主要參考文章：
Android Retrofit 2.0 的詳細(xì) 使用攻略（含實(shí)例講解）
Android：手把手帶你 深入讀懂 Retrofit 2.0 源碼

主要過(guò)程：
1、建造者模式- 初始化所需 變量
2、使用retrofit.create(AccessApi.class) 生成動(dòng)態(tài)代理對(duì)象，調(diào)用方法時(shí)，解析方法上的 注解+參數(shù)+參數(shù)的注解 生成ServiceMethod 并緩存
3、ServiceMethod 中能獲取到retrofit初始化的變量，ServiceMethod.invoke(...) 中使用 這些變量發(fā)起請(qǐng)求callAdapter.adapt(new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
4、以rxjava為例，ServiceMethod.invoke(...) 返回來(lái)Observable， 訂閱者->callAdapter->OkHttpCall->okhttp3.Call->網(wǎng)絡(luò)請(qǐng)求->okhttp3.Call->OkHttpCall->converter數(shù)據(jù)解析->callAdapter->訂閱者

1、初始化

主要是初始化以下 Retrofit 變量

HttpLoggingInterceptor loggerInterceptor = new HttpLoggingInterceptor();//打印 信息
loggerInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
OkHttpClient client = new OkHttpClient.Builder().addNetworkInterceptor(loggerInterceptor).build();
// 初始化Retrofit
mRetrofit = new Retrofit.Builder()
    .client(client)
    .baseUrl(baseUrl)
    .addConverterFactory(GsonConverterFactory.create(buildGson()))
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();

1.1 Retrofit.Builder() 變量

private final Platform platform; //平臺(tái)類型
private @Nullable okhttp3.Call.Factory callFactory;//OkHttpClient

private @Nullable HttpUrl baseUrl;
//自己添加的 GsonConverterFactory
private final List<Converter.Factory> converterFactories = new ArrayList<>();
//自己添加的 RxJava2CallAdapterFactory
private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();
//沒(méi)賦值 就是用 platform.defaultCallbackExecutor() = MainThreadExecutor
private @Nullable Executor callbackExecutor;
private boolean validateEagerly;

平臺(tái)類型 有 Android， java8；這里主要看android

1.2 Retrofit.Builder.build() 得到Retrofit 實(shí)例

Retrofit(okhttp3.Call.Factory callFactory, HttpUrl baseUrl,
List<Converter.Factory> converterFactories, List<CallAdapter.Factory> callAdapterFactories,
@Nullable Executor callbackExecutor, boolean validateEagerly) {

  this.callFactory = callFactory;//OkHttpClient
  this.baseUrl = baseUrl;
  this.converterFactories = converterFactories; // Copy+unmodifiable at call site.
  //BuiltInConverters + 自己添加的 + android平臺(tái) 
  //BuiltInConverters、GsonConverterFactory、 OptionalConverterFactory（api24以上有）
  this.callAdapterFactories = callAdapterFactories; // Copy+unmodifiable at call site.
  //自己添加的+android平臺(tái)  RxJava2CallAdapterFactory,CompletableFutureCallAdapterFactory（>api24）、ExecutorCallAdapterFactory
  this.callbackExecutor = callbackExecutor;//android 平臺(tái)來(lái)說(shuō)就是 MainThreadExecutor
  this.validateEagerly = validateEagerly;//作用：是否提前對(duì)業(yè)務(wù)接口中的注解進(jìn)行驗(yàn)證轉(zhuǎn)換的標(biāo)志位

}

2、使用

構(gòu)建用了動(dòng)態(tài)代理， 最終是通過(guò)okhttp
2.1 ServiceMethod 構(gòu)建

public class JavaBean {}
public interface AccessApi {
  @GET("openapi.do?keyfrom=Yanzhikai&key=2032414398&type=data&doctype=json&version=1.1&q=car")
  Call<JavaBean> getCall();
}
AccessApi NetService = retrofit.create(AccessApi.class);
Call<JavaBean> call = NetService.getCall();

retrofit.create(AccessApi.class) 生成 代理對(duì)象

public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);//提前解析所有方法，加入緩存
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
       new InvocationHandler() {
        private final Platform platform = Platform.get();
        private final Object[] emptyArgs = new Object[0];

        @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
        throws Throwable {//返回 接口方法的返回參數(shù)，即方法返回的結(jié)果
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
                return method.invoke(this, args);
            }
            //1.8之后接口可以有默認(rèn)的實(shí)現(xiàn)
            if (platform.isDefaultMethod(method)) {
                return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
        }
       });
}

loadServiceMethod(…) —> ServiceMethod.parseAnnotations(this, method); 根據(jù)方法的注解 生成 ServiceMethod實(shí)例

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
    //解析方法的注解，方法參數(shù)的注解，加上各種校驗(yàn)注解使用的正確性，構(gòu)造出RequestFactory，待新建請(qǐng)求時(shí)使用

    Type returnType = method.getGenericReturnType();
    if (Utils.hasUnresolvableType(returnType)) {
        throw methodError(method,"Method return type must not include a type variable or wildcard: %s", returnType);
    }
    if (returnType == void.class) {
        throw methodError(method, "Service methods cannot return void.");
    }
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}

HttpServiceMethod<ResponseT, ReturnT> extends ServiceMethod<ReturnT>
返回了 new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter)

參1、requestFactory = RequestFactory.parseAnnotations(retrofit, method);
參2、callFactory = retrofit.callFactory//OkHttpClient
參3、callAdapter 從 retrofit.callAdapterFactories 中 根據(jù)返參和注解，查找并生成 合適的CallAdapter；
參4、responseConverter 從 retrofit.converterFactories中 根據(jù)返參和注解，查找并生成 合適的Converter

2.2 ServiceMethod 方法的調(diào)用

@Override ReturnT invoke(Object[] args) {
return callAdapter.adapt(
    new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}

OkHttpCall中會(huì)新建 原始請(qǐng)求，okhttp3 的請(qǐng)求

private okhttp3.Call createRawCall() throws IOException {
  okhttp3.Call call = callFactory.newCall(requestFactory.create(args));
  if (call == null) {
    throw new NullPointerException("Call.Factory returned null.");
  }
  return call;
}

OkHttpCall.enqueue(final Callback<T> callback) --> 調(diào)用 okhttp3.Call.enqueue(new okhttp3.Callback() {...}
結(jié)果返回后通過(guò) responseConverter.convert(...) 將okhttp3 返回的數(shù)據(jù) 解析 為 想要的返回值（如Gson 轉(zhuǎn)型）最后是一層一層回調(diào)出去。