java应用之ConcurrentModificationException

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(); }
		}
	}

  

 

  
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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; } }

  

 

  
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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; }

  

 

  
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/** * 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; }

  

 

  
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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(); }
		}
	}

  

 

  
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附录参考资料：

Java迭代foreach原理解析

文章来源: smilenicky.blog.csdn.net，作者：smileNicky，版权归原作者所有，如需转载，请联系作者。

原文链接：smilenicky.blog.csdn.net/article/details/79786087