Retrofit2.0 源码解析

首先介绍下Retrofit基本用法，先创建接口，注解申明、请求方式Post/Get等

public interface Service {

    @POST("list")
    Call loadRepo();

}

基本使用如下

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://www.jianshu.com/u/c030e2bc8731")
                .build();

        Service service = retrofit.create(Service.class);
        Call userCall = service.loadRepo();

        userCall.enqueue(new Callback() {
            @Override
            public void onResponse(Call call, Response response) {

            }

            @Override
            public void onFailure(Call call, Throwable t) {

            }
        });

上面是简单的网络请求流程，那么我们来分析具体是怎么实现的，直接看源码

Retrofit.Builder()方法

首先Retrofit.Builder()中Platform.get()最后调用findPlatform()得到平台信息，可以看到支持Android、Java8平台,其中还对Converter.Factory 设置默认值。

  private static Platform findPlatform() {
    try {
      Class.forName("android.os.Build");
      if (Build.VERSION.SDK_INT != 0) {
        return new Android();
      }
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("java.util.Optional");
      return new Java8();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
  }

Retrofit.Builder().builder()方法

    public Retrofit build() {
      //baseUrl必须不为空
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
      //OkHttpClient对象
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }
      //将回调传递，此时platform为Android 平台，在Retrofit.Builder()中初始化
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List adapterFactories = new ArrayList<>(this.adapterFactories);
      //将callbackExecutor转化成ExecutorCallAdapterFactory并添加到adapterFactories 集合中
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

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

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

1）以上可见baseUrl必须传，null则抛出异常。
2）若未传入OkHttpClient则默认创建一个，此处callFactory即OkHttpClient对象，OkHttpClient实现了okhttp3.Call.Factory接口。
3）callbackExecutor，是用来将回调切换到主线程中去，此处利用platform对象，对平台进行判断，判断主要是利用Class.forName("")进行查找，如果是Android平台，会自定义一个Executor对象MainThreadExecutor，并且利用Looper.getMainLooper()实例化一个handler对象，在Executor内部通过handler.post(runnable)，代码如下：

  static class Android extends Platform {
    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

    static class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

      @Override public void execute(Runnable r) {
        handler.post(r);
      }
    }
  }

4)adapterFactories，这个对象主要用于对Call进行转化。创建callbackExecutor即MainThreadExecutor对象，MainThreadExecutor
5）converterFactories 转换器工厂，该对象用于转化数据，例如将返回的responseBody转化为对象等；当然不仅仅是针对返回的数据，还能用于一般备注解的参数的转化例如@Body标识的对象做一些操作等。

retrofit.create(Service.class)

  public  T create(final Class service) {
    Utils.validateServiceInterface(service);
    ......
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class[] { service },
        new InvocationHandler() {
            ......
          @Override public Object invoke(Object proxy, Method method, Object[] args)
              throws Throwable {
            ......
            ServiceMethod serviceMethod = (ServiceMethod) loadServiceMethod(method);
            OkHttpCall