Retrofit2+rxjava2源码解析（一）：Retrofit2的源码解析

背景

最近闲来无事，突然有了兴致想写一篇文章。之前一直在用retrofit2+rxjava2的网络框架，对里面的实现逻辑一直很好奇，翻了一遍网上的源码解析，基本上都是拆开单独的解析。于是花了差不多有一周的时间将里面的实现逻辑整理了一遍。
第一次写，排版问题请多见谅。

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retrofit2的实现原理

retrofit2严格意义上说其实并不算是一个网络框架，因为里面发起网络请求用的依然是OKHttp。那么retrofit2到底是个什么？请随我仔细阅读以下的源码应该就能知道了。

话不多说先上代码：
1、定义了接口，然后通过注解的方式封装了地址和请求参数

/**
 * Created by chendanfeng on 2018/3/19.
 * 定义的请求服务器的API的接口
 */
public interface APIFunction {
    /**
     * 登录请求
     * @param map 请求参数
     * @return
     */
    @POST(HttpConfig.REQUEST_LOGIN)
    Call> login(@Body Map map);

}

上图可以看出注解主要用了@POST和@Body。@POST代表的是请求方式是post，@Body代表的是请求体

2、初始化retrofit

        //共通参数拦截器
        HeaderParamInterceptor commonParamInterceptor = new HeaderParamInterceptor();

        // 初始化okhttp
        OkHttpClient client = new OkHttpClient.Builder()
                .addInterceptor(commonParamInterceptor)
                .build();

        // 初始化Retrofit
        mRetrofit = new Retrofit.Builder()
                .client(client)
                .baseUrl(HttpConfig.IP)
                .addConverterFactory(GsonConverterFactory.create())//添加gson转换器
                .build();


        // 初始化Service
        mApiFunction = mRetrofit.create(APIFunction.class);

2.1 先看下Retrofit的Builder方法:

    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());
    }

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

class Platform {
  private static final Platform PLATFORM = findPlatform();

  static Platform get() {
    return PLATFORM;
  }

  //因为是Android设备，所以这里返回的肯定是Android对象
  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();
  }
  ...代码省略...

  static class Android extends Platform {
        ...代码省略...
  }
}

请记住此时返回的Android对象，之后会有用处。

2.2 然后着重看下retrofit2的build方法：

    /**
     * Create the {@link Retrofit} instance using the configured values.
     * 

     * Note: If neither {@link #client} nor {@link #callFactory} is called a default {@link
     * OkHttpClient} will be created and used.
     */
    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 adapterFactories = new ArrayList<>(this.adapterFactories);
      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);
    }
  }

看源码发现：
如果没有callFactory的话就会new出一个OkHttp，换句话说如果OkHttp没有什么特殊要求的话，在初始化retrofit的时候client方法其实可以不需要调用。

另外这里的platform就是Android对象，下面这段便是Android类的方法，所以build方法里面的callbackExecutor便是MainThreadExecutor对象，adapterFactories所add进去的CallAdapter.Factory就是ExecutorCallAdapterFactory对象。

    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
      if (callbackExecutor == null) throw new AssertionError();
      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);
      }
    }

2.3 最后看一下Retrofit的create方法：

  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