Does a "private" modifier also ensure Thread safety in Java

Question

Reading through this excellent article about safe construction techniques by Brain Goetz, I got confused by a comment given inside the listing 5. Here is the code snippet:

public class Safe { 

 private Object me;
 private Set set = new HashSet();
 private Thread thread;

 public Safe() { 
 // Safe because "me" is not visible from any other thread
 me = this;

// Safe because "set" is not visible from any other thread
 set.add(this);

// Safe because MyThread won't start until construction is complete
// and the constructor doesn't publish the reference
   thread = new MyThread(this);
}

public void start() {
   thread.start();
 }

private class MyThread(Object o) {

private Object theObject;

public MyThread(Object o) { 
  this.theObject = o;
}

...
 }
}

Why does he say this about me ? // Safe because "me" is not visible from any other thread. Can't two threads simultanouesly access the instance variable me?

me is private so how come threads can access it? – amicngh Jul 02 '12 at 08:22 — amicngh, Jul 02 '12 at 08:22

score 3 · Accepted Answer · answered Jul 02 '12 at 08:31

The only point is that while the constructor is running, you are not leaking this by setting it to a private property. After the constructor completes and the caller receives the this reference of the object, which is obviously the me reference as well, it can publish it to any thread, so indeed it will be accessible to any thread. But that was not the point Goetz was making.

score 0 · Answer 2 · answered Jul 02 '12 at 08:24

0

private modifier by itself does not ensure thread safety. What does, though, is other threads not being able to access/see your object. Here, the instance of Safe is being passed to MyThread, and isn't being stored/exposed elsewhere.

answered Jul 02 '12 at 08:24

DPlusV

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score 0 · Answer 3 · answered Jul 02 '12 at 09:09

Note the following points about this code :

me is private
No other public methods indirectly allow access to me for outside code.

These two together allow the thread-safety for the me reference. If the class is redesigned to have another public method as below :

public Safe someMethodForPublicAcess(){ return me; }

Then the thread-safety is gone, even if the member me is private. This happens because an external code can now call this public method and pass me to multiple threads which can do anything they like.

Does a "private" modifier also ensure Thread safety in Java

3 Answers3