java – JSP没有抛出NullPointerException

发布时间：2020-12-15 01:30:46 所属栏目：大数据来源：网络整理

导读：我有控制器： @RequestMapping(method = RequestMethod.GET)public String getViewRailwayService(@RequestParam long id,Model model) { model.addAttribute("railwayService",railwayServiceRepository.findOne(id)); return "admin/railwayService/view";

我有控制器：

@RequestMapping(method = RequestMethod.GET)
public String getViewRailwayService(@RequestParam long id,Model model) {
    model.addAttribute("railwayService",railwayServiceRepository.findOne(id));
    return "admin/railwayService/view";
}

和jsp页面：

...


它运行正常,但我很困惑,当railwayServiceRepository.findOne(id)返回null时,NullPointerException不会抛出.


最佳答案
不确定StackOverflow wiki on Expression Language是否值得信赖(我一直试图在官方规格中找到它,但还没有运气),但是：


EL relies on the JavaBeans specification when it comes to accessing properties. In JSP,the following expression:
${user.name}
does basically the same as the following in “raw” scriptlet code (the below example is for simplicity,in reality the reflection API is used to obtain the methods and invoke them):

<%
  User user = (User) pageContext.findAttribute("user");
  if (user != null) {
    String name = user.getName();
    if (name != null) {
      out.print(name);
    }
  }
%>


(…) Please note that it thus doesn’t print “null” when the value is null nor throws a NullPointerException unlike as when using scriptlets. In other words,EL is null-safe.


                        （编辑：李大同）
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