GsonBuilder - 从接口获取对象

Question

我有一个像这样声明的 GsonFactory ：

public class GsonFactory {
    public Gson create() {
        return new GsonBuilder()
                .setPrettyPrinting()
                .registerTypeAdapter(MyInterface.class, new MyInstanceCreator())
                .create();
    }

    private static final GsonFactory instance = new GsonFactory();

    private GsonFactory() {
    }

    public static Gson getInstance(){
        return instance.create();
    }
}

我还有 InstanceCreator 类：

public class MyInstanceCreator implements InstanceCreator<MyInterface> {
    @Override
    public StrategySymbol createInstance(Type type) {
        return new MyInterfaceImpl();
    }
}

class MyClass {
  private List<MyInterface> myList;

  public List<MyInterface> getMyList() { return myList; }

  public void setMyList(List<MyInterface> myList) { this.myList = myList; }
}

interface MyInterface {
  Long getValue1();
  void setValue1(Long value1);
}

class MyInterfaceImpl implements MyInterface {
  private Long value1;

  @Override
  public Long getValue1() {
     return value1;
  }

  @Override
  public void setValue1(Long value1) {
     this.value1 = value1
  }
}

这段代码似乎实现得很好，但是如果我尝试解析 JSON with value1：

MyClass obj = GsonFactory.getInstance().fromJson("{'myList': [{'value1':8}]}", MyClass.class);

返回的对象带有 MyInterfaceImpl 的实例，但字段 value1 始终为 null。正如我所看到的，似乎 Gson 在 interface 中查找字段（无），而不是在实现 的 class 中查找字段接口。

有谁知道如何解决这个问题？

Answer 1

InstanceCreator 可以仅创建未定义无参数构造函数的类的实例。它不处理反序列化。 Gson 仍然无法弄清楚对象的具体数据类型。

解决方案。接口的自定义解串器

您需要为您的接口定义一个自定义反序列化器，并在工厂中注册这个新型适配器。

public class MyInterfaceDeserializer implements JsonDeserializer<MyInterface> {
    @Override
    public MyInterface deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        return jsonDeserializationContext.deserialize(jsonElement, MyInterfaceImpl.class);
    }
}

public class GsonFactory {
    public Gson create() {
        return new GsonBuilder()
                .setPrettyPrinting()
                .registerTypeAdapter(MyInterface.class, new MyInterfaceDeserializer())
                .create();
    }
}

或通用解串器

public class InterfaceDeserializer<T, E> implements JsonDeserializer<T> {
    private final Class<E> implementationClass;

    public InterfaceDeserializer(Class<E> implementationClass) {
        this.implementationClass = implementationClass;
    }

    @Override
    public T deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        return jsonDeserializationContext.deserialize(jsonElement, implementationClass);
    }
}

public class GsonFactory {
    public Gson create() {     
        return new GsonBuilder()
                .setPrettyPrinting()                
               .registerTypeAdapter(MyInterface.class, new InterfaceDeserializer<MyInterface, MyInterfaceImpl>(MyInterfaceImpl.class))
                .create();
}

或

public class MyInterfaceDeserializer implements JsonDeserializer<MyInterface> {
    @Override
    public MyInterface deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        MyInterface myInterface = new MyInterfaceImpl();
        JsonObject jObject = jsonElement.getAsJsonObject();
        myInterface.setValue1(jObject.get("value1").getAsLong());
        return myInterface;
    }
}

public class GsonFactory {
    public Gson create() {
        return new GsonBuilder()
                .setPrettyPrinting()
                .registerTypeAdapter(MyInterface.class, new MyInterfaceDeserializer())
                .create();
    }
}

另一个选项
使用 RuntimeTypeAdapterFactory 但此解决方案需要 json 中的额外 type 属性来定义确切的子类型，请参阅文档。

示例：

    public Gson create() {
        RuntimeTypeAdapterFactory<MyInterface> runtimeTypeAdapterFactory = RuntimeTypeAdapterFactory.of(MyInterface.class).registerSubtype(MyInterfaceImpl.class, "MyInterfaceImpl");
        return new GsonBuilder()
                .setPrettyPrinting()
                .registerTypeAdapterFactory(runtimeTypeAdapterFactory)
                .create();
    }

JSON 必须包含类型字段：

{
  myList: [
    {
      value1: 8,
      type: MyInterfaceImpl
    }
  ]
}

请注意 gson-extras 库。此工件位于 CronApp 存储库。