Retrofit基本介绍

Retrofit是Square开源的一个适用于Android和Java的类型安全的HTTP网络框架。其本质是对OKHttp的进一步封装，通过接口的方式进行网络请求，结合了注解、动态代理等技术实现模块解耦，简化了网络请求的上层逻辑。从Android4.4开始，http的底层实现已替换成了OKHttp，Retrofit也因此成为了Android平台上主流的网络框架之一。

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

Retrofit使用方法

Step1：声明Retrofit API请求接口

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

• 请求类型：以@GET/POST/PUT/DELETE注解声明
• 请求相对URL：以请求方法注解内的Value值表示
• 请求参数：以接口方法@Path参数表示

Step2：构建Retrofit对象并创建API接口实例

    // 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);

• 通过baseUrl接口声明接口请求域名；
• 配置对象的序列化和反序列化转换工厂；

Step3：通过API接口方法创建Call实例并发起请求

    // 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的实现就是这么简单优雅，业务层只需要像调用普通接口一样即可完成网络请求。

Retrofit源码解析

首先要构建一个Retrofit实例，其作用是把接口方法注解转换为OKHttp请求。

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，生产并处理请求的工厂（默认创建OKHttpClient）
• callbackExecutor，用以处理callback线程调度（Android默认回调到主线程）
• callAdapterFactories，用以生产CallAdapter以处理Response类型转换（默认使用ExecutorCallAdapterFactory）
• converterFactories，用以生产Converter处理http请求响应的数据转换（默认使用BuiltInConverters）

Retrofit实例化完成，创建API接口动态代理实现。

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