当外部名称可变时，如何使用 GSON/Retrofit 获取内部 JSON 对象？

Question

我有一个来自 API 的 JSON 响应，如下所示：

{"asalas77":
    {"id":23519033,"name":"Asalas77","profileIconId":22,"revisionDate":1487214366000,"summonerLevel":30}
}

我需要从中提取内部对象。我尝试使用这个问题Get nested JSON object with GSON using retrofit 中所示的解串器，但它对我不起作用。

public class SummonerDeserializer implements JsonDeserializer<Summoner> {
@Override
public Summoner deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
        throws JsonParseException {
    long id = je.getAsJsonObject().get("id").getAsLong();
    String name = je.getAsJsonObject().get("name").getAsString();
    int profileIconId = je.getAsJsonObject().get("profileIconId").getAsInt();
    long revisionDate = je.getAsJsonObject().get("revisionDate").getAsLong();
    long summonerLevel = je.getAsJsonObject().get("summonerLevel").getAsLong();
    Summoner s = new Summoner();
    s.setId(id);
    s.setName(name);
    s.setProfileIconId(profileIconId);
    s.setRevisionDate(revisionDate);
    s.setSummonerLevel(summonerLevel);

    return s;
}
}

但问题是我无法从 JsonElement je 访问内部字段，而名称 asalas77 是一个变量（它是一个搜索查询），所以我无法直接提取内部对象。

Answer 1

您必须有一个包装类，以免与反序列化策略发生冲突。假设如下：

final class SummonerResponse {

    private final Summoner summoner;

    private SummonerResponse(final Summoner summoner) {
        this.summoner = summoner;
    }

    static SummonerResponse summonerResponse(final Summoner summoner) {
        return new SummonerResponse(summoner);
    }

    Summoner getSummoner() {
        return summoner;
    }

}

然后您可以创建自定义响应反序列化器：

final class SummonerWrapperDeserializer
        implements JsonDeserializer<SummonerResponse> {

    private static final JsonDeserializer<SummonerResponse> summonerDeserializer = new SummonerWrapperDeserializer();

    private SummonerWrapperDeserializer() {
    }

    static JsonDeserializer<SummonerResponse> getSummonerResponseDeserializer() {
        return summonerDeserializer;
    }

    @Override
    public SummonerResponse deserialize(final JsonElement jsonElement, final Type type, final JsonDeserializationContext context)
            throws JsonParseException {
        // Pick the root as a JSON object
        final JsonObject outerJsonObject = jsonElement.getAsJsonObject();
        // And check how many properties does it have
        final Iterable<? extends Entry<String, JsonElement>> outerJsonObjectEntries = outerJsonObject.entrySet();
        if ( outerJsonObject.size() != 1 ) {
            throw new JsonParseException("Expected one property object, the actual properties are: " + getPropertyName(outerJsonObjectEntries));
        }
        // If it has only one property, just get the property and take its inner value
        final JsonElement innerJsonElement = outerJsonObjectEntries.iterator().next().getValue();
        // Once it's obtained, just delegate the parsing to a downstream parser - no need to create Summoner instances by hands
        return summonerResponse(context.deserialize(innerJsonElement, Summoner.class));
    }

    private static Set<String> getPropertyName(final Iterable<? extends Entry<String, JsonElement>> entries) {
        final Set<String> keys = new LinkedHashSet<>();
        for ( final Entry<String, JsonElement> entry : entries ) {
            keys.add(entry.getKey());
        }
        return keys;
    }

}

或者节省一些内存（JSON（反）序列化程序需要一些内存，因为它们使用 JSON 树）并创建一个更底层的类型适配器：

final class SummonerResponseTypeAdapterFactory
        implements TypeAdapterFactory {

    private static final TypeAdapterFactory summonerResponseTypeAdapterFactory = new SummonerResponseTypeAdapterFactory();

    private SummonerResponseTypeAdapterFactory() {
    }

    static TypeAdapterFactory getSummonerResponseTypeAdapterFactory() {
        return summonerResponseTypeAdapterFactory;
    }

    @Override
    public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
        // Check if we can handle SummonerResponse. Classes can be compared with `==`
        if ( typeToken.getRawType() == SummonerResponse.class ) {
            final TypeAdapter<SummonerResponse> typeAdapter = getSummonerResponseTypeAdapter(gson);
            @SuppressWarnings("unchecked")
            final TypeAdapter<T> castTypeAdapter = (TypeAdapter<T>) typeAdapter;
            return castTypeAdapter;
        }
        return null;
    }

}

final class SummonerResponseTypeAdapter
        extends TypeAdapter<SummonerResponse> {

    private final Gson gson;

    private SummonerResponseTypeAdapter(final Gson gson) {
        this.gson = gson;
    }

    static TypeAdapter<SummonerResponse> getSummonerResponseTypeAdapter(final Gson gson) {
        return new SummonerResponseTypeAdapter(gson);
    }

    @Override
    @SuppressWarnings("resource")
    public void write(final JsonWriter out, final SummonerResponse summonerResponse)
            throws IOException {
        // The incoming object may be null
        if ( summonerResponse == null && gson.serializeNulls() ) {
            out.nullValue();
            return;
        }
        // Generate the inner object
        out.beginObject();
        out.name(summonerResponse.getSummoner().name);
        gson.toJson(summonerResponse.getSummoner(), Summoner.class, out);
        out.endObject();
    }

    @Override
    public SummonerResponse read(final JsonReader in)
            throws IOException {
        // is it a null?
        if ( in.peek() == NULL ) {
            return null;
        }
        // make sure that the inner read JSON contains an inner object
        in.beginObject();
        // ignore the name
        in.nextName();
        // delegate parsing to the backing Gson instance in order to apply downstream parsing
        final Summoner summoner = gson.fromJson(in, Summoner.class);
        // check if there are more properties within the inner object
        if ( in.peek() == NAME ) {
            throw new MalformedJsonException("Unexpected: " + in.nextName());
        }
        // consume the "}" token
        in.endObject();
        return summonerResponse(summoner);
    }

}

那么上面的任何一个选项都可以这样使用：

final Gson gson = new GsonBuilder()
        .registerTypeAdapter(SummonerResponse.class, getSummonerResponseDeserializer())
        .create();
final SummonerResponse summonerResponse = gson.fromJson(JSON, SummonerResponse.class);
final Summoner summoner = summonerResponse.getSummoner();
out.println(summoner.id + " => " + summoner.name);

或

final Gson gson = new GsonBuilder()
        .registerTypeAdapterFactory(getSummonerResponseTypeAdapterFactory())
        .create();
final SummonerResponse summonerResponse = gson.fromJson(JSON, SummonerResponse.class);
final Summoner summoner = summonerResponse.getSummoner();
out.println(summoner.id + " => " + summoner.name);
out.println(gson.toJson(summonerResponse));

输出是

23519033 => 阿萨拉斯77

和

23519033 => 阿萨拉斯77
{"Asalas77":{"id":23519033,"name":"Asalas77","profileIconId":22,"revisionDate":1487214366000,"summonerLevel":30}}

分别。