java中出现的ConcurrentModificationException产生原因，解决方式及其它

JAVA开发中有时会出现ConcurrentModificationException，该异常是由于在遍历Collection的时候做了删除或者增加的操作，此时原来统计的size和新的size并不一致，产生了该异常。下面是stackOverflow上的解释：

引用

Here's why: As it is says in the Javadoc:
    The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

This check is done in the next() method of the iterator (as you can see by the stacktrace). But we will reach the next() method only if hasNext() delivered true, which is what is called by the for each to check if the boundary is met. In your remove method, when hasNext() checks if it needs to return another element, it will see that it returned two elements, and now after one element was removed the list only contains two elements. So all is peachy and we are done with iterating. The check for concurrent modifications does not occur, as this is done in the next() method which is never called.

Next we get to the second loop. After we remove the second number the hasNext method will check again if can return more values. It has returned two values already, but the list now only contains one. But the code here is:

public boolean hasNext() {
        return cursor != size();
}

1 != 2, so we continue to the next() method, which now realizes that someone has been messing with the list and fires the exception.

Hope that clears your question up.

要解决该问题，网上也有不少文章。大都谈的是在一个线程中遍历Collection，同时进行删除操作，建议是使用Iterator#remove()方法，而不是其remove(Obj)方法。但如果是多线程的情况下，即使一个线程中使用了Iterator#remove()方法改变了某个Collection,而另一个线程正在遍历该Collection,还是会产生相同的问题。
所以解决办法是需要给两个线程中使用到该Collection的地方都加锁，可以锁定Collection,或者锁定包含Collection的类。

而解决在遍历时会增加的情况，可以将List改为CopyOnWriteArrayList，Map改为相应的同步版本ConcurrentHashMap 即可。