Question

I need some clarificaton on how the syncronized keyword works in java 6.

Using the following example class.

class Carrier {

   private String[] _collection = new String[2];

   public Carrier() {
       this._collection[0] = "abc";
       this._collection[1] = "123";
   }

   public syncronized void change(int cId) {
       Thread.sleep(3000);
       this._collection[cId] = "changed";
   }

}

Now, some place in the application, referencing the same object instance of the Carrier class, the .change() method is called, possibly at the same time.

...carrier.change(1);

…

...carrier.change(1);

Will the syncronized keyword prevent asyncronized execution of the method? Will it simply queue the call to .change(), waiting for each one to complete?

Answer 1

Yes, it will prevent the two method calls from executing at the same time.

That is really what the main use of the synchronized keyword is.

When one synchronized method runs, it obtains a ‘lock’ on the object it is called on. That means no other synchronized code can be run on that object while that method holds the lock. Once the method is done executing, it will release the lock, and other synchronized methods can obtain a lock on that object.