在Java中使用Gson建立自訂實例的方法？

在將JSON 字串解析為Java 物件或從Java 物件解析JSON 字串時，預設情況下Gson 嘗試透過呼叫預設建構子來建立Java 類別的實例。如果Java類別不包含預設建構函式或我們想在建立Java物件時進行一些初始配置，我們需要建立並註冊自己的實例建立器。

我們可以建立自訂實例建立器在Gson 中使用InstanceCreator介面並且需要實作createInstance(Type type)方法。

語法

T createInstance(Type type)

範例

import java.lang.reflect.Type;
import com.google.gson.*;
public class CustomInstanceCreatorTest {
   public static void main(String args[]) {
      GsonBuilder gsonBuilder = new GsonBuilder();
      gsonBuilder.registerTypeAdapter(Course.class, new CourseCreator());
      Gson gson = gsonBuilder.create();
      String jsonString = "{'course1':'Core Java', 'course2':'Advanced Java'}";
      Course course = gson.fromJson(jsonString, Course.class);
      System.out.println(course);
   }
}
// Course class
class Course {
   private String course1;
   private String course2;
   private String technology;
   public Course(String technology) {
      this.technology = technology;
   }
   public void setCourse1(String course1) {
      this.course1 = course1;
   }
   public void setCourse2(String course2) {
      this.course2 = course2;
   }
   public String getCourse1() {
      return course1;
   }
   public String getCourse2() {
      return course1;
   }
   public void setTechnology(String technology) {
      this.technology = technology;
   }
   public String getTechnology() {
      return technology;
   }
   public String toString() {
      return "Course[ " +
             "course1 = " + course1 +
             ", course2 = " + course2 +
             ", technology = " + technology +
             " ]";
   }
}
// CourseCreator class
class CourseCreator implements InstanceCreator {
   @Override
   public Course createInstance(Type type) {
      Course course = new Course("Java");
      return course;
   }
}

輸出

Course[ course1 = Core Java, course2 = Advanced Java, technology = Java ]

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