Unit testing a parallel Java program

Question

I'm trying to write a JUnit test. My problem is that my threads can't see the object I have created in the sequential bit of code (code before starting the threads).

public class MyTest implements Runnable {

    private MyClass mc;

    /**
     * @throws InterruptedException
     */
    @Test
    public void parallelTest() throws InterruptedException {
        this.mc = new MyClass();
        Thread thread1 = new Thread(new MyTest());
        Thread thread2 = new Thread(new MyTest());
        thread1.join();
        thread2.join();
        thread1.start();
        thread2.start();
        // TODO the test
    }

    public void run() {
       if(mc != null) {
          System.out.println("ph not null");
       } else {
          System.out.println("ph null"); // THIS CODE GETS EXECUTED
       }
       // some code
    }
}

See the comment in the run method above. My object is null but I want both threads to be able to access the MyClass object. How come they see null? I tried using a constructor but I think the interface prevented me from passing a parameter to the constructor.

Many thanks.

Answer 1

Each runnable instance is created using new MyTest(). And the MyTest class doesn't have any constructor initializing the field private MyClass mc;. So it has its default value: null.

Each object has its own instance fields. That's a basic principle of OO.

Answer 2

You initialize the .mc filed of this but then you create two new instances of MyTest (in which .mc is not initialized, that is: it is null) and pass these instances to your threads.

The solution is therefore quite simple: pass this to the threads.

[EDIT]

Also, I had to switch the order of the .start() and the .join() calls - waiting for a thread to finish (i.e., join) is meaningful only if the thread has been previously started.

package p1;

import org.junit.Test;

public class MyTest implements Runnable {

  private MyClass mc;

  /**
   * @throws InterruptedException
   */
  @Test
  public void parallelTest() throws InterruptedException {
      this.mc = new MyClass();
      Thread thread1 = new Thread(this);
      Thread thread2 = new Thread(this);
      thread1.start();
      thread2.start();
      thread1.join();
      thread2.join();
  }

  public void run() {
     if(mc != null) {
        System.out.println("ph not null");
     } else {
        System.out.println("ph null"); // THIS CODE GETS EXECUTED
     }
     // some code
  }
}

When I run it I get this output:

ph not null
ph not null

Answer 3

I worked it out :). I'll give some +1s because there were some further problems I had:

1. @Jeffrey: Yes, start and join were the wrong way round
2. @JB Nizet: Thanks.. I had a crazy typo in my question (edited question)

@Itay Maman had the same idea as me but it didn't work - JUnit wouldn't let me pass a parameter into its constructor.

Solution was not to use a constructor but use @BeforeClass annotation and an init method to instantiate a MyClass.

public class MyTest implements Runnable {

    private static MyClass mc;

    @BeforeClass
    public static void init() {
        mc = new MyClass();
    }

    @Test
    public void parallelTest() throws InterruptedException {
        Thread thread1 = new Thread(new MyTest());
        Thread thread2 = new Thread(new MyTest());
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        // my assertion about mc object goes here
    }

    public void run() {
        // code to test
    }
}

This unit test still doesn't test the code perfectly because the execution path is non-deterministic so it could sometimes pass and sometimes fail. Perhaps I will add some finer grain tests too.