Gson 使用 TypeAdapter 或 Json Deserializer 将数据从错误列表

Question

让我们从例子开始：如果数据正确，应该是（Beijing cities为空）{   "code":200,   "msg":"success",   "data":[      {         "id":1,         "name":"Beijing",         "cities":[]      },      {         "id":2,         "name":"Guangdong",         "cities":[            {               "id":1,               "name":"Guangzhou"            }         ]      }   ]}现在我得到了错误的数据。（Beijing cities为空）{   "code":200,   "msg":"success",   "data":[      {         "id":1,         "name":"Beijing",         "cities":null      },      {         "id":2,         "name":"Guangdong",         "cities":[            {               "id":1,               "name":"Guangzhou"            }         ]      }   ]}我正在使用Retrofit2 ResponseBodyConverter实体类：public class Result<T> {    private int code;    private String msg;    private T data;    // getters, setters}public class Province {    private int id;    private String name;    private List<City> cities;}public class City {    private int id;    private String name;}

Answer 1

我找到了CollectionTypeAdapterFactory源Gson代码。我尝试修改它，它已经过测试并且很有用。

public class CollectionTypeAdapterFactory implements TypeAdapterFactory {

    private final ConstructorConstructor constructorConstructor;

    public CollectionTypeAdapterFactory(ConstructorConstructor constructorConstructor) {

        this.constructorConstructor = constructorConstructor;

    }

    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {

        Type type = typeToken.getType();

        Class<? super T> rawType = typeToken.getRawType();

        if (!Collection.class.isAssignableFrom(rawType)) {

            return null;

        }

        Type elementType = $Gson$Types.getCollectionElementType(type, rawType);

        TypeAdapter<?> elementTypeAdapter = gson.getAdapter(TypeToken.get(elementType));

        ObjectConstructor<T> constructor = constructorConstructor.get(typeToken);

        @SuppressWarnings({"unchecked", "rawtypes"}) // create() doesn't define a type parameter

                TypeAdapter<T> result = new Adapter(gson, elementType, elementTypeAdapter, constructor);

        return result;

    }

    private static final class Adapter<E> extends TypeAdapter<Collection<E>> {

        private final TypeAdapter<E> elementTypeAdapter;

        private final ObjectConstructor<? extends Collection<E>> constructor;

        public Adapter(Gson context, Type elementType,

                       TypeAdapter<E> elementTypeAdapter,

                       ObjectConstructor<? extends Collection<E>> constructor) {

            this.elementTypeAdapter =

                    new TypeAdapterRuntimeTypeWrapper<E>(context, elementTypeAdapter, elementType);

            this.constructor = constructor;

        }

        public Collection<E> read(JsonReader in) throws IOException {

            if (in.peek() == JsonToken.NULL) {

                in.nextNull();

                //In the source code is return null, I changed to return an empty collection

                return constructor.construct();

            }

            Collection<E> collection = constructor.construct();

            in.beginArray();

            while (in.hasNext()) {

                E instance = elementTypeAdapter.read(in);

                collection.add(instance);

            }

            in.endArray();

            return collection;

        }

        public void write(JsonWriter out, Collection<E> collection) throws IOException {

            if (collection == null) {

                out.nullValue();

                return;

            }

            out.beginArray();

            for (E element : collection) {

                elementTypeAdapter.write(out, element);

            }

            out.endArray();

        }

    }

}

在源代码中TypeAdapterRuntimeTypeWrapper是受保护的，我们必须进行复制。

  public class TypeAdapterRuntimeTypeWrapper<T> extends TypeAdapter<T> {

      private final Gson context;

      private final TypeAdapter<T> delegate;

      private final Type type;

      TypeAdapterRuntimeTypeWrapper(Gson context, TypeAdapter<T> delegate, Type type) {

          this.context = context;

          this.delegate = delegate;

          this.type = type;

      }

      @Override

      public T read(JsonReader in) throws IOException {

          return delegate.read(in);

      }

      @SuppressWarnings({"rawtypes", "unchecked"})

      @Override

      public void write(JsonWriter out, T value) throws IOException {

          TypeAdapter chosen = delegate;

          Type runtimeType = getRuntimeTypeIfMoreSpecific(type, value);

          if (runtimeType != type) {

              TypeAdapter runtimeTypeAdapter = context.getAdapter(TypeToken.get(runtimeType));

              if (!(runtimeTypeAdapter instanceof ReflectiveTypeAdapterFactory.Adapter)) {

                  // The user registered a type adapter for the runtime type, so we will use that

                  chosen = runtimeTypeAdapter;

              } else if (!(delegate instanceof ReflectiveTypeAdapterFactory.Adapter)) {

                  // The user registered a type adapter for Base class, so we prefer it over the

                  // reflective type adapter for the runtime type

                  chosen = delegate;

              } else {

                  // Use the type adapter for runtime type

                  chosen = runtimeTypeAdapter;

              }

          }

          chosen.write(out, value);

      }

      private Type getRuntimeTypeIfMoreSpecific(Type type, Object value) {

          if (value != null

                  && (type == Object.class || type instanceof TypeVariable<?> || type instanceof Class<?>)) {

              type = value.getClass();

          }

          return type;

      }

  }

如何使用

Gson gson = new GsonBuilder().serializeNulls()

           .registerTypeAdapterFactory(

             new CollectionTypeAdapterFactory(new ConstructorConstructor(new HashMap<>()))

             )

           .create();

Result<List<Province>> result = gson.fromJson(jsonStr, new TypeToken<Result<List<Province>>>() {}.getType());

印刷：

Result{code=200, msg='success', data=[Province{id=1, name='Beijing', cities=[]}, Province{id=2, name='Guangdong', cities=[City{id=1, name='Guangzhou'}]}]}