Question

In a lot of of the Java source, (for example LinkedBlockingDeque) I see things like this;

final ReentrantLock lock = new ReentrantLock();

public void putLast(E e) throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
       // do stuff
    } finally {
        lock.unlock();
    }
}

I understand the basic pattern (lock, unlock in finally) but my question is why make an assignment to a locally scoped Lock variable before using it? Why do this instead of the following?

final ReentrantLock lock = new ReentrantLock();

public void putLast(E e) throws InterruptedException {
    this.lock.lock();
    try {
       // do stuff
    } finally {
        lock.unlock();
    }
}

Would it affect optimisations? Could the first example prevent lock coarsening?

EDIT after comments: Please don’t add an answer if you don’t really know why this is the case. This is from the Java source, the @author tag is Doug Lea so I’m pretty sure it’s there for a reason. Please don’t point out that the code is simply equivalent.

Thanks

Answer 1

When you assign to local variable in method, compiler can do some optimizations.
see In ArrayBlockingQueue, why copy final member field into local final variable?