Retrofit2.5如何发送请求和处理响应？

Retrofit对请求方法的解析

Retrofit对我们在接口类中定义的方法是如何转换成了请求，又如何处理响应。这一切都在它的loadServiceMethod方法中

代码基于Retrofit2.5.0

  ServiceMethod loadServiceMethod(Method method) {
    ServiceMethod result = serviceMethodCache.get(method);
    if (result != null) return result;

    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = ServiceMethod.parseAnnotations(this, method);
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }

主要是调用ServiceMethod.parseAnnotations

  static  ServiceMethod parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);//这里就是对请求的解析，也就是我们定义的接口方法

    Type returnType = method.getGenericReturnType();//获取返回的类型
    //省略无关代码

    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

RequestFactory.parseAnnotations这个方法就是对我们定义的接口方法的解析，在里面解析注解和使用反射得到了关于请求的所有信息。

响应的处理

在完成了对请求的处理后构建了RequestFactory类，这个类里面包含了请求的所有信息：

final class RequestFactory {
  static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
    return new Builder(retrofit, method).build();
  }

  private final Method method;
  private final HttpUrl baseUrl;
  final String httpMethod;
  private final @Nullable String relativeUrl;
  private final @Nullable Headers headers;
  private final @Nullable MediaType contentType;
  private final boolean hasBody;
  private final boolean isFormEncoded;
  private final boolean isMultipart;
  private final ParameterHandler[] parameterHandlers;

接下来继续来看ServiceMethod的parseAnnotations剩下的语句

ServiceMethod.parseAnnotations

  static  ServiceMethod parseAnnotations(Retrofit retrofit, Method method) {
    //省略无关代码

    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);//对响应的处理
  }

HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)

  static  HttpServiceMethod parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter callAdapter = createCallAdapter(retrofit, method);//创建请求适配器
    Type responseType = callAdapter.responseType();
    //省略无关代码
    Converter responseConverter =
        createResponseConverter(retrofit, method, responseType);//创建响应数据转换器（Gson）

    okhttp3.Call.Factory callFactory = retrofit.callFactory;//Okhttp请求客户端
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
  }

创建请求适配器CallAdapter

首先进入第一行
HttpServiceMethod.createCallAdapter

  private static  CallAdapter createCallAdapter(
      Retrofit retrofit, Method method) {
    Type returnType = method.getGenericReturnType();//获取返回的类型
    Annotation[] annotations = method.getAnnotations();//获取注解
    try {
      //noinspection unchecked
      return (CallAdapter) retrofit.callAdapter(returnType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create call adapter for %s", returnType);
    }
  }

在获取了方法的返回类型和注解之后，调用

retrofit.callAdapter

  public CallAdapter callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }
  
    public CallAdapter nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
      Annotation[] annotations) {
      //空判断
    checkNotNull(returnType, "returnType == null");
    checkNotNull(annotations, "annotations == null");

    int start = callAdapterFactories.indexOf(skipPast) + 1;
    for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
      CallAdapter adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
      if (adapter != null) {
        return adapter;
      }
    }
  }

这里面是一个循环获取请求适配器工厂数组callAdapterFactories，在retrofit创建的时候，我们可以知道往这个数组里面加的是Android平台默认的ExecutorCallAdapterFactory

public Retrofit build() {
      List callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));   
}
static class Android extends Platform {
    @Override List defaultCallAdapterFactories(
        @Nullable Executor callbackExecutor) {
      if (callbackExecutor == null) throw new AssertionError();
      //这里可以看到返回的类
      ExecutorCallAdapterFactory executorFactory = new ExecutorCallAdapterFactory(callbackExecutor);
      return Build.VERSION.SDK_INT >= 24
        ? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
        : singletonList(executorFactory);
    }    
}

在刚才的循环里，调用了它的get方法，实际就是调用

ExecutorCallAdapterFactory.get

  @Override public @Nullable CallAdapter get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
      return null;
    }
    final Type responseType = Utils.getCallResponseType(returnType);//根据返回类型获取泛型里面的想要获取的响应类型
    return new CallAdapter>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public Call