Why CopyOnWriteArrayList use local variable referenced to instance variable?

Question

Following is jdk concurrent's definition of CopyOnWriteArrayList:

private volatile transient Object[] array;
transient final ReentrantLock lock = new ReentrantLock();


public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

Why add function defines another lock(final ReentrantLock lock = this.lock;) which also referenced to the instance variable(lock) as local virable, instead of directly using the instance variable?

Can add function be implemented as:

int len = array.length;
Object[] newElements = Arrays.copyOf(array, len + 1);
newElements[len] = o;
array = newElements;