your app. Use the new feature by calling Executors.newVirtualThreadExecutor.

try (
        ExecutorService executorService = Executors.newVirtualThreadExecutor() ;
)
{ … }

Example class

I have written a class named Incrementor to be similar to yours. Using such a class looks like this:

        Incrementor incrementor = new Incrementor();
        try (
                ExecutorService executorService = Executors.newVirtualThreadExecutor() ;
        )
        {
            for ( int i = 0 ; i < 10 ; i++ )
            {
                executorService.submit( ( ) -> {
                    int newValueA = incrementor.addA();
                    int newValueB = incrementor.addB();
                    System.out.println( "Thread " + Thread.currentThread().getId() + " incremented a & b to: " + newValueA + " & " + newValueB + " at " + Instant.now() );
                } );
            }
        }

Coordinating a & b

Caveat: As others commented, such code does not atomically increment both a & b together in synch. Apparently that is not a need of yours, so I ignore the issue. You can see that behavior in action, in the example run output shown at bottom below, where two threads interleaved during their access to a & b. Excerpted here:

Thread 24 incremented a & b to: 10 & 9 at 2021-02-09T02:21:30.270246Z
Thread 23 incremented a & b to: 9 & 10 at 2021-02-09T02:21:30.270246Z

Full class code

Pull together all this code.

Notice the simplicity of the code when (a) under Loom, and (b) using Atomic… constants. No need for semaphores, latches, CompletableFuture, nor calling ExecutorService#shutdown.

package work.basil.example;

import java.time.Instant;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class Incrementor
{
    // Member fields
    final AtomicInteger a , b ;

    // Constructor
    public Incrementor ( )
    {
        this.a = new AtomicInteger();
        this.b = new AtomicInteger();
    }

    // Logic
    public int addA ( )
    {
        return this.a.incrementAndGet();
    }

    public int addB ( )
    {
        return this.b.incrementAndGet();
    }
}

And a main method to demonstrate using that class.

    public static void main ( String[] args )
    {
        // Exercise this class by instantiating, then incrementing ten times.
        System.out.println( "INFO - `main` starting the demo. " + Instant.now() );

        Incrementor incrementor = new Incrementor();
        try (
                ExecutorService executorService = Executors.newVirtualThreadExecutor() ;
        )
        {
            for ( int i = 0 ; i < 10 ; i++ )
            {
                executorService.submit( ( ) -> {
                    int newValueA = incrementor.addA();
                    int newValueB = incrementor.addB();
                    System.out.println( "Thread " + Thread.currentThread().getId() + " incremented a & b to: " + newValueA + " & " + newValueB + " at " + Instant.now() );
                } );
            }
        }

        System.out.println( "INFO - At this point all submitted tasks are done/failed/canceled, and executor service is shutting down. " + Instant.now() );
        System.out.println( "incrementor.a.get() = " + incrementor.a.get() );
        System.out.println( "incrementor.b.get() = " + incrementor.b.get() );
        System.out.println( "INFO - `main` ending. " + Instant.now() );
    }

When run.

INFO - `main` starting the demo. 2021-02-09T02:21:30.173816Z
Thread 18 incremented a & b to: 4 & 4 at 2021-02-09T02:21:30.245812Z
Thread 14 incremented a & b to: 1 & 1 at 2021-02-09T02:21:30.242306Z
Thread 20 incremented a & b to: 6 & 6 at 2021-02-09T02:21:30.246784Z
Thread 21 incremented a & b to: 8 & 8 at 2021-02-09T02:21:30.269666Z
Thread 22 incremented a & b to: 7 & 7 at 2021-02-09T02:21:30.269666Z
Thread 17 incremented a & b to: 3 & 3 at 2021-02-09T02:21:30.243580Z
Thread 24 incremented a & b to: 10 & 9 at 2021-02-09T02:21:30.270246Z
Thread 23 incremented a & b to: 9 & 10 at 2021-02-09T02:21:30.270246Z
Thread 16 incremented a & b to: 2 & 2 at 2021-02-09T02:21:30.242335Z
Thread 19 incremented a & b to: 5 & 5 at 2021-02-09T02:21:30.246646Z
INFO - At this point all submitted tasks are done/failed/canceled, and executor service is shutting down. 2021-02-09T02:21:30.279542Z
incrementor.a.get() = 10
incrementor.b.get() = 10
INFO - `main` ending. 2021-02-09T02:21:30.285862Z