ArrayList 并发操作 ConcurrentModificationException 异常

1、故障现象

ArrayList在迭代的时候如果同时对其进行修改就会抛出java.util.ConcurrentModificationException异常

2、故障代码

public class ArrayListTest {

    public static void main(String[] args) {

        List<String> lists = new ArrayList<>();
        lists.add("a");
        lists.add("b");

        Iterator<String> iterator = lists.iterator();
        while (iterator.hasNext()) {
            String next = iterator.next();
            if (next == "b") {
                lists.remove(next);
            }
        }
    }
}

异常截图

3、导致原因

通过查看异常，发现异常出现的位置在 java.util.ArrayList类的内部类Itr中的checkForComodification方法中

/**
 * 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;

    Itr() {}

    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();
    }
}

通过查看代码发现如果 modCount和expectedModCount不相等就会导致抛出异常。

modCount是修改记录数，expectedModCount是期望修改记录数，初始化时expectedModCount=modCount。

ArrayList集合的add和remove操作都有对modCount++操作，就会导致expectedModCount和modCount值不相等从而产生==ConcurrentModificationException异常==

4、解决方案

解决方案1：使用iterator的remove操作替代ArrayList集合自己的remove操作

public class ArrayListTest {

    public static void main(String[] args) {
        List<String> lists = new ArrayList<>();
        lists.add("a");
        lists.add("b");

        Iterator<String> iterator = lists.iterator();
        while (iterator.hasNext()) {
            String next = iterator.next();
            if (next == "b") {
                iterator.remove();
            }
        }
    }
}

分析：通过查看iterator的remove方法发现，其实还是调用了ArrayList集合的remove方法移除元素，但是会使

expectedModCount=modeCount所以不会抛出==ConcurrentModificationException异常==

解决方案2：使用JUC concurrent包中CopyOnWriteArrayList并发集合类

public class ArrayListTest {

    public static void main(String[] args) {
        List<String> lists = new CopyOnWriteArrayList<>();
        lists.add("a");
        lists.add("b");

        Iterator<String> iterator = lists.iterator();
        while (iterator.hasNext()) {
            String next = iterator.next();
            if (next == "b") {
                lists.remove(next);
                System.out.println(lists);
            }
        }
    }
}

因为迭代器中没有checkForComodification操作，并且集合的add和remove方法中都通过ReentrantLock加锁保证并发操作下的安全性。

5、优化建议

在对ArrayList集合进行并发操作时尽量使用CopyOnWriteArrayList集合类代替