修改retUser接口

src/com/hellogithub/servlet/retUserServlet.java

@@ -42,6 +42,10 @@ public class retUserServlet extends HttpServlet {
 
         PrintWriter writer = resp.getWriter();
         String username = req.getParameter("username");
+        if(username ==null)
+        {
+            return ;
+        }
         String jsonString="";
         jsonString = JSONObject.toJSONString(userService.selectUserByName(username));
         if(jsonString.equals(""))