Java: Multithreading accessing main thread variables/methods

Question

I was testing the following code, and I was wondering how come the threads could access the the increment method?

I was thinking this since thread1 and thread2 are objects created from an anonymous class that don't inherit worker class how can they access the increment() method? what is the theory behind it?

public class Worker {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

    public void run() {
        Thread thread1 = new Thread(new Runnable() {
            public void run() {
                for(int i = 0; i < 10000; i++) {
                    increment();
                }
            }
        });
        thread1.start();

        Thread thread2 = new Thread(new Runnable() {
            public void run() {
                for(int i = 0; i < 10000; i++) {
                    increment();
                }
            }
        });
        thread2.start();

        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("Count is: " + count);
    }
}

Anonymous classes have a reference to their enclosing class. — assylias
– assylias, Commented Sep 26, 2013 at 17:08

John Vint · Accepted Answer · 2013-09-26 17:09:03Z

4

Since the Runnables are non-static inner classes , the Worker.this is implicitly inherited into the Runnable instances. So that what is really happening is

public void run(){
   Worker.this.increment();
}

If the class were static this wouldn't be the case.

answered Sep 26, 2013 at 17:09

John Vint

40.3k7 gold badges81 silver badges109 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Sotirios Delimanolis · Accepted Answer · 2013-09-26 17:26:49Z

The Java Language Specification (JLS) states this about Inner Classes and Enclosing Instances

Inner classes include local (§14.3), anonymous (§15.9.5) and non-static member classes (§8.5).

and about Inner Classes and Enclosing Instances

An inner class C is a direct inner class of a class O if O is the immediately lexically enclosing class of C and the declaration of C does not occur in a static context.

and finally about Qualified this

Any lexically enclosing instance (§8.1.3) can be referred to by explicitly qualifying the keyword this.

Let C be the class denoted by ClassName. Let n be an integer such that C is the n'th lexically enclosing class of the class in which the qualified this expression appears.

The value of an expression of the form ClassName.this is the n'th lexically enclosing instance of this.

That's why you can access members of Worker.

The call

new Runnable()

within a Worker instance method, creates an inner class for Worker. So Worker is the 0'th lexically enclosing class of that Runnable. With regards to the JLS quotes above, replace ClassName with Worker and you have access to your method

Worker.this.increment()

It's done implicitly by the compiler

Jose Cifuentes · Accepted Answer · 2013-09-26 17:10:53Z

1

An anonymous inner class has access to the enclosing class instance fields and methods. In short, every instance of the inner class keeps a reference to the enclosing class precisely so that it is able to access its fields. If you want more info about inner classes, static classes, etc... check out the Java Tutorials on that topic (Nested Classes) at: http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html Also, make sure to understand the dangers associated with non-static inner classes, as they can be troublesome and create memory leaks if you hand out references to the inner instance to code outside of the enclosing class.

answered Sep 26, 2013 at 17:10

Jose Cifuentes

5966 silver badges21 bronze badges

Comments

Thorn G · Accepted Answer · 2013-09-26 17:09:17Z

0

Because it's a non-static inner class, it captures a reference to the Worker instance it is created within. That means you can invoke methods of the worker inside the anonymous class.

answered Sep 26, 2013 at 17:09

Thorn G

12.8k2 gold badges48 silver badges56 bronze badges

Comments

Alexei Kaigorodov · Accepted Answer · 2013-09-26 17:12:50Z

0

The "theory" is described in inner classes. The practice is, inner class constructors have hidden parameter which refers to outer class instance, and that parameter is saved in a hidden field. This field can be accessed with Worker.this construct from inner class methods.

answered Sep 26, 2013 at 17:12

Alexei Kaigorodov

13.6k1 gold badge24 silver badges38 bronze badges

2 Comments

M. Carlo Bramini Over a year ago

this means that anonymous classes are always treated as inner classe?

Alexei Kaigorodov Over a year ago

yes, JLS clearly states that Inner classes include local, anonymous and non-static member classes.

Collectives™ on Stack Overflow

Java: Multithreading accessing main thread variables/methods

5 Answers 5

Comments

Comments

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related