Java_collection ConcurrentModificationException异常是如何产生的？

梦里梦外; 2022-11-14 10:29 140阅读 0赞

### Java\_collection ConcurrentModificationException ###

*  ConcurrentModificationException异常是如何产生的？
 *  哇呀呀，iter2.next() 如何判断正在被修改呢？

# ConcurrentModificationException异常是如何产生的？ #

ConcurrentModificationException：当方法检测到对象的并发修改，但不允许这种修改时，抛出此异常。

**示例：**

public class ConcurrentModificationExceptionTest { 
    
        public static void main(String[] args) { 
            List<String> list = new ArrayList();
            list.add("aaa");
            list.add("bbb");
            list.add("ccc");
            list.add("ddd");
            list.add("eee");
            ListIterator<String> iter1 = list.listIterator();
            ListIterator<String> iter2 = list.listIterator();
            iter1.next();
            iter1.remove();
            iter2.next(); //报错行
        }
    }

**执行结果：**

Exception in thread "main" java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911)
    	at java.util.ArrayList$Itr.next(ArrayList.java:861)
    	at cn.ling.collection.ConcurrentModificationExceptionTest.main(ConcurrentModificationExceptionTest.java:27)
    
    Process finished with exit code 1

# 哇呀呀，iter2.next() 如何判断正在被修改呢？ #

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();
        }
    }
    //modCount != expectedModCount 不相等就抛异常
    final void checkForComodification() { 
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }

**上面示例为什么不一致呢？**

remove方法修改了 modCount

public E remove(int index) { 
       rangeCheck(index);
    
        modCount++;
        E oldValue = elementData(index);
    
        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
    
        return oldValue;
    }

当执行iter1.remove(); 时，modCount++; 所以在iter2.next();执行时，checkForComodification校验失败。

public E next() { 
     checkForComodification();
      int i = cursor;
      if (i >= SubList.this.size)
          throw new NoSuchElementException();
      Object[] elementData = ArrayList.this.elementData;
      if (offset + i >= elementData.length)
          throw new ConcurrentModificationException();
      cursor = i + 1;
      return (E) elementData[offset + (lastRet = i)];
    }

如果将ListIterator iter2 = list.listIterator();代码放在iter2.next();前面就不会有这种错误了。