异常java.util.ConcurrentModificationException

控制台出现提示如下：

java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
    at java.util.HashMap$KeyIterator.next(Unknown Source)
    at cn.whu.sendimage.pm.Crawler.reGet(Crawler.java:64)
    at cn.whu.sendimage.pm.timer.ReGetTasker.run(ReGetTasker.java:18)
    at cn.whu.sendimage.pm.timer.Tasker.run(Tasker.java:22)
    at java.util.TimerThread.mainLoop(Unknown Source)
    at java.util.TimerThread.run(Unknown Source)

---

源代码：

//这个是一个map：键是时间（一个小时），如20180624020000；值是此时间段（一个小时）需要重新请求的url.
private static final Map<String, Set<String>> errorAndRequestAgain = new HashMap<String, Set<String>>();
//重新请求所有map中的url.如果key中没有值，那么就删除此键值对。
for (String key : errorAndRequestAgain.keySet()) {

            System.out.println(key);
            System.out.println(errorAndRequestAgain.get(key).size());
            System.out.println(errorAndRequestAgain.get(key));
            if (errorAndRequestAgain.get(key).size() == 0) {
                errorAndRequestAgain.remove(key);
            } else {
                logger.info("还有"+key+"时刻未处理，共"+errorAndRequestAgain.get(key).size()+"站点，准备重新下载");
                JSONArray httpJson = getHttpJson(new ArrayList<String>(errorAndRequestAgain.get(key)), key);
                List<String[]> list = jsonArrayTOList(httpJson);
                String fileName = key + ".txt";
                ToFileUtil.WriteStringToTxtFile(list, downfilePath + fileName);
            }

        }

---

更改为：

Iterator<Entry<String, Set<String>>> it = errorAndRequestAgain.entrySet().iterator();  
        while(it.hasNext()){  
            Entry<String, Set<String>> entry=it.next();  
            String key=entry.getKey(); 

            if (errorAndRequestAgain.get(key).size() == 0) {
                it.remove();
            }else {
                logger.info("还有"+key+"时刻未处理，共"+errorAndRequestAgain.get(key).size()+"站点，准备重新下载");
                JSONArray httpJson = getHttpJson(new ArrayList<String>(errorAndRequestAgain.get(key)), key);
                List<String[]> list = jsonArrayTOList(httpJson);
                String fileName = key + ".txt";
                ToFileUtil.WriteStringToTxtFile(list, downfilePath + fileName);
            }


        }