浅谈fail-fast机制
2018-09-05 07:47:04来源:博客园 阅读 ()
fail-fast机制即为快速失败机制,个人认为是一种防护措施,在集合结构发生改变的时候,使尽全力抛出ConcurrentModificationException,所以该机制大部分用途都是用来检测Bug的;
下面的代码可以引发fail-fast
1 public static void main(String[] args) { 2 List<String> list = new ArrayList<>(); 3 for (int i = 0 ; i < 10 ; i++ ) { 4 list.add(i + ""); 5 } 6 Iterator<String> iterator = list.iterator(); 7 int i = 0 ; 8 while(iterator.hasNext()) { 9 if (i == 3) { 10 list.remove(3); 11 //list.add("11"); 添加元素同样会引发 12 } 13 System.out.println(iterator.next()); 14 i ++; 15 } 16 }
fail-fast原理
每个集合都会实现可遍历的接口,以上述代码为例,集合调用iterator();方法的时候,其实是返回了一个new Itr();
/** * Returns an iterator over the elements in this list in proper sequence. * * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>. * * @return an iterator over the elements in this list in proper sequence */ public Iterator<E> iterator() { return new Itr(); }
以下是Itr源码
/** * An optimized version of AbstractList.Itr */ 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 = ArrayList.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 { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }
Itr有3个重要属性;
cursor是指集合遍历过程中的即将遍历的元素的索引
lastRet是cursor -1,默认为-1,即不存在上一个时,为-1,它主要用于记录刚刚遍历过的元素的索引。
expectedModCount它初始值就为ArrayList中的modCount(modCount是抽象类AbstractList中的变量,默认为0,而ArrayList 继承了AbstractList ,所以也有这个变量,modCount用于记录集合操作过程中作的修改次数)
由源码可以看出,该异常就是在调用next()的时候引发的,而调用next()方法的时候会先调用checkForComodification(),该方法判断expectedModCount与modCount是否相等,如果不等则抛异常了
那么问题就来了,初始化的时候expectedModCount就被赋值为modCount,而且源码当中就一直没有改变过,所以肯定是modCount的值变了
arrayList继承了abstractList,abstractList有modCount属性,通过以下源码我们可以看到,当ArrayList调用add、remove方法,modCount++
/** * Inserts the specified element at the specified position in this * list. Shifts the element currently at that position (if any) and * any subsequent elements to the right (adds one to their indices). * * @param index index at which the specified element is to be inserted * @param element element to be inserted * @throws IndexOutOfBoundsException {@inheritDoc} */ public void add(int index, E element) { rangeCheckForAdd(index); ensureCapacityInternal(size + 1); // Increments modCount!! System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; } /** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ 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; }
所以由此可见,对集合的操作中若modCount发生了改变,则会引发fail-fast机制;同时可以看出如果想要移除集合某元素,可以使用迭代器的remove方法,则不会引发fail-fast;
发表该文章也参考了许多另一片文章的内容,详情地址:https://blog.csdn.net/zymx14/article/details/78394464
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