java 多线程操作List,已经做了同步synchronized,还会有ConcurrentModificationException,知道为什么吗?

如题,最近项目里有个模块我做了异步处理方面的事情,在code过程中发现一个颠覆我对synchronized这个关键字和用法的地方,请问各位java开发者们是否对此有一个合理的解释,不多说,我直接贴出问题代码:

 

(事实证明这是一个坑,各位读者,如果有兴趣,可以先不看答案,自己看看能不能发现这个坑)

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

public class ConcurrentList {
    //private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
    private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (TEST_LIST) {
                        TEST_LIST.add("11");
                    }
                    System.out.println("Thread1 running");
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (TEST_LIST) {
                        for (String at : TEST_LIST) {
                            TEST_LIST.add("22");
                        }
                    }
                    System.out.println("Thread2 running");
                }
            }
        }).start();
    }
}

输出结果是:

Thread1 running
Exception in thread "Thread-1" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java:343)
    at com.free4lab.lol.ConcurrentList$2.run(ConcurrentList.java:40)
    at java.lang.Thread.run(Thread.java:619)
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running
Thread1 running

 

 

 

-----------------------------------分隔线,以下是解释--------------------------------

 

 

 

问题明了了:

以上问题不是并发的问题,是ArrayList的问题,是个坑!且看如下代码,以及运行结果:

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

public class ConcurrentList {
    //private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
    private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());

    public static void main(String[] args) {
        TEST_LIST.add("111");
        TEST_LIST.add("222");
        for (String at : TEST_LIST) {
            System.out.println(at);
            TEST_LIST.add("333");
            System.out.println("add over");
        }
    }
}

结果是:

111
add over
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java:343)
    at com.free4lab.lol.ConcurrentList.main(ConcurrentList.java:15)

分析:我们发现迭代了一次之后就抛出所谓的并发修改异常,不过这里没有多线程,看下源代码就知道了

list.add的时候执行了,修改了modCount,循环外面一次add到第一次迭代不会有问题,因为初始化的时候在AbstractList中int expectedModCount = modCount;,

/**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
    ensureCapacity(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
    }

public void ensureCapacity(int minCapacity) {
    modCount++;
    int oldCapacity = elementData.length;
    if (minCapacity > oldCapacity) {
        Object oldData[] = elementData;
        int newCapacity = (oldCapacity * 3)/2 + 1;
            if (newCapacity < minCapacity)
        newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = Arrays.copyOf(elementData, newCapacity);
    }
    }

public E next() {
            checkForComodification();
        try {
        E next = get(cursor);
        lastRet = cursor++;
        return next;
        } catch (IndexOutOfBoundsException e) {
        checkForComodification();
        throw new NoSuchElementException();
        }
    }

这样迭代器next()第一次 checkForComodification() 是不会抛出异常的,第二次才会抛出异常,因为在checkForComodification()里检查了

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

这样,在循环迭代中,进行了一次add操作,修改了modcount变量,再次迭代的时候,异常就throw出来了!

 

如果非要进行这样的操作,那么声明list为CopyOnWriteArrayList,就ok!因为用了copyonwrite技术

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

public class ConcurrentList {
    private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
    //private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());

    public static void main(String[] args) {
        TEST_LIST.add("111");
        TEST_LIST.add("222");
        for (String at : TEST_LIST) {
            System.out.println(at);
            TEST_LIST.add("333");
            System.out.println("add over");
        }
    }
}

输出是正确的:

111
add over
222
add over

 

额外再说一点,也可以用iterator迭代,不过同样也无法调用next()方法(我注释掉了),这样程序就是死循环了,不断的加,不断的迭代。所以我感觉如果需要在迭代中增加元素,真正有用的还是CopyOnWriteArrayList,不过实际中,如果CopyOnWriteArrayList代价太高,可能我们可以申请一个临时list存放,在迭代后合并到主list中!

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class ConcurrentList {
    //private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
    private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());

    public static void main(String[] args) {
        TEST_LIST.add("111");
        TEST_LIST.add("222");
        Iterator iterator  = TEST_LIST.iterator();  
        while(iterator.hasNext()){
            //System.out.println(iterator.next());
            TEST_LIST.add("333");
            System.out.println("add over");
        }  
    }
}

 

萌萌的IT人,IT人的乐园

posted @ 2013-10-12 16:30 yanghuahui 阅读(...) 评论(...) 编辑 收藏