ConcurrentModificationException原因及排除

最新推荐文章于 2021-07-14 17:55:02 发布

转载最新推荐文章于 2021-07-14 17:55:02 发布 · 128 阅读

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原文链接：http://www.cnblogs.com/it-worker365/p/6861237.html

文章标签：

#java

本文探讨了在Java中遍历并修改集合时遇到的ConcurrentModificationException异常，通过示例展示了如何使用CopyOnWriteArrayList和迭代器的方法来避免该异常。

如何产生，一边遍历一边修改元素，产生iter后再修改原结构，如下，无论是for中或iter都会产生ConcurrentModificationException

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * Created by itworker365 on 5/16/2017.
 */
public class ConcurrentModifyException {
    public static void main(String[] args) throws InterruptedException {
        List<String> a = new ArrayList<String>();
        a.add("a");
        a.add("b");
        a.add("c");
        final List<String> list = new ArrayList<String>(a);
        final Iterator<String> it = list.iterator();
        //cause ConcurrentModifyException,list.iterator已经生成iterator对象，此时再更改原结构报错
        a.add("d");
        Thread t = new Thread(new Runnable() {
            int count = -1;
            @Override
            public void run() {
                if (list.contains("a")) {
                    list.remove("a");
                }
            }
        });
        t.setDaemon(true);
        t.start();

        for (String s : list) {
            //cause ConcurrentModifyException,边读边改报错
            System.out.println(list.hashCode() + "   " + s);
        }
//        while (it.hasNext()) {
//            //cause ConcurrentModifyException,边读边改报错
//            System.out.println(it.next());
//        }
    }
}

产生原因：

        public E next() {
            checkForComodification();

            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

解决办法：

1. 用CopyOnWriteArrayList, 可以发现在打印hashcode的时候并不相同，这种结构在修改元素时使用Arrays.CopyOf复制元素，然后在新List中修改之后修改引用

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

/**
 * Created by itworker365 on 5/16/2017.
 */
public class ConcurrentModifyException {
    public static void main(String[] args) throws InterruptedException {
        List<String> a = new ArrayList<String>();
        a.add("a");
        a.add("b");
        a.add("c");
        final List<String> list = new CopyOnWriteArrayList<String>(a);
        final Iterator<String> it = list.iterator();
        Thread t = new Thread(new Runnable() {
            int count = -1;
            @Override
            public void run() {
                if (list.contains("a")) {
                    list.remove("a");
                }
            }
        });
        t.setDaemon(true);
        t.start();
        for (String s : list) {
            System.out.println(list.hashCode() + "   " + s);
        }
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

CopyOnWriteArrayList关键实现,一看既懂，需要注意的是内存使用，可能复制后会出发gc，因为写操作完成并且引用指向新数组之前，其他并发读取线程只能看到旧数据，所以只是最终一致性，不能实时。

private volatile transient Object[] array;

    public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

    final void setArray(Object[] a) {
        array = a;
    }

2. 使用Iterator去除，迭代器支持的功能有限

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

/**
 * Created by itworker365 on 5/16/2017.
 */
public class ConcurrentModifyException {
    public static void main(String[] args) throws InterruptedException {
        List<String> a = new ArrayList<String>();
        a.add("a");
        a.add("b");
        a.add("c");
        final List<String> list = new ArrayList<String>(a);
        final Iterator<String> it = list.iterator();
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                while (it.hasNext()) {
                    String xx = it.next();
                    it.remove();
                }
            }
        });
        t.setDaemon(true);
        t.start();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}