控制考试时间，自动更新考试时间

src/main/java/edu/nju/service/ExtraService.java

@@ -154,10 +154,22 @@ public class ExtraService {
 
 	public Task saveTask(String id){
 		Task task=taskDao.findById(id);
-		if(task!=null)
-			return task;
+		if(task!=null) {
+			long oldTime=task.getEnd_time();
+			//考试时间已过
+			if(oldTime>System.currentTimeMillis()){
+				return getAndSaveTaskInfo(id);
+			}else {
+				return task;
+			}
+		}else{
+			return getAndSaveTaskInfo(id);
+		}
+	}
+
+	private Task getAndSaveTaskInfo(String id){
 		String result = HTTP.sendGet("http://www.mooctest.net/api/exam/" + id + "/info", "");
-		if (result != null && !result.equals("")) {
+		if (!result.equals("")) {
 			JSONObject json = new JSONObject(result);
 			long beginTime = json.getLong("beginTime");
 			long endTime = json.getLong("endTime");
@@ -166,9 +178,9 @@ public class ExtraService {
 			Task newTask = new Task(id, name,beginTime, endTime, totalMins, totalMins);
 			taskDao.save(newTask);
 			return newTask;
+		}else{
+			return null;
 		}
-		return null;
-//		return taskDao.findById(id);
 	}
 
 	public boolean updateTask(String id,double writeMins){