摘要：從開始，的底層實現已替換成了，也因此成為了平臺上主流的網絡框架之一。，負責把接口方法注解轉換為表示包括等，負責把轉換為，并且包裝了的接口調用。負責把轉為，使其具備線程調度能力。其內部接口負責把請求響應結果轉換可識別的數據格式。

Retrofit是Square開源的一個適用于Android和Java的類型安全的HTTP網絡框架。其本質是對OKHttp的進一步封裝，通過接口的方式進行網絡請求，結合了注解、動態代理等技術實現模塊解耦，簡化了網絡請求的上層邏輯。從Android4.4開始，http的底層實現已替換成了OKHttp，Retrofit也因此成為了Android平臺上主流的網絡框架之一。

http://square.github.io/retro...

Step1：聲明Retrofit API請求接口

    public interface GitHub {
        @GET("/repos/{owner}/{repo}/contributors")
        Call> contributors(
            @Path("owner") String owner,
            @Path("repo") String repo);
    }

請求類型：以@GET/POST/PUT/DELETE注解聲明

請求相對URL：以請求方法注解內的Value值表示

請求參數：以接口方法@Path參數表示

Step2：構建Retrofit對象并創建API接口實例

    // Create a very simple REST adapter which points the GitHub API.
    Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("https://api.github.com")
        .addConverterFactory(GsonConverterFactory.create())
        .build();

    // Create an instance of our GitHub API interface.
    GitHub github = retrofit.create(GitHub.class);

通過baseUrl接口聲明接口請求域名；

配置對象的序列化和反序列化轉換工廠；

Step3：通過API接口方法創建Call實例并發起請求

    // Create a call instance for looking up Retrofit contributors.
    Call> call = github.contributors("square", "retrofit");

    // Fetch and print a list of the contributors to the library.
    List contributors = call.execute().body();

Retrofit的實現就是這么簡單優雅，業務層只需要像調用普通接口一樣即可完成網絡請求。

首先要構建一個Retrofit實例，其作用是把接口方法注解轉換為OKHttp請求。

Retrofit.Builder.build

public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }

      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List converterFactories =
          new ArrayList<>(1 + this.converterFactories.size());

      // 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());
      converterFactories.addAll(this.converterFactories);

      return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
    }
  }

callFactory，生產并處理請求的工廠（默認創建OKHttpClient）

callbackExecutor，用以處理callback線程調度（Android默認回調到主線程）

callAdapterFactories，用以生產CallAdapter以處理Response類型轉換（默認使用ExecutorCallAdapterFactory）

converterFactories，用以生產Converter處理http請求響應的數據轉換（默認使用BuiltInConverters）

Retrofit實例化完成，創建API接口動態代理實現。

Retrofit.create

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