Why do we make a copy of final instance variable in a method in Java?

Question

Possible Duplicate:
In ArrayBlockingQueue, why copy final member field into local final variable?

For instance:

private final Object o;

public void doSomething(){

    final Object local = this.o;
    //access methods of local;

}

This practice is followed in lots of java classes (such as ArrayBlockingQueue). What's the benefit of this?