Java ConcurrentModificationException异常分析

项目中有一个需求：

服务器在检测到设备接入后给设备下发配置。在服务器异常重启的场景下，服务器检测到终端建链时底层通信模块还没有初始化完成，此时需要将已经建链设备的建链通知消息缓存起来，创建线程，待通信模块初始化完成后给设备下发配置。因频繁涉及节点增删，这里用LinkedList缓存触发建链通知的设备。

代码如下，在运行时抛出java.util.ConcurrentModificationException异常： 

package com.ancha.helloworld;import java.util.LinkedList;import com.ancha.oss.api.util.logging.DebugPrn;public class LinkStateReadyAction{    private static DebugPrn             debugPrn                  = new DebugPrn(LinkStateReadyAction.class.getName());        private static LinkStateReadyAction instance                  = null;    private LinkedList<String>          linkReadyNes              = new LinkedList<String>();        private static final long           WAIT_FOR_LINK_STATE_READY = 5000;    private static final long           ACTION_PEROID             = 100;        private LinkStateReadyAction()    {        Thread thread = new NeStateReadyProc();        thread.setName("Link-State-Ready-Proc");        thread.start();    }        public static synchronized LinkStateReadyAction getInstance()    {        if (instance == null)        {            instance = new LinkStateReadyAction();        }        return instance;    }        public void addNe(String ne)    {        linkReadyNes.add(ne); //此处应该有同步保护    }        private class NeStateReadyProc extends Thread    {        public void run()        {            try            {                Thread.sleep(WAIT_FOR_LINK_STATE_READY);                while (true)                {                    try                    {                        synchronized (linkReadyNes)                        {                            if (linkReadyNes.size() > 0)                            {                                for (String ne : linkReadyNes) //linkReadyNes中节点较多时，线程耗时较长，新的终端建链消息阻塞在addNe方法                                {                                    debugPrn.info(ne);                                    linkReadyNes.remove(ne); // WARN: 在for循环中，不允许进行节点的增删操作！                                }                            }                        }                    }                    catch (Exception e)                    {                        debugPrn.info("exception in ne state ready proc: ", e);                    }                    finally                    {                        try                        {                            Thread.sleep(ACTION_PEROID);                        }                        catch (Exception e)                        {                            debugPrn.warn("sleep error: ", e);                        }                    }                }            }            catch (Exception e)            {                debugPrn.warn("sleep error: ", e);            }        }    }        public static void main(String[] args)    {        debugPrn.info(" T-E-S-T ");                int i = 0;        while (true)        {            try            {                LinkStateReadyAction.getInstance().addNe(String.valueOf(i++));                Thread.sleep(ACTION_PEROID);            }            catch (InterruptedException e)            {                debugPrn.info("", e);            }        }    }}

抛出的异常：

2015-08-21 10:47:14,718 INFO [com.ancha.helloworld.LinkStateReadyAction] exception in ne state ready proc: java.util.ConcurrentModificationExceptionat java.util.LinkedList$ListItr.checkForComodification(LinkedList.java:761)at java.util.LinkedList$ListItr.next(LinkedList.java:696)at com.ancha.helloworld.LinkStateReadyAction$NeStateReadyProc.run(LinkStateReadyAction.java:56)

 

代码存在的问题：

1、由于服务器需要支持10万台设备的并发容量，当服务器重启后，短时间内会有大量设备建链并触发建链消息，List没有长度限制会有内存溢出的风险。

2、addNe中没有检查是否已经存在相同的元素，网元频繁断链时会导致List中添加大量相同的元素。

3、addNe()对List的操作没有添加同步保护，会抛出ConcurrentModificationException异常！

4、在for循环中遍历所有元素，给每个终端下发配置，会导致线程耗时过长，在同步保护下，主线程addNe操作会被长时间阻塞，在不允许长时间阻塞的消息响应机制里，消息时序会被打乱。

5、慎重在for循环中执行集合元素的增删操作。

for循环中对List执行执行remove操作，每当删除一个元素时，集合的size方法的值都会减小1，这将直接导致集合中元素的索引重新排序，进一步说，就是剩余所有元素的索引值都减1，如下图所示，而for循环语句的局部变量i仍然在递增，这将导致删除操作发生跳跃，遗漏元素。

