java.util.ConcurrentModificationException理解

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java.util.ConcurrentModificationException这个异常想必大家都遇到过，

可以通过源码找到根因，容器类，比如ArrayList、HashMap、HashSet，循环方法中可以得知有个modCount发生变化，当同时遍历容器对象，同时增加或者删除元素，就会抛出ConcurrentModificationException，源码如下：

    @Overridepublic void forEach(BiConsumer<? super K, ? super V> action) {Node<K,V>[] tab;if (action == null)throw new NullPointerException();if (size > 0 && (tab = table) != null) {int mc = modCount;// Android-changed: Detect changes to modCount early.for (int i = 0; (i < tab.length && mc == modCount); ++i) {for (Node<K,V> e = tab[i]; e != null; e = e.next)action.accept(e.key, e.value);}if (modCount != mc)throw new ConcurrentModificationException();}}

    /*** Removes all of the mappings from this map.* The map will be empty after this call returns.*/public void clear() {Node<K,V>[] tab;modCount++;if ((tab = table) != null && size > 0) {size = 0;for (int i = 0; i < tab.length; ++i)tab[i] = null;}}

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {Node<K,V>[] tab; Node<K,V> p; int n, i;if ((tab = table) == null || (n = tab.length) == 0)n = (tab = resize()).length;if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);else {Node<K,V> e; K k;if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))e = p;else if (p instanceof TreeNode)e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);else {for (int binCount = 0; ; ++binCount) {if ((e = p.next) == null) {p.next = newNode(hash, key, value, null);if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1sttreeifyBin(tab, hash);break;}if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))break;p = e;}}if (e != null) { // existing mapping for keyV oldValue = e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;}}++modCount;if (++size > threshold)resize();afterNodeInsertion(evict);return null;}

因此，不要同时遍历容器对象，又同时修改里面的元素，

注意点：

1.这个异常不是只在多线程才会触发，单线程同样会触发，只要满足同时遍历容器对象，又同时修改里面的元素；

2.容器类的迭代器中同时遍历和修改元素，也会触发该异常；

3.打印容器对象时，会调用该对象的toString()方法，这个方法会遍历该容器对象，如果刚好遇到该对象在同时修改元素，也会触发该异常。

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