浅谈fail-fast机制

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

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        for (int i = 0 ; i < 10 ; i++ ) {
            list.add(i + "");
        }
        Iterator<String> iterator = list.iterator();
        int i = 0 ;
        while(iterator.hasNext()) {
            if (i == 3) {
                list.remove(3);
                //list.add("11");   添加元素同样会引发
            }
            System.out.println(iterator.next());
            i ++;
        }
    }

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