 修改后代码如下：

package com.ancha.helloworld;import java.util.LinkedList;import com.ancha.oss.api.util.logging.DebugPrn;public class LinkStateReadyAction{    private static DebugPrn             debugPrn                  = new DebugPrn(LinkStateReadyAction.class.getName());        private static LinkStateReadyAction instance                  = null;    private LinkedList<String>          linkReadyNes              = new LinkedList<String>();        private static final long           WAIT_FOR_LINK_STATE_READY = 5000;    private static final long           ACTION_PEROID             = 100;        private LinkStateReadyAction()    {        Thread thread = new NeStateReadyProc();        thread.setName("Link-State-Ready-Proc");        thread.start();    }        public static synchronized LinkStateReadyAction getInstance()    {        if (instance == null)        {            instance = new LinkStateReadyAction();        }        return instance;    }        public void addNe(String ne)    {        synchronized (linkReadyNes)        {            linkReadyNes.add(ne); //此处应该有同步保护        }    }        private class NeStateReadyProc extends Thread    {        public void run()        {            try            {                Thread.sleep(WAIT_FOR_LINK_STATE_READY);                while (true)                {                    try                    {                        synchronized (linkReadyNes)                        {                            if (linkReadyNes.size() > 0)                            {                                for (String ne : linkReadyNes) //linkReadyNes中节点较多时，线程耗时较长，新的终端建链消息阻塞在addNe方法                                {                                    debugPrn.info(ne);                                    linkReadyNes.remove(ne); // WARN: 在for循环中，不允许进行节点的增删操作！                                }                            }                        }                    }                    catch (Exception e)                    {                        debugPrn.info("exception in ne state ready proc: ", e);                    }                    finally                    {                        try                        {                            Thread.sleep(ACTION_PEROID);                        }                        catch (Exception e)                        {                            debugPrn.warn("sleep error: ", e);                        }                    }                }            }            catch (Exception e)            {                debugPrn.warn("sleep error: ", e);            }        }    }        public static void main(String[] args)    {        debugPrn.info(" T-E-S-T ");                int i = 0;        while (true)        {            try            {                LinkStateReadyAction.getInstance().addNe(String.valueOf(i++));                Thread.sleep(ACTION_PEROID);            }            catch (InterruptedException e)            {                debugPrn.info("", e);            }        }    }}

修改后代码解决了前面提到的问题2、3、4、5，但问题1依然存在 。

重构代码，其中LinkedBlockingQueue是一个线程安全的队列。 

package com.ancha.helloworld;import java.util.concurrent.LinkedBlockingQueue;import java.util.concurrent.TimeUnit;import com.ancha.oss.api.util.logging.DebugPrn;public class LinkReadyActionRefactor{    private static DebugPrn                debugPrn                  = new DebugPrn(                                                                             LinkReadyActionRefactor.class.getName());        private static LinkReadyActionRefactor instance                  = null;    private LinkedBlockingQueue<String>    linkReadyNes              = new LinkedBlockingQueue<String>();        private static final long              WAIT_FOR_LINK_STATE_READY = 1000;    private static final long              ACTION_PEROID             = 10;        private LinkReadyActionRefactor()    {    }        public static synchronized LinkReadyActionRefactor getInstance()    {        if (instance == null)        {            instance = new LinkReadyActionRefactor();            instance.initNeStateReadyProc();        }        return instance;    }        private void initNeStateReadyProc()    {        Thread thread = new NeStateReadyProc();        thread.setName("Link-State-Ready-Proc");        thread.start();    }        public void addNe(String ne)    {        try        {            linkReadyNes.put(ne);        }        catch (InterruptedException e)        {            debugPrn.warn("insert ne failed.", e);        }    }        public String popNe()    {        try        {            //等待30分钟，如果30分钟内队列无数据，则返回null.            return linkReadyNes.poll(30, TimeUnit.MINUTES);        }        catch (InterruptedException e)        {            debugPrn.warn("pop ne failed, ", e);            return null;        }    }        private class NeStateReadyProc extends Thread    {        public void run()        {            try            {                Thread.sleep(WAIT_FOR_LINK_STATE_READY);                while (true)                {                    try                    {                        String ne = popNe();                        if (ne != null)                        {                            //NeStateReadyOperation(ne);                            debugPrn.info(ne);                        }                    }                    catch (Exception e)                    {                        debugPrn.info("ne state ready action failed. ", e);                    }                }            }            catch (InterruptedException e)            {                debugPrn.warn("sleep error: ", e);            }        }    }        public static void main(String[] args)    {        debugPrn.info(" T-E-S-T ");                int i = 0;        while (true)        {            try            {                LinkReadyActionRefactor.getInstance().addNe(String.valueOf(i++));                Thread.sleep(ACTION_PEROID);            }            catch (InterruptedException e)            {                debugPrn.info("", e);            }        }    }}

 

新手代码，是不是还有很多问题啊