🥎 CopyOnWriteArrayList

1. 类注释

1. 概述

   • 通过 写时复制 保证线程安全的 ArrayList

2. Iterator

   • iterator 保证弱一致性，在期间执行 增、删、修改 等操作时不会抛出 ConcurrentModificationException 异常
   • 不支持对 iterator 自身元素进行 remove set add 操作，若进行，则抛出 UnsupportedOperationException 异常

3. 支持元素类型

   • 支持所有类型元素，包括 null

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2. 类图

public class CopyOnWriteArrayList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
// ......
}

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3. 属性

    private static final long serialVersionUID = 8673264195747942595L;

    /** The lock protecting all mutators */
    final transient ReentrantLock lock = new ReentrantLock();

    /** The array, accessed only via getArray/setArray. */
    private transient volatile Object[] array;

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4. 构造函数

    /**
     * Creates an empty list.
     */
    public CopyOnWriteArrayList() {
        setArray(new Object[0]);
    }
    /**
     * Sets the array.
     */
    final void setArray(Object[] a) {
        array = a;
    }    
    
    /**
     * Creates a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection of initially held elements
     * @throws NullPointerException if the specified collection is null
     */
    public CopyOnWriteArrayList(Collection<? extends E> c) {
        Object[] elements;
        if (c.getClass() == CopyOnWriteArrayList.class)
            elements = ((CopyOnWriteArrayList<?>)c).getArray();
        else {
            elements = c.toArray();
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elements.getClass() != Object[].class)
                elements = Arrays.copyOf(elements, elements.length, Object[].class);
        }
        setArray(elements);
    }

    /**
     * Creates a list holding a copy of the given array.
     *
     * @param toCopyIn the array (a copy of this array is used as the
     *        internal array)
     * @throws NullPointerException if the specified array is null
     */
    public CopyOnWriteArrayList(E[] toCopyIn) {
        setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
    }

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5. 常用方法

1. 插入元素

1) add(E e)

    /**
     * reL 保证线程安全 & 写时复制
     * 可以看到，这里添加元素就没有 ArrayList 的扩容概念，添加一个数组长度加 1，每次正好填满数组
     *
     * @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); // 写时复制
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

2) add(int index, E element)

   /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            if (index > len || index < 0)
                throw new IndexOutOfBoundsException("Index: "+index+", Size: "+len);
            Object[] newElements;
            int numMoved = len - index;
            if (numMoved == 0)
                newElements = Arrays.copyOf(elements, len + 1);
            else {
                newElements = new Object[len + 1];
                System.arraycopy(elements, 0, newElements, 0, index);
                System.arraycopy(elements, index, newElements, index + 1,numMoved);
            }
            newElements[index] = element;
            setArray(newElements);
        } finally {
            lock.unlock();
        }
    }

2. 删除元素

1) remove(int index)

    /**
     * 返回被删除元素的值
     * reL 保证现场安全 + 写时复制
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            E oldValue = get(elements, 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();
        }
    }

2) remove(Object o)

    /**
     * 删除首次出现该元素的下标所在的元素
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified element
     */
    public boolean remove(Object o) {
        Object[] snapshot = getArray();
        int index = indexOf(o, snapshot, 0, snapshot.length);
        return (index < 0) ? false : remove(o, snapshot, index);
    }

    /**
     * 返回 o 在数组 elements[] 的 [index, fence) 中首次出现的下标
     * @param o element to search for
     * @param elements the array
     * @param index first index to search
     * @param fence one past last index to search
     * @return index of element, or -1 if absent
     */
    private static int indexOf(Object o, Object[] elements, int index, int fence) {
        if (o == null) {
            for (int i = index; i < fence; i++)
                if (elements[i] == null)
                    return i;
        } else {
            for (int i = index; i < fence; i++)
                if (o.equals(elements[i]))
                    return i;
        }
        return -1;
    }
    
