Creating Object in a thread safety way

Question

Directly from this web site, I came across the following description about creating object thread safety.

Warning: When constructing an object that will b

Answer 1

1. Let us assume, you have such class:

   class Sync {
       public Sync(List list) {
           list.add(this);
           // switch
           // instance initialization code
       }
   
       public void bang() { }
   }
   

2. and you have two threads (thread #1 and thread #2), both of them have a reference the same List list instance.

3. Now thread #1 creates a new Sync instance and as an argument provides a reference to the list instance:

   new Sync(list);
   

4. While executing line // switch in the Sync constructor there is a context switch and now thread #2 is working.

5. Thread #2 executes such code:

   for(Sync elem : list)
       elem.bang();
   

6. Thread #2 calls bang() on the instance created in point 3, but this instance is not ready to be used yet, because the constructor of this instance has not been finished.

Therefore,

• you have to be very careful when calling a constructor and passing a reference to the object shared between a few threads
• when implementing a constructor you have to keep in mind that the provided instance can be shared between a few threads