1. 返回静态页面

直接在这里返回的，那么返回的就是静态页面的名称，比如

@RequestMapping("/resp")
@Controller
//@ResponseBody
public class RespController {
    @RequestMapping("/Hi")
    public String sayHi(){
        return "/hello.html";
    }
}

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.
0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>hello,spring mvc</title>
    <script src="index.js"></script>
</head>
<body>
<h1>Hello,Spring MVC.</h1>
</body>
</html>

可以看到前面返回的内容是 hello.html，这里通过一级目录 sayhi，也可以访问到这个页面，说明返回的虽然是 String 类型，但实际上是一个页面

2. 返回非静态页面

2.1 @ResponseBody 返回页面内容

@Controller
@ResponseBody
public class TestController {

    @RequestMapping("/Hi")
    public String sayHi() {
        return "hello.html";
    }
}

2.2 @RestController = @ResponseBody + @Controller

@RequestMapping("/resp")
//@Controller
//@ResponseBody
@RestController
public class RespController {
    @RequestMapping("/Hi")
    public String sayHi(){
        return "/hello.html";
    }
}

2.3 示例:实现简单计算的功能

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.
0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>document</title>
    <script src="index.js"></script>
</head>
<body>
<form action="/calc">
    <div style="margin-top:100px;text-align: center;">
        <h1>计算器</h1>
        数字1：<input name="num1" type="text"><br>
        数字2：<input name="num2" type="text"><br>
        <input type="submit" value=" 点击相加 ">
    </div>
</form>
</body>
</html>

@Controller
@ResponseBody
public class CalcController {
    @RequestMapping("/calc")
    public String calc(Integer num1,Integer num2){
        if (num1 == num2 || num2 == null){
            return "参数错误";
        }
        return "结果=" + (num1 + num2);
    }
}

3. 返回JSON对象

    @RequestMapping("/respjson")
    public HashMap<String, String> respJson() {
        HashMap<String, String> map = new HashMap<>();
        map.put("Java", "Java Value");
        map.put("MySQL", "MySQL Value");
        map.put("Redis", "Redis Value");
        return map;
    }

3.1 实现登录功能，返回 JSON 对象

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.
0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <script src="js/jquery-2.1.4.min.js"></script>
    <title>Document</title>
    <script>
        // ajax 提交
        function mysub() {
            // 判空
            // var username = $("#username");
            //这里注意如果在js中有多个 jquery 那么这里的$，是不合适的，$相当于小名，jQuery相当于大名，最好使用大名
            var username = jQuery("#username");
            var password = jQuery("#password");
            if(jQuery.trim(username.val()) == "") {
                alert("请先输入用户名!");
                username.focus; // 让光标重置到此元素
                return;
            }
            if(jQuery.trim(password.val()) == "") {
                alert("请先输入密码!");
                password.focus();
                return;
            }
            jQuery.ajax({
                url:"/user/login2",
                type:"POST",
                data:{"username":username.val(),
            "password":password.val()},
                success:function(result) {
                    alert(JSON.stringify(result));
                }
            });
        }
    </script>
</head>
<body>
<div style="text-align: center;">
    <h1>登录</h1>
    ⽤户：<input id="username">
    <br>
    密码：<input id="password" type="password">
    <br>
    <input type="button" value=" 提交 " onclick="mysub()" style="margin-top: 20px;margin-left: 50px;">
</div>
</body>
</html>

@Slf4j
@Controller
@ResponseBody  
@RequestMapping("/user") 
public class UserController {

    @RequestMapping("/login2")
    public HashMap<String, Object> login2(String username, String password) {
        HashMap<String, Object> result = new HashMap<>();
        int state = 200;
        int data = -1; // 等于1，登录成功，否则登录失败
        String msg = "未知错误";
        if(StringUtils.hasLength(username) && StringUtils.hasLength(password)) {
          if(username.equals("admin") && password.equals("admin")) {
              data = 1;
              msg = "";
          } else {
              msg = "用户名或密码错误!";
          }
        } else { //说明参数为空
            msg = "非法参数";
        }
        result.put("state", state);
        result.put("data", data);
        result.put("msg",msg);
        return result;
    }
}

4. 请求转发(forward)或请求重定向(redirect)

4.1 请求转发：forward

@Controller
public class TestController {

    //请求转发方式1
    @RequestMapping("/fw")
    public String myForward() {
        return "forward:/hello.html";
    }

    //请求转发方式2
    @RequestMapping("/fw2")
    public void myForward2(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.getRequestDispatcher("/hello.html").forward(request,response);
    }
}

4.2 请求重定向：redirect

@Controller
public class TestController {

    //请求重定向方式1
    @RequestMapping("/rd")
    public String myRedirect() {
        return "redirect:/hello.html";
    }
    
    //请求重定向方式2
    @RequestMapping("/rd2")
    public void myRedirect2(HttpServletResponse response) throws IOException {
        response.sendRedirect("/hello.html");
    }
}