spring boot里增加表单验证hibernate-validator并在freemarker模板里显示错误信息(推荐)

public class Person implements Serializable {
 @NotNull
 @Length(min = 3, max = 10) // username长度在3-10之间
 private String username;
 @NotNull
 @Min(18) // 年龄最小要18岁
 private Integer age;
 @NotNull
 // 使用正则来验证字段，message: 设置验证失败的信息
 @Pattern(regexp = "[\\w-\\.]+@([\\w-]+\\.)+[a-z]{2,3}", message = "邮箱格式不正确")
 private String email;
 public String getEmail() {
  return email;
 }
 public void setEmail(String email) {
  this.email = email;
 }
 public String getUsername() {
  return username;
 }
 public void setUsername(String username) {
  this.username = username;
 }
 public Integer getAge() {
  return age;
 }
 public void setAge(Integer age) {
  this.age = age;
 }
}

@Controller
public class WebController extends WebMvcConfigurerAdapter {
 @Override
 public void addViewControllers(ViewControllerRegistry registry) {
  //添加一个路由并设置页面名字
  registry.addViewController("/results").setViewName("results");
 }
 @GetMapping("/")
 public String showForm(Person person) {
  return "form";
 }
 @PostMapping("/")
 public String checkPersonInfo(@Valid Person person, BindingResult bindingResult, RedirectAttributes redirectAttributes) {
  // 使用BindingResult来验证表单数据的正确性
  if (bindingResult.hasErrors()) {
   // 将提交的表单内容原封不动的返回到页面再展示出来
   redirectAttributes.addFlashAttribute("person", person);
   return "form";
  }
  return "redirect:/results";
 }
}

spring:
 freemarker:
  settings:
   auto_import: /spring.ftl as spring

<form action="/" method="post">
 <div class="form-group">
  <label for="username">username</label>
  <@spring.bind "person.username"/>
  <input type="text" id="username" name="username" value="${person.username!}" class="form-control"
      placeholder="username"/>
  <span class="text-danger"><@spring.showErrors ""/></span>
 </div>
 <div class="form-group">
  <label for="age">age</label>
  <@spring.bind "person.age"/>
  <input type="number" id="age" name="age" value="${person.age!}" class="form-control" placeholder="age"/>
  <span class="text-danger"><@spring.showErrors ""/></span>
 </div>
 <div class="form-group">
  <label for="email">email</label>
  <@spring.bind "person.email"/>
  <input type="text" id="email" name="email" value="${person.email!}" class="form-control"
      placeholder="email"/>
  <span class="text-danger"><@spring.showErrors ""/></span>
 </div>
 <input type="submit" value="submit" class="btn btn-sm btn-primary"/>
</form>