    /**
     * A version of remove(Object) using the strong hint that given
     * recent snapshot contains o at the given index.
     */
    private boolean remove(Object o, Object[] snapshot, int index) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            // 说明前面在查找 o 在数组下标的过程中，数组已经被其他线程所改变
            // 此时需要重新寻找 index 的正确位置
            if (snapshot != current) findIndex: {
                int prefix = Math.min(index, len);
                for (int i = 0; i < prefix; i++) {
                    if (current[i] != snapshot[i] && eq(o, current[i])) {
                        index = i;
                        break findIndex;
                    }
                }
                if (index >= len)
                    return false;
                if (current[index] == o)
                    break findIndex;
                // 【疑问，这里为什么不直接使用该函数，前面还需要加判断】
                index = indexOf(o, current, index, len); 
                if (index < 0)
                    return false;
            }
            Object[] newElements = new Object[len - 1];
            System.arraycopy(current, 0, newElements, 0, index);
            System.arraycopy(current, index + 1, newElements, index, len - index - 1);
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }
    
    /**
     * 判断两个元素是否相等
     */
    private static boolean eq(Object o1, Object o2) {
        return (o1 == null) ? o2 == null : o1.equals(o2);
    }

3. 修改元素

    /**
     * reL 加锁 + 写时复制
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    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();
        }
    }

4. 查找元素

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        return get(getArray(), index);
    }

5. 序列化 & 反序列化

    /**
     * 只序列化有用的部分
     *
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     * @serialData The length of the array backing the list is emitted
     *               (int), followed by all of its elements (each an Object)
     *               in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

        s.defaultWriteObject();

        Object[] elements = getArray();
        // Write out array length
        s.writeInt(elements.length);

        // Write out all elements in the proper order.
        for (Object element : elements)
            s.writeObject(element);
    }

    /**
     * Reconstitutes this list from a stream (that is, deserializes it).
     * @param s the stream
     * @throws ClassNotFoundException if the class of a serialized object
     *         could not be found
     * @throws java.io.IOException if an I/O error occurs
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {

        s.defaultReadObject();

        // bind to new lock
        resetLock();

        // Read in array length and allocate array
        int len = s.readInt();
        SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, len);
        Object[] elements = new Object[len];

        // Read in all elements in the proper order.
        for (int i = 0; i < len; i++)
            elements[i] = s.readObject();
        setArray(elements);
    }

6. Iterator

    /**
     * Iterator 是创建 iterator 时的一个快照，因此不需要保证线程安全
     * 不提供 remove 方法
     *
     * <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);
    }
    
    static final class COWIterator<E> implements ListIterator<E> {
    
        private final Object[] snapshot; // 数组快照
        private int cursor; // 当前下标

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

        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;
        }
    }

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6. CopyOnWriteArraySet

1. 基于 CopyOnWriteArrayList 实现

public class CopyOnWriteArraySet<E> extends AbstractSet<E> implements java.io.Serializable {
    private static final long serialVersionUID = 5457747651344034263L;

    private final CopyOnWriteArrayList<E> al;

    // 构造函数
    public CopyOnWriteArraySet() {
        al = new CopyOnWriteArrayList<E>();
    }

    /**
     * 构造函数
     */
    public CopyOnWriteArraySet(Collection<? extends E> c) {
        if (c.getClass() == CopyOnWriteArraySet.class) {
            @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc = (CopyOnWriteArraySet<E>)c;
            al = new CopyOnWriteArrayList<E>(cc.al);
        }
        else {
            al = new CopyOnWriteArrayList<E>();
            al.addAllAbsent(c);
        }
    }
    
    // ----------- 常用方法 -----------------
    public boolean add(E e) {
        return al.addIfAbsent(e);
    }
    
    public boolean contains(Object o) {
        return al.contains(o);
    }
    
    public boolean remove(Object o) {
        return al.remove(o);
    }
    // ...