目前比较流行的使用Retrofit的方法是Retrofit + RxJava + Gson, 之前的Retrofit学习(二)已经分析过Retrofit是如何结合RxJava，这一篇主要看一下同Gson的结合

要想在Retrofit中使用Gson, 需要添加Gson对应的Converter:

    OkHttpclient client = new OkHttpClient.Builder().build();
    Retrofit retrofit = new Retrofit.Builder()
        .client(client)
        .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
        //添加Gson
        .addConverterFactory(GsonConverterFactory.create())
        .baseUrl("http://localhost:4567/")
        .build();

GsonConverterFactory

public final class GsonConverterFactory extends Converter.Factory {

    public static GsonConverterFactory create() {
        return create(new Gson());
    }

    public static GsonConverterFactory create(Gson gson) {
        return new GsonConverterFactory(gson);
    }

    private final Gson gson;

    private GsonConverterFactory(Gson gson) {
        if (gson == null) throw new NullPointerException("gson == null");
        this.gson = gson;
    }

    @Override
    public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
                                                            Retrofit retrofit) {
        TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
        return new GsonResponseBodyConverter<>(gson, adapter);
    }

    @Override
    public Converter<?, RequestBody> requestBodyConverter(Type type,
                                                          Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
        TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
        return new GsonRequestBodyConverter<>(gson, adapter);
    }
}

从之前的Retrofit学习(一)中我们可以知道，每次调用接口方法时都会通过ServiceMethod.Builder.build()创建一个ServiceMethod对象, ServiceMethod.Builder.build()会初始化responseConverter, 经过一系列调用最终会调用Converter.Factory.responseBodyConverter，如果在初始化Retrofit时添加了GsonConverterFactory，则会相应地调用GsonConverterFactory.responseBodyConverter

GsonConverterFactory.responseBodyConverter

    public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[]     annotations,Retrofit retrofit) {
        TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
        return new GsonResponseBodyConverter<>(gson, adapter);
    }

从之前的Retrofit学习(一)和Retrofit学习(二)可以知道，最终的步骤都会是OkHttpCall.execute或OkHttpCall.enqueue，不论是enqueue还是execute,都会调用OkHttpCall.parseResponse方法，该方法最主要的作用是从okhttp3.Response中得到okhttp3.ResponseBody，之后调用ServiceMethod.toResponse

ServiceMethod.toResponse

    R toResponse(ResponseBody body) throws IOException {
        return responseConverter.convert(body);
    }

调用Converter.convert转换结果，这里的Convert就是GsonResponseBodyConverter

GsonResponseBodyConverter.convert

    public T convert(ResponseBody value) throws IOException {
        JsonReader jsonReader = gson.newJsonReader(value.charStream());
        try {
            return adapter.read(jsonReader);
        } finally {
            value.close();
        }
    }