Retrofit2.5怎么做到网络请求的处理的？

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 <T> ServiceMethod<T> 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 <T> ServiceMethod<T> 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 <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);//创建请求适配器
    Type responseType = callAdapter.responseType();
    //省略无关代码
    Converter<ResponseBody, ResponseT> 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 <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> 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<ResponseT, ReturnT>) 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<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));   
}
static class Android extends Platform {
    @Override List<? extends CallAdapter.Factory> 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<Object, Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public Call<Object> adapt(Call<Object> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }

我们定义的返回类型一般是泛型的形式，如Call或者Observable，所以Utils.getCallResponseType就是取出泛型里面的类型。最后将得到的类型传入新建的CallAdapter

    return new CallAdapter<Object, Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public Call<Object> adapt(Call<Object> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };

自此就完成了CallAdapter的创建，这里记住它的adapt方法，里面是返回的ExecutorCallbackCall，这个后面会调用，再来继续看。

HttpServiceMethod的parseAnnotations第一行代码到此就完了，继续往下执行：

创建响应数据转换器（如Gson）和请求客户端

HttpServiceMethod.parseAnnotations

  static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
    Type responseType = callAdapter.responseType();//获取具体的响应类型
    //省略无关代码
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
  }

注意这里传给responseConverter的responseType是我们刚才获取Call泛型里面的具体类型，也就是我们想要拿到的实体类类型。

这个过程跟刚才创建CallAdapter的过程很像，就连方法名都很像，所以来看看createResponseConverter方法：

HttpServiceMethod.createResponseConverter

  private static <ResponseT> Converter<ResponseBody, ResponseT> createResponseConverter(
      Retrofit retrofit, Method method, Type responseType) {
    Annotation[] annotations = method.getAnnotations();
    try {
      return retrofit.responseBodyConverter(responseType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create converter for %s", responseType);
    }
  }

通过method的getAnnotations拿到注解列表，然后跟返回类型传入responseBodyConverter方法：

retrofit.responseBodyConverter

  public <T> Converter<ResponseBody, T> responseBodyConverter(Type type, Annotation[] annotations) {
    return nextResponseBodyConverter(null, type, annotations);
  }

  public <T> Converter<ResponseBody, T> nextResponseBodyConverter(
      @Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
    checkNotNull(type, "type == null");
    checkNotNull(annotations, "annotations == null");

    int start = converterFactories.indexOf(skipPast) + 1;
    for (int i = start, count = converterFactories.size(); i < count; i++) {
      Converter<ResponseBody, ?> converter =
          converterFactories.get(i).responseBodyConverter(type, annotations, this);
      if (converter != null) {
        //noinspection unchecked
        return (Converter<ResponseBody, T>) converter;
      }
    }

这个过程简直跟刚才创建CallAdapter的过程一模一样，也是循环从converterFactories取出Converter，根据之前Retrofit的创建时我们传入的是GsonConverterFactory，所以是调用的它的responseBodyConverter方法：

GsonConverterFactory.responseBodyConverter

  public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }

这里的代码就很面熟了，通过TypeToken获取类型，之后返回一个Gson解析器，它的convert就是调用Gson进行泛型的解析。

GsonResponseBodyConverter

final class GsonResponseBodyConverter<T> implements Converter<ResponseBody, T> {
  private final Gson gson;
  private final TypeAdapter<T> adapter;

  GsonResponseBodyConverter(Gson gson, TypeAdapter<T> adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }

  @Override public T convert(ResponseBody value) throws IOException {
    JsonReader jsonReader = gson.newJsonReader(value.charStream());
    try {
      T result = adapter.read(jsonReader);
      if (jsonReader.peek() != JsonToken.END_DOCUMENT) {
        throw new JsonIOException("JSON document was not fully consumed.");
      }
      return result;
    } finally {
      value.close();
    }
  }
}

到这里就完成了响应字符串的解析器的创建，接下来会执行

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);

创建okhttp请求的客户端，这里命名为callFactory，在Retrofit创建的时候我们传入的okhttpClient就是赋值给了callFactory，最后将创建好的对象传入HttpServiceMethod返回。

到此为止就执行完了HttpServiceMethod的parseAnnotations方法，在这个方法里面，创建了几个重要的东西

• 对请求和响应做具体处理的CallAdapter类，具体为ExecutorCallAdapterFactory，这里要注意它的adapt方法返回ExecutorCallbackCall类
• 响应数据解析工厂Converter，具体为GsonResponseBodyConverter
• okhttp请求客户端

发送请求

终于到了这一步了，在做好了所有的准备工作之后，最终Retrofit怎么来发送请求呢，首先一步步回到最开始的地方：
在HttpServiceMethod.parseAnnotations方法是在ServiceMethod的parseAnnotations调用的

ServiceMethod.parseAnnotations

abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);

    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.");
    }

    //在这里调用并返回
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

  abstract T invoke(Object[] args);
}

然后ServiceMethod.parseAnnotations方法是在Retrofit的loadServiceMethod调用的

然后loadServiceMethod是在Retrofit的create方法里面创建动态代理的invoke调用的

Retrofit.create

  public <T> T create(final Class<T> service) {
    //省略无关代码
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          //省略无关代码

