CopyOnWriteArrayList

    1. final transient ReentrantLock lock = new ReentrantLock(); //写锁
    2. private transient volatile Object[] array; //指向数组的引用

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    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();
        }
    }
/**
     * Appends all of the elements in the specified collection to the end
     * of this list, in the order that they are returned by the specified
     * collection's iterator.
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     * @see #add(Object)
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] cs = (c.getClass() == CopyOnWriteArrayList.class) ?
            ((CopyOnWriteArrayList<?>)c).getArray() : c.toArray();
        if (cs.length == 0)
            return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (len == 0 && cs.getClass() == Object[].class)
                setArray(cs);
            else {
                Object[] newElements = Arrays.copyOf(elements, len + cs.length);
                System.arraycopy(cs, 0, newElements, len, cs.length);
                setArray(newElements);
            }
            return true;
        } finally {
            lock.unlock();
        }
    }

   /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        return get(getArray(), index);
    }
    @SuppressWarnings("unchecked")
    private E get(Object[] a, int index) {
        return (E) a[index];
    }
    /**
     * Gets the array.  Non-private so as to also be accessible
     * from CopyOnWriteArraySet class.
     */
    final Object[] getArray() {
        return array;
    }
    public void forEach(Consumer<? super E> action) {
        if (action == null) throw new NullPointerException();
        Object[] elements = getArray();
        int len = elements.length;
        for (int i = 0; i < len; ++i) {
            @SuppressWarnings("unchecked") E e = (E) elements[i];
            action.accept(e);
        }
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * {@code remove} method.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new COWIterator<E>(getArray(), 0);
    }
   /**
     * {@inheritDoc}
     *
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * {@code remove}, {@code set} or {@code add} methods.
     */
    public ListIterator<E> listIterator() {
        return new COWIterator<E>(getArray(), 0);
    }
    static final class COWIterator<E> implements ListIterator<E> {
        /** Snapshot of the array */
        private final Object[] snapshot;
        /** Index of element to be returned by subsequent call to next.  */
        private int cursor;

        private COWIterator(Object[] elements, int initialCursor) {
            cursor = initialCursor;
            snapshot = elements;
        }

        public boolean hasNext() {
            return cursor < snapshot.length;
        }

        public boolean hasPrevious() {
            return cursor > 0;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (! hasNext())
                throw new NoSuchElementException();
            return (E) snapshot[cursor++];
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            if (! hasPrevious())
                throw new NoSuchElementException();
            return (E) snapshot[--cursor];
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code remove}
         *         is not supported by this iterator.
         */
        public void remove() {
            throw new UnsupportedOperationException();
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code set}
         *         is not supported by this iterator.
         */
        public void set(E e) {
            throw new UnsupportedOperationException();
        }

        /**
         * Not supported. Always throws UnsupportedOperationException.
         * @throws UnsupportedOperationException always; {@code add}
         *         is not supported by this iterator.
         */
        public void add(E e) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            Object[] elements = snapshot;
            final int size = elements.length;
            for (int i = cursor; i < size; i++) {
                @SuppressWarnings("unchecked") E e = (E) elements[i];
                action.accept(e);
            }
            cursor = size;
        }
    }