2019独角兽企业重金招聘Python工程师标准>>>

先看怎么发生

List<Integer> list=new ArrayList<>();
for(int i=0;i<10;i++){
    list.add(i);
}
Iterator<Integer> it=list.iterator();
while(it.hasNext()){
    Integer str=it.next();
    if(str==5)
    list.remove(str);//不通过it.remove()方法删除，而是通过list.remove()方法删除元素
}

这段代码，就会发生java.util.ConcurrentModificationException异常
一下看看实现：
先看 AbstractList 类有这样一个变量

  /**
     * The number of times this list has been <i>structurally modified</i>.
     * Structural modifications are those that change the size of the
     * list, or otherwise perturb it in such a fashion that iterations in
     * progress may yield incorrect results.
     *  他代表list被修改的次数（一般指 add，remove 次数。某个元素重赋值，一般不计数 ）
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *  这个变量，一般在是  iterator ， list iterator 实现时用到的，并且在他们的 next，remove，add，previous 等方法中，会利用它决定是否抛出ConcurrentModificationException异常。
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     * 也可以在子类中利用这个字段，提供快速失败机制。具体在像add,remove方法中，增加modCount的值，每次调用加1。然后利用它，决定是否抛出ConcurrentModificationExceptions
     */
    protected transient int modCount = 0;

接着看ArrayList类方法

/**
     * Returns an iterator over the elements in this list in proper sequence.
     * 这个方法，返回一个本身list的一个iterator
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();//关键这个类
    }

再看 Itr类

 /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return  //有个潜台词，默认值是0，list当前的下标
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;//初始化时候，会把list原来的修改次数，赋给expectedModCount

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

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();//会抛出java.util.ConcurrentModificationException异常
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;//每次，移动下标加1，下次的游标位置
            return (E) elementData[lastRet = i];//lastRet 赋值本次的坐标。
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();//会抛出java.util.ConcurrentModificationException异常

            try {
                ArrayList.this.remove(lastRet);//根据下标删除元素，ArrayList.this.remove()方法，会modCount++;
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;//让后又把最新的modCount 赋给expectedModCount，这样保证，通过Itr remove方法删除元素，不会抛ConcurrentModificationException异常，因为expectedModCount == modCount
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {//抛出java.util.ConcurrentModificationException异常的方法
            if (modCount != expectedModCount)//如果modCount 不等于expectedModCount 就抛出这个异常
                throw new ConcurrentModificationException();
        }
    }

最后，再回来说ConcurrentModificationException异常，直接一点，它的作用是，一个集合(list 实现，比如ArrayList,LinkedList,Vertor 等AbstractList子类)，
在用它自己的Iterator对象 在玩转它本身时(next，remove，add，previous 调用)时，如果发现有其他方式(非这个Iterator 对象)，
修改这个list对象，就会（通过比较modCount, expectedModCount值）抛出ConcurrentModificationException异常。由于 Iterator 对象不是线程安全的，在多线程中用it.remove()删除元素，同样可以抛出 ConcurrentModificationException异常 ！

转载于:https://my.oschina.net/u/146130/blog/871820