Java中的CopyOnWriteArrayList原理详解

final transient ReentrantLock lock = new ReentrantLock();
/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;
/**
 * Gets the array.  Non-private so as to also be accessible
 * from CopyOnWriteArraySet class.
 */
final Object[] getArray() {
    return array;
}
/**
 * Sets the array.
 */
final void setArray(Object[] a) {
    array = a;
}
/**
 * Creates an empty list.
 */
public CopyOnWriteArrayList() {
    setArray(new Object[0]);
}

private transient volatile Object[] array;

final transient ReentrantLock lock = new ReentrantLock();

private E get(Object[] a, int index) {
    return (E) a[index];
}
public E get(int index) {
    return get(getArray(), index);
}

public E set(int index, E element) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        E oldValue = get(elements, index);
        if (oldValue != element) {
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len);
            newElements[index] = element;
            setArray(newElements);
        } else {
            // Not quite a no-op; ensures volatile write semantics
            setArray(elements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}

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();
    }
}

public E remove(int index) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        E oldValue = get(elemnts, index);
        int numMoved = len - index - 1;
        if (numMoved == 0)
            setArray(Arrays.copyOf(elements, len - 1));
        else {
            Object[] newElements = new Object[len - 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index + 1, newElements, index,
                             numMoved);
            setArray(newElements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}