CopyOnWriteArrayList集合排序异常问题

1、集合自定义排序实现

对List集合的自定义排序想必大家都知道要使用如下的方式，通过实现Comparator接口并实现compare方法来实现。

/**
         * 
         * @方法名 changeChain
         * @描述    改变请求链
         * @参数
         * @返回类型 void
         * @作者 cymiao
         */
        public static void changeChain(Class<? extends Chain> type){
            List<? extends Chain> list = DynamicLoader.getInstance().getChainList(type);
            Collections.sort(list, new Comparator<Chain>(){

                @Override
                public int compare(Chain chain1, Chain chain2) {
                    int orderO = ((OrderAnnotation)chain1.getClass().getAnnotation(OrderAnnotation.class)).value();
                    int orderT = ((OrderAnnotation)chain2.getClass().getAnnotation(OrderAnnotation.class)).value();
                    if(orderO > orderT){
                        return 1;
                    }else if(orderO < orderT){
                        return -1;
                    }
                    return 0;
                }
                
            });
            List<Chain> aList = new CopyOnWriteArrayList<Chain>();
            aList.addAll(list);
            DynamicLoader.getInstance().setChainList(aList, type);
        }

2、CopyOnWriteArrayList集合排序异常问题

在不同版本的JDK时，CopyOnWriteArrayList集合使用如上方法排序的时候会抛出UnsupportedOperationException异常，这是为什么呢？下来让我们通过研究CopyOnWriteArrayList类的源码来说明这个问题:

(1)、我们先来分析下JDK1.6中Collections.sort的实现逻辑

 如下JDK1.6的源码所示，在自定义排序时，先使用Arrays.sort对集合的Object数组进行排序，然后通过调用集合的listIterator()方法获取ListIterator对象，并调用对象的set(E e)方法赋值，我们再看CopyOnWriteArrayList集合的源码，发现listIterator()方法返回的是new COWIterator<E>(getArray(), 0);对象，而这个对象的set方法没有具体实现而是直接抛UnsupportedOperationException异常，所以问题很显而易见了，在JDK1.6版本下是不能对CopyOnWriteArrayList集合使用Comparator来进行自定义排序的。

public static <T> void sort(List<T> paramList, Comparator<? super T> paramComparator)
  {
    Object[] arrayOfObject = paramList.toArray();
    Arrays.sort(arrayOfObject, paramComparator);
    ListIterator localListIterator = paramList.listIterator();
    for (int i = 0; i < arrayOfObject.length; i++) {
      localListIterator.next();
      localListIterator.set(arrayOfObject[i]);
    }
  }

/**
     * {@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();
        }
    }

(2)、接下来我们分析下JDK1.8中Collections.sort的实现逻辑

如下Collections类的源码所示，1.8版本中修改了集合排序的逻辑，不是直接在Collections类中排序，而是调用集合对象自己的sort方法实现排序了，在JDK1.6中CopyOnWriteArrayList是没有sort方法实现的，在1.8中添加了sort的实现，源码如下，集合的Object数组排序完成以后，直接把排序后的数据赋值给了集合中的Object数据，所以在JDK1.8中能对CopyOnWriteArrayList集合使用Comparator来进行自定义排序的。

/**
     * Sorts the specified list according to the order induced by the
     * specified comparator.  All elements in the list must be <i>mutually
     * comparable</i> using the specified comparator (that is,
     * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
     * for any elements {@code e1} and {@code e2} in the list).
     *
     * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
     * not be reordered as a result of the sort.
     *
     * <p>The specified list must be modifiable, but need not be resizable.
     *
     * @implNote
     * This implementation defers to the {@link List#sort(Comparator)}
     * method using the specified list and comparator.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be sorted.
     * @param  c the comparator to determine the order of the list.  A
     *        {@code null} value indicates that the elements' <i>natural
     *        ordering</i> should be used.
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator.
     * @throws UnsupportedOperationException if the specified list's
     *         list-iterator does not support the {@code set} operation.
     * @throws IllegalArgumentException (optional) if the comparator is
     *         found to violate the {@link Comparator} contract
     * @see List#sort(Comparator)
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    public static <T> void sort(List<T> list, Comparator<? super T> c) {
        list.sort(c);
    }

public void sort(Comparator<? super E> c) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            Object[] newElements = Arrays.copyOf(elements, elements.length);
            @SuppressWarnings("unchecked") E[] es = (E[])newElements;
            Arrays.sort(es, c);
            setArray(newElements);
        } finally {
            lock.unlock();
        }
    }

/**
     * Sets the array.
     */
    final void setArray(Object[] a) {
        array = a;
    }