总结
主要用于集合类中,以确保 在遍历集合时如果集合本身被修改,则会立即抛出ConcurrentModificationException
异常。
原理
- 内部计数器:每个集合对象都有一个内部计数器,称为
modCount
(modification count)。每当集合结构被修改(比如添加或删除元素),这个计数器就会增加。 - 迭代器检查:当通过迭代器遍历集合时,迭代器会在每次调用
next()
方法之前检查当前的modCount
是否与创建迭代器时的expectedModCount
相匹配。如果不匹配,说明集合已经被修改了,这时迭代器就会抛出ConcurrentModificationException
。
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { transient Object[] elementData; private int size; // add public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; // size ++ return true; } private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // modCount++ // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } // remove public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } private void fastRemove(int index) { modCount++; // modCount++ int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work // --size } 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; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = 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 { this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (modCount != expectedModCount) // modCount与expectedModCount比较 throw new ConcurrentModificationException(); } } }
示例
单线程
当使用iterator遍历List & 使用List的remove,将会ConcurrentModificationException;
List<String> list = new ArrayList<>(); list.add("Apple"); list.add("Banana"); list.add("Cherry"); list.add("Banana"); Iterator<String> iterator = list.iterator(); while (iterator.hasNext()) { String fruit = iterator.next(); // 在遍历过程中修改集合 if ("Banana".equals(fruit)) { list.remove(fruit); } } System.out.println(list);
list的remove会修改modCount,导致modCount 与 expectedModCount不相等;
解决
1、使用iterator遍历时,使用iterator的remove,将不会修改modCount;
2、多线程环境,使用线程安全集合类;
3、同步控制,保证只有一个线程能访问集合;
标签:index,int,elementData,list,---,modCount,Fail,JavaSE,size From: https://www.cnblogs.com/anpeiyong/p/18427204