注解形式开发Controller方法的返回值类型

下面为方法返回类型的几种形式:

    //形式一:返回值为String,代表要访问的URL地址
    @RequestMapping("test1")
    public String test1(){
        return "index";
    }

    //形式二:返回ModelAndView
    //构造可以传URL地址,和传入一个Map集合到request域中
    //JSP取Map集合时为:${requestScope.admin.username }
    //不是:${requestScope.map.admin.username }
    @RequestMapping("test2")
    public ModelAndView test2(){
        Map<String,User> map = new HashMap<String, User>();
        map.put("admin",new User("admin","1234"));
        ModelAndView mv = new ModelAndView("index",map);
        //mv.addObject("map",map);
        return mv;
    }

    //形式三:返回值为String,但形参为Map集合,键值最好为:String,Object类
    //JSP取Map集合时为:${requestScope.admin.username }
    //不是:${requestScope.map.admin.username }
    @RequestMapping("test3")
    public String test3(Map<String,Object> map){
        map.put("admin",new User("admin","1234"));
        return "index";
    }

    //形式四:返回值为String,形参为Model
    @RequestMapping("test4")
    public String test4(Model model,HttpServletRequest request){

        //添加一个List集合
        List<User> list = new ArrayList<User>();
        list.add(new User("list5","1234"));
        model.addAttribute("list",list);

        //添加一个Map集合
        Map<String,User> map = new HashMap<String, User>();
        map.put("user1",new User("map","1234"));
        //注意:下面两个方法不同
        //addAllAttributes在JSP中取值时为:${requestScope.user1.username }
        model.addAllAttributes(map);
        //addAttribute在JSP中取值时为:${requestScope.map.user1.username }
        model.addAttribute("map",map);

        //添加一个对象
        model.addAttribute("user",new User("admin","1234"));

        return "index";
    }

    //形式五:返回值为void,利用Ajax返回,参数为HttpServletResponse
    @RequestMapping("test5")
    public void test5(HttpServletResponse response) throws IOException{
        response.setContentType("application/json");
        response.getWriter().write("{\"json\":\"json\"}");
    }


    //形式六:返回值为void,利用Ajax返回,形参为PrintWriter
    @RequestMapping("test6")
    public void test6(HttpServletResponse response ,PrintWriter pw){
        response.setContentType("application/json");
        pw.write("{\"json\":\"json\"}");
    }