在第一次注冊并登陸的時候，服務(wù)端會返回用戶的信息。如果沒有默認值或返回的數(shù)據(jù)為null時，我們使用GsonConverterFactory轉(zhuǎn)換器就會報空指針異常。以下方法針對返回的是String的數(shù)據(jù)做一次轉(zhuǎn)換。

一個普通的Service創(chuàng)建

 private static Retrofit userService = null;

    public static UserService userApi() {
        if (userService == null) {
            Gson gson = new GsonBuilder().registerTypeAdapterFactory(new NullStringToEmptyAdapterFactory()).create();
            userService = new Retrofit.Builder()
                    .baseUrl(API_BASE_URL)
                    .client(mOkHttpClient)
                    .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .build();
        }
        return userService.create(UserService.class);
    }

   private static class NullStringToEmptyAdapterFactory<T> implements TypeAdapterFactory {
        @SuppressWarnings("unchecked")
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            Class<T> rawType = (Class<T>) type.getRawType();
            if (rawType != String.class) {
                return null;
            }
            return (TypeAdapter<T>) new StringNullAdapter();
        }
    }

    private static class StringNullAdapter extends TypeAdapter<String> {
        @Override
        public String read(JsonReader reader) throws IOException {
            if (reader.peek() == JsonToken.NULL) {
                reader.nextNull();
                return "";
            }
            return reader.nextString();
        }

        @Override
        public void write(JsonWriter writer, String value) throws IOException {
            if (value == null) {
                writer.nullValue();
                return;
            }
            writer.value(value);
        }
    }

這樣就把model屬性值為null的String，轉(zhuǎn)換為""。