java.util.ConcurrentModificationException原因

我们要写个遍历Map集合，删除指定key值的方法，我们估计会这样写。
刚开始我习惯上会写上map.remove(entry.getKey())，remove集合的一个值。但是写了之后，会发现java.util.ConcurrentModificationException异常。

private static void deleteKeyOfMap(String mkey,Map<String,Object> paramsMap){
        Iterator<Map.Entry<String,Object>> iter = map.entrySet().iterator();
        while(iter.hasNext()){
            Map.Entry<String, Object> entry = iter.next();
            if(entry.getKey().equals(k)){
                map.remove(entry.getKey());
                //iter.remove();
            }
        }
    }

这是什么异常呢？
总结起来其实是集合的安全性决定的，可以看看HashMap的源码

private abstract class HashIterator<E> implements Iterator<E> {
        Entry<K,V> next;        // next entry to return
        int expectedModCount;   // For fast-fail
        int index;              // current slot
        Entry<K,V> current;     // current entry

        HashIterator() {
            expectedModCount = modCount;
            if (size > 0) { // advance to first entry
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
        }

        public final boolean hasNext() {
            return next != null;
        }

        final Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();

            if ((next = e.next) == null) {
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
            current = e;
            return e;
        }

        public void remove() {
            if (current == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Object k = current.key;
            current = null;
            HashMap.this.removeEntryForKey(k);
            expectedModCount = modCount;
        }
    }

找到源码关键的一句expectedModCount = modCount;
初始化时，modCount赋值给expectedModCount。

跟一下代码：

 public void remove() {
            if (current == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Object k = current.key;
            current = null;
            HashMap.this.removeEntryForKey(k);//在跟一下removeEntryForKey方法
            expectedModCount = modCount;
        }

/**
     * Removes and returns the entry associated with the specified key
     * in the HashMap.  Returns null if the HashMap contains no mapping
     * for this key.
     */
    final Entry<K,V> removeEntryForKey(Object key) {
        if (size == 0) {
            return null;
        }
        int hash = (key == null) ? 0 : hash(key);
        int i = indexFor(hash, table.length);
        Entry<K,V> prev = table[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                modCount++;//modCount+1
                size--;
                if (prev == e)
                    table[i] = next;
                else
                    prev.next = next;
                e.recordRemoval(this);
                return e;
            }
            prev = e;
            e = next;
        }

        return e;
    }

从源码可以看出，调用HashMap的remove方法modCount加1了，所以会导致
expectedModCount != modCount;

这就是java.util.ConcurrentModificationException出现的原因

集合本身这样设计是为了安全性考虑，在Iterator遍历时，不允许被调用remove等等方法

所以正确的代码应该这样改

private static void deleteKeyOfMap(String mkey,Map<String,Object> paramsMap){
        Iterator<Map.Entry<String,Object>> iter = map.entrySet().iterator();
        while(iter.hasNext()){
            Map.Entry<String, Object> entry = iter.next();
            if(entry.getKey().equals(k)){
                //map.remove(entry.getKey());
                iter.remove();
            }
        }
    }

附录参考资料：

Java迭代foreach原理解析