Spring提供的一个参数校验框架，使用预定义的注解完成参数校验
1、引入依赖

<!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-validation -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
    <version>3.2.5</version>
</dependency>

2、在参数前面添加@Pattern注解

@PostMapping("/register")
public Result register(@Pattern(regexp = "^\\S{5,16}$") String username, @Pattern(regexp = "^\\S{5,16}$")String password) {
    // 查询用户
    User u = userService.findByUserName(username);
    if (u == null) {
        // 注册
        userService.register(username, password);
        return Result.success();
    }else {
        return Result.error("用户名已存在！");
    }
}

3、在Controller类上添加@Validated注解