Thread safe access to private field

Question

So I have the following scenario (can't share the actual code, but it would be something like this):

public class Test
{
   private Object obj;

   public void init()
   {
       service.registerListener(new InnerTest());
   }

   public void readObj()
   {
       // read obj here
   }

   private class InnerTest implements Listener
   {
       public synchronized void updateObj()
       {
           Test.this.obj = new Object();
           // change the obj
       }
   }
}

The InnerTest class is registered as listener in a service. That Service is running in one thread the calls to readObj() are made from a different thread, hence my question, to ensure consistency of the obj is it enough to make the UpdateObj() method synchronized?

Answer 1

I would suggest using another object as a lock to ensure that the class only blocks when the obj is accessed:

public class Test
{
   private final Object lock = new Object();
   private Object obj;

   public void init()
   {
       service.registerListener(new InnerTest());
   }

   public void readObj()
   {
       synchronized(lock){
           // read obj here
       }
   }

   private class InnerTest implements Listener
   {
       public void updateObj()
       {
           synchronized(Test.this.lock){
               Test.this.obj = new Object();
               // change the obj
           }
       }
   }
}

Then use that lock in all methods that need to have consistent access to obj . In your current example the readObj and updateObj methods.

Also as stated in the comments, using synchronized on the method level in your InnerTest class, will not really work as you probably intended. That is, because synchronized methods will use a synchronized block on the this variable. Which just blocks your InnerTest class. But not the outer Test class.