Thread scope in java for object instance

Question

I understand that objects are stored in heap space. with more details here : https://www.hackerearth.com/practice/notes/runtime-data-areas-of-java/

in the following code the object param the reference of it is stored in thread stack, but the object itself is stored in heap :

private void foo(Object param) { 
        ....
    }

To ask my question , At first I will start with code :

public class Thread1 implements Runnable {
Test test = new Test();

public void run() {
    test=new Test(); // This will affect other thread , the object reference is changed here
    System.out.println(test.id());
   }
    }

in the code above all the threads from the same instance of Thread1 will have the same reference( let's say variable) of test , which means changing the reference of test will affect other thread:

  Runnable runnable=new Thread1();
    Thread thread1=new Thread(runnable);
    thread1.start();
    Thread t2=new Thread(runnable);
    thread2.start();

The question here , test will be stored in heap. but how does thread access it ? ( I don't think it will have reference in stack, because changing the value inside thread won't affect the other in this case). if the thread can access that variable directly ( let's say no reference in stack) what scope will it have ? ( I mean shouldn't it be limited to it's own variable )

miskender · Accepted Answer · 2018-12-15 18:07:59Z

1

When you pass your Runnable instance to Thread constructor, it will store this object in a private field. When you start your thread, the runnable instance you passed as parameter will be called. And thread will access your test object through this Runnable.(*)

If you want to know, how different threads can have its own copy of Test object You should check ThreadLocal.

Example:

public class Thread1 implements Runnable { 
   // each thread will have it's own copy of test object in this case
   private ThreadLocal<Test> test = new ThreadLocal<Test>();

    public void run() {
        // this line wont affect the others test instance.
        test.set( new Test() );

   }

}

(*) To understand this clearly you can think this as, you are just passing an object to another object.

answered Dec 15, 2018 at 18:07

miskender

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