Gson 不遵守生成器中的规则

Question

我有一个名为 Student 的 class，它有一些必填字段，其他字段是可选的。为了确保 class Student 的对象在没有提供必填字段的情况下被创建，我使用了一个构建器。这是 class Student:

package prj1.dob;

import java.util.Date;

public class Student {
   private final int id;
   private final String name;
   private final String surname;
   private final Date dob;
   private final Double gpa;
   private final String email;

   public static class Builder {
      //required fields
      private int id;
      private String name;
      private String surname;

      //optional fields
      private Date dob;
      private Double gpa;
      private String email;

      public Builder(int id, String name, String surname) {
         this.id = id;
         this.name = name;
         this.surname = surname;
      }

      public Builder dob(Date dob) {
         this.dob = dob;
         return this;
      }

      public Builder gpa(Double gpa) {
         this.gpa = gpa;
         return this;
      }

      public Builder email(String email) {
         this.email = email;
         return this;
      }

      public Student build() throws Exception {
         return new Student(this);
      }
    }

    private Student(Builder b) throws Exception {
         this.id = b.id;
         this.name = b.name;
         this.surname = b.surname;
         this.dob = b.dob;
         this.gpa = b.gpa;
         this.email = b.email;
         validate();
     }

   private void validate() throws Exception {
      if (id < 10 || name == null || surname == null) {
        throw new Exception("You should provide an id, name and surname.");
   }
 }

 @Override
 public String toString() {
  return "Student [id=" + id + ", name=" + name + ", surname=" + surname + ", dob=" + dob + ", gpa=" + gpa  + ", email=" + email + "]";
    }
}

有了 class Student 的这个实现，我希望不应该有创建无效 Student 对象的方法（即必填字段不应该是 null).但是，下面提供的代码不会抛出 Exception，当我打印 Student 的内容时，必填字段是 null.

JSON 对象：

{"id":-1,"name":null,"surname":null,"dob":null,"gpa":null,"email":null}

这是从 JSON:

创建 POJO 的代码

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  try {
       String param = request.getParameter("std").toString();
       Gson gson = new GsonBuilder().setDateFormat("yyyy-MM-dd").serializeNulls().create();
       Student student = gson.fromJson(param, Student.class);
       // student.validate();
       response.getWriter().write(student.toString());
  } catch (Exception ex) { 
       response.getWriter().write(ex.getMessage());
  }
}

我希望它抛出一个 Exception 但它创建了一个 Student 的对象，当打印时这是内容：

Student [id=-1, name=null, surname=null, dob=null, gpa=null, email=null]

这是 Gson 库中的错误还是我做错了什么？

如果这是应该的，那么使用 builder 方法有什么意义，因为仍然可以在不遵循规则的情况下创建 class 的对象（提供必填字段）?

Answer 1

GSON 使用 reflection 而不是您的构建器来构建 Student。

虽然您无法阻止 Student 的其他用户随意使用反射创建东西，但您可以提供一个 JsonDeserializer，它知道 Student 的规则并使用您的构建器创建一个。

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