Retrofit的原理

Retrofit是目前比较流行的网络框架，它封装了okhttp作为底层的网络通讯框架, 使用动态代理方式来解析注解, 今天来扒一扒它的运行原理.
通常Retrofit的使用分为如下四步:
第一步: 定义网络接口

public interface PostRequest_Interface {
    @POST("translate?doctype=json&jsonversion=&type=&keyfrom=&model=&mid=&imei=&vendor=&screen=&ssid=&network=&abtest=")
    @FormUrlEncoded
    Call getCall(@Field("i") String targetSentence);
}

第二步: 创建Retrofit

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://fanyi.youdao.com/") // 设置 网络请求 Url
                .addConverterFactory(GsonConverterFactory.create()) //设置使用Gson解析(记得加入依赖)
                .build();

第三步: 创建代理对象

PostRequest_Interface request = retrofit.create(PostRequest_Interface.class);

第四步: 创建Call对象

Call call = request.getCall("I love you");

第五步:执行异步网络请求

call.enqueue(new Callback() {
            //请求成功时回调
            @Override
            public void onResponse(Call call, Response response) {
                System.out.println(response.body().getTranslateResult().get(0).get(0).getTgt());
            }

            //请求失败时回调
            @Override
            public void onFailure(Call call, Throwable throwable) {
                System.out.println("请求失败");
                System.out.println(throwable.getMessage());
            }
        });

下面通过解析源码来看看，整个内部实现原理是怎样的.
我们从第二步代码开始

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://fanyi.youdao.com/") // 设置 网络请求 Url
                .addConverterFactory(GsonConverterFactory.create()) //设置使用Gson解析(记得加入依赖)
                .build();

这里首先会实例化一个

public Builder() {
      this(Platform.get());
}

Builder(Platform platform) {
      this.platform = platform;
      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
}

其实这就是以build模式创建一个Retrofit实例. 接下来我们看下第三步创建代理对象

PostRequest_Interface request = retrofit.create(PostRequest_Interface.class);

下面代码比较关键，也是Retrofit的精髓所在.

image.png

public  T create(final Class service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            ServiceMethod serviceMethod =
                (ServiceMethod) loadServiceMethod(method);
            OkHttpCall