java - 为什么 Gson.toJson 会将泛型字段序列化为空的 JSON 对象？

Question

我有一个包含 T 类型字段的泛型类，Gson 将该字段序列化为一个空对象。我在下面包含了代码来演示这个问题。读回 JSON 似乎没问题(只要您提供正确的类型标记)。

import java.lang.reflect.Type;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonIssue {
    static class AbstractThing {
        private String fieldA = "valueA";

        public String getFieldA() {
            return fieldA;
        }

        public void setFieldA(String fieldA) {
            this.fieldA = fieldA;
        }

        @Override
        public String toString() {
            return "AbstractThing [fieldA=" + fieldA + "]";
        }
    }

    static class Thing extends AbstractThing {
        private String fieldB = "valueB";

        @Override
        public String toString() {
            return "Thing [fieldB=" + fieldB + ", fieldA=" + getFieldA() + "]";
        }
    }

    static class Wrapper<T extends AbstractThing> {
        private T abstractThing;
        private String standardField = "standard value";

        public Wrapper(T abstractThing) {
            this.abstractThing = abstractThing;
        }

        @Override
        public String toString() {
            return "Wrapper [abstractThing=" + abstractThing + ", standardField=" + standardField + "]";
        }
    }

    public static void main(String[] args) {
        Wrapper<Thing> wrapper = new Wrapper<Thing>(new Thing());

        Gson gson = new Gson();
        String json = gson.toJson(wrapper);
        System.out.println(json);
        // prints : {"abstractThing":{},"standardField":"standard value"}
        // so standardField is correctly serialized but not abstractThing.

        // but if we manually construct the expected json string, and parse it back, we get the expected object structure
        json = "{\"standardField\": \"some text\", " +
                "\"abstractThing\":{\"fieldB\" : \"arb value\", \"fieldA\" : \"another arb value\"}}";

        Type type = new TypeToken<Wrapper<Thing>>() {}.getType();
        Object fromJson = gson.fromJson(json, type);
        System.out.println(fromJson);
        // prints : Wrapper [abstractThing=Thing [fieldB=arb value, fieldA=another arb value], standardField=some text]
        // which is as expected
    }
}

Answer 1

来自他们的文档:

When you call toJson(obj), Gson calls obj.getClass() to get information on the fields to serialize. Similarly, you can typically pass MyClass.class object in the fromJson(json, MyClass.class) method. This works fine if the object is a non-generic type. However, if the object is of a generic type, then the Generic type information is lost because of Java Type Erasure

You can solve this problem by specifying the correct parameterized type for your generic type. You can do this by using the TypeToken class.

他们为 List<T> 给出了以下示例:

Type listType = new TypeToken<List<String>>() {}.getType();
gson.toJson(myStrings, listType);

因此对于您的代码，您需要...

Type myType = new TypeToken<Wrapper<Thing>>() {}.getType();
String json = gson.toJson(wrapper, myType);

https://sites.google.com/site/gson/gson-user-guide#TOC-Serializing-and-Deserializing-Gener