如何验证Spring-Boot映射的实体

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

import javax.validation.constraints.NotEmpty;
import javax.validation.constraints.Size;

public class User {
    @NotEmpty(message = "用户名不能为空")
    private String username;

    @Size(min = 6, max = 12, message = "密码长度必须在6到12个字符之间")
    private String password;

    // 省略其他字段及getter/setter方法
}

import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
@Validated
public class UserController {
    @PostMapping("/users")
    public void createUser(@Valid @RequestBody User user) {
        // 处理创建用户的逻辑
    }
}

import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.RestControllerAdvice;

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public String handleValidationException(MethodArgumentNotValidException e) {
        return e.getBindingResult().getFieldError().getDefaultMessage();
    }
}