从源码级理解JAVA的并发修改异常

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首先先铺垫一下

   在Itr初始化(用ArrayList内部方法new出来的public Iterator<E> iterator()),时int expectedModCount = modCount   (此变量定义在java.util.AbstractList类中, 为已从结构上修改此列表的次数).   modeCount初始值为0,但是ArrayList的方法:        add(两个重载方法)都会调用方法 ensureCapacityInternal(size + 1);  会增加modCount的值;        remove  remove(int index) 会直接modCount++;                 remove(Object o) 会调用 fastRemove(int index) 使modCount++         其他的更改集合长度的都会改变modCount值;

再看下面的代码:
我们在调用Itr(ArrayList的)的next方法时,第一行会检测checkForComodification();

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
如果我们在遍历时(或者forPlus++遍历集合)时修改了集合的长度,那么modCount的值发生改变,则
检测不会过,直接抛并发修改异常;
而ListItr为Itr的子类但每次add时虽然修改了集合长度,但及时expectedModCount = modCount;所以用迭代器的方法在遍历时(或者forPlus++遍历集合)修改集合长度不会并发修改异常

private class Itr implements Iterator<E> {        int cursor;       // index of next element to return        int lastRet = -1; // index of last element returned; -1 if no such        int expectedModCount = modCount; //初始值为0        public boolean hasNext() {            return cursor != size;        }        @SuppressWarnings("unchecked")        public E next() {            checkForComodification();            int i = cursor;            if (i >= size)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                throw new ConcurrentModificationException();            cursor = i + 1;            return (E) elementData[lastRet = i];        }        public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }

   private class ListItr extends Itr implements ListIterator<E> {        ListItr(int index) {            super();            cursor = index;        }        public boolean hasPrevious() {            return cursor != 0;        }        public int nextIndex() {            return cursor;        }        public int previousIndex() {            return cursor - 1;        }        @SuppressWarnings("unchecked")        public E previous() {            checkForComodification();            int i = cursor - 1;            if (i < 0)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                throw new ConcurrentModificationException();            cursor = i;            return (E) elementData[lastRet = i];        }        public void set(E e) {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.set(lastRet, e);            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        public void add(E e) {            checkForComodification();            try {                int i = cursor;                ArrayList.this.add(i, e);                cursor = i + 1;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }    }

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