          @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
              throws Throwable {
            //省略无关代码
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
          }
        });
  }

可以看到，在loadServiceMethod之后调用了invoke方法，也就是ServiceMethod的invoke方法，点进去之后发现它是个ServiceMethod是个抽象类，它的invoke需要实现类来重写：

abstract class ServiceMethod<T> {
    abstract T invoke(Object[] args);
}

ServiceMethod类只有一个实现类，也就是我们刚才看到过的HttpServiceMethod，所以实际上调用的是它的invoke方法：

HttpServiceMethod.invoke

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

在这里只有一行代码，实际上是调用callAdapter.adapt，那这个callAdapter是什么呢？刚才我们已经知道它是ExecutorCallAdapterFactory创建的，然后在创建的callAdapter的adapt方法返回ExecutorCallbackCall类，所以来看看这个类：

ExecutorCallbackCall是ExecutorCallAdapterFactory的静态内部类

ExecutorCallbackCall

  static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override public void enqueue(final Callback<T> callback) {
      checkNotNull(callback, "callback == null");

      delegate.enqueue(new Callback<T>() {
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
              } else {
                callback.onResponse(ExecutorCallbackCall.this, response);
              }
            }
          });
        }

        @Override public void onFailure(Call<T> call, final Throwable t) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              callback.onFailure(ExecutorCallbackCall.this, t);
            }
          });
        }
      });
    }

    @Override public boolean isExecuted() {
      return delegate.isExecuted();
    }

    @Override public Response<T> execute() throws IOException {
      return delegate.execute();
    }

    @Override public void cancel() {
      delegate.cancel();
    }

    @Override public boolean isCanceled() {
      return delegate.isCanceled();
    }

    @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
    @Override public Call<T> clone() {
      return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
    }

    @Override public Request request() {
      return delegate.request();
    }
  }

在这个类里面，我们就看到了很多很熟悉的方法，enqueue方法发送请求，实际的请求和处理就是在这里面，但是还有疑惑就是具体的处理我们还是没看到，callbackExecutor和delegate这两个熟悉才是关键所在：

callbackExecutor

这个是在ExecutorCallAdapterFactory的构造方法中传入的，它的构造方法又是在Platform的defaultCallAdapterFactories方法中调用的，看到Platform，自然可以想到它的Android平台类，所以这个callbackExecutor实际上是MainThreadExecutor

    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }
    
    static class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

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

其实代码很简单，使用了我们最常用的Handler，让代码在主线程中执行，也就是我们通常做的在子线程请求网络，主线程中更新UI。

delegate

delegate实际是在HttpServiceMethod的invoke方法里传入：

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

实际上是OkHttpCall这个类，这个类里面就是发送请求和处理响应的具体处理，代码很多，我们只看看构造方法和enqueue

  OkHttpCall(RequestFactory requestFactory, Object[] args,
      okhttp3.Call.Factory callFactory, Converter<ResponseBody, T> responseConverter) {
    this.requestFactory = requestFactory;//请求工厂，里面包含了请求的所有信息
    this.args = args;//参数
    this.callFactory = callFactory;//请求客户端，okhttp
    this.responseConverter = responseConverter;//响应的json解析器（Gson）
  }

OkHttpCall.enqueue

  @Override public void enqueue(final Callback<T> callback) {
    checkNotNull(callback, "callback == null");

    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          call = rawCall = createRawCall();
        } catch (Throwable t) {
          throwIfFatal(t);
          failure = creationFailure = t;
        }
      }
    }

    if (failure != null) {
      callback.onFailure(this, failure);
      return;
    }

    if (canceled) {
      call.cancel();
    }

    call.enqueue(new okhttp3.Callback() {
      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
        Response<T> response;
        try {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          throwIfFatal(e);
          callFailure(e);
          return;
        }

        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      @Override public void onFailure(okhttp3.Call call, IOException e) {
        callFailure(e);
      }

      private void callFailure(Throwable e) {
        try {
          callback.onFailure(OkHttpCall.this, e);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }
    });
  }

代码其实很简单，就是使用okhttp请求，然后在响应方法onResponse做相应的解析和处理，最后完成网络请求。

总结

Retrofit对响应的解析大概可分为

1. 创建处理适配器CallAdapter，具体实现类为ExecutorCallbackCall，在这里面进行发送请求和处理响应等相关的操作，最终是调用okhttp进行网络请求
2. 创建响应数据转换器Converter，具体实现类为GsonResponseBodyConverter，用于对返回json数据的解析，这个是我们自己传入的Gson解析工厂，Retrofit也支持其他的第三方解析库
3. 发送请求，具体的操作类是OkHttpCall来发送请求和处理响应，MainThreadExecutor来做回调的处理，在主线程中处理已经转化好的实体类。

聊技术 聊电影 聊人生 什么都聊的公众号