Java synchronized keyword - Does it protect a class method from being executed at the same time?

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, the second thread will block while the first thread is executing. That's because they both try to acquire the monitor on the same object - via this, in this case. So your change method is equivalent to:

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

Personally I don't like synchronizing on "this" as it means any other code with access to the object itself could acquire the same monitor. I prefer to create an object just for locking which only code within the class has access to:

private final Object lock = new Object();

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

That will still have the same effect in terms of two threads calling change on the same object, because both threads will still be acquiring the same monitor - just the one associated with the lock-specific object.