Java ConcurrentModificationException

Question

删除倒数第二个元素时，没有ConcurrentModificationException

List<String> myList1 = new ArrayList<String>();
Collections.addAll(myList1, "str1","str2","str3","str4","str5");
for(String element : myList1){//no ConcurrentModificationException here
if(element.equalsIgnoreCase("str4"))
    myList1.remove("str4");
}
System.out.println(myList1);

但是，当删除其他元素时，会出现一个ConcurrentModificationException

List<String> myList2 = new ArrayList<String>();
Collections.addAll(myList2, "str1","str2","str3","str4","str5");
for(String element : myList2){//ConcurrentModificationException here
if(element.equalsIgnoreCase("str1"))
    myList2.remove("str1");
}
System.out.println(myList2);

原因何在？

Answer 1

我也看到了同样的事情，

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Launcher 
{
    public static void main(String[] args) 
    {
        doThis();
        doThat();
    }

    private static void doThis()
    {
        System.out.println("dothis");
        try
        {
            List<String> myList1 = new ArrayList<String>();
            Collections.addAll(myList1, "str1","str2","str3","str4","str5");
            for(String element : myList1){//no ConcurrentModificationException here
            if(element.equalsIgnoreCase("str4"))
                myList1.remove("str4");
            }
            System.out.println(myList1);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    private static void doThat()
    {
        System.out.println("dothat");
        try
        {
            List<String> myList2 = new ArrayList<String>();
            Collections.addAll(myList2, "str1","str2","str3","str4","str5");
            for(String element : myList2){//ConcurrentModificationException here
            if(element.equalsIgnoreCase("str1"))
                myList2.remove("str1");
            }
            System.out.println(myList2);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }
}

它输出，

dothis
[str1, str2, str3, str5]
dothat
java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
    at java.util.AbstractList$Itr.next(Unknown Source)
    at com.foo.Launcher.doThat(Launcher.java:41)
    at com.foo.Launcher.main(Launcher.java:12)

我找到了。

Answer 2

Java使用modCount(修改计数)和expectedCount来测试列表是否有修改。

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

在这两种情况下，删除后的modCount为6，但expectedModCount为5。

问题出在hasNext()。

public boolean hasNext() {
    return cursor != size;
}

列表使用光标和大小来检查是否有下一个元素。hasNext()发生在checkForComodification之前，因为checkForComodification()是在next()方法中调用的。

    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];
    }

因此，当您删除倒数第二个元素时，cursor=4和size=4也会被删除。hasNext()返回false。跳出循环并打印结果。

Answer 3

javac为for-each构建的实际代码是

    Iterator<String> i = myList1.iterator();
    while(i.hasNext()) {
        String element = i.next();
        if (element.equalsIgnoreCase("str4"))
            myList1.remove("str4");
    }

这是ArrayList Iterator.hasNext的实现

    public boolean hasNext() {
        return cursor != size;
    }

正如我们所看到的，hasNext()不检查并发修改，所以当我们删除倒数第二个元素时，循环结束而没有注意到问题。

实际上，奇怪的是next()和remove()会检查并发修改，而hasNext()不会。快速失效迭代器应该检测bug，但是我们的bug没有被注意到。