Android Retrofit原理深入探索

目录

• 序章
• Retrofit构建过程
• 创建网络请求接口实例过程
• 执行请求过程
• 总结

序章

首先引入依赖

    implementation 'com.squareup.retrofit2:retrofit:2.9.0'

在原理之前，我们先来回忆一下Retrofit的基本使用

1、定义接口

interface MyService {
    @GET("gallery/{imageType}/response")
    fun getImages(@Path("imageType") imageType: String): Call>
}

2、构建Retrofit，创建网络请求接口类实例

        val retrofit = Retrofit.Builder()
            .baseUrl(BASE_URL)
            .build()
        val myService = retrofit.create(MyService::class.java)

3、生成Call，执行请求

        val resp = myService.getImages("banner")
        resp.enqueue(object : Callback> {
            override fun onResponse(call: Call>, response: Response>) {
                TODO("Not yet implemented")
            }
            override fun onFailure(call: Call>, t: Throwable) {
                TODO("Not yet implemented")
            }
        })

这样一个基本的网络请求就搞定了，使用很简洁，正是因为其内部使用了大量的设计模式和优秀的架构设计，才得以使其如此方便地进行网络请求，下面我们就一起来探索探索Retrofit的设计之美。

Retrofit构建过程

使用了建造者模式通过内部静态类Builder构建一个Retrofit实例，这里只列出了部分方法，其他类似

public static final class Builder {
    private final Platform platform;
    // 网络请求工厂，工厂方法模式
    private @Nullable okhttp3.Call.Factory callFactory;
    // 网络请求地址
    private @Nullable HttpUrl baseUrl;
    // 数据转换器工厂的集合
    private final List converterFactories = new ArrayList<>();
    // 网络请求适配器工厂的集合，默认是ExecutorCallAdapterFactory
    private final List callAdapterFactories = new ArrayList<>();
    // 回调方法执行器，用于切换线程
    private @Nullable Executor callbackExecutor;
    // 一个开关，为true则会缓存创建的ServiceMethod
    private boolean validateEagerly;
    //......
    public Builder baseUrl(String baseUrl) {
      Objects.requireNonNull(baseUrl, "baseUrl == null");
      return baseUrl(HttpUrl.get(baseUrl));
    }
    public Builder baseUrl(HttpUrl baseUrl) {
      Objects.requireNonNull(baseUrl, "baseUrl == null");
      List pathSegments = baseUrl.pathSegments();
      if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
        throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
      }
      this.baseUrl = baseUrl;
      return this;
    }
    // 将一个含有Gson对象实例的GsonConverterFactory放入数据转换器工厂
    public Builder addConverterFactory(Converter.Factory factory) {
      converterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }
    //......
}

通过build，我们上面Builder类中的参数对象都配置到了Retrofit对象中。

    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 callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
      // Make a defensive copy of the converters.
      List converterFactories =
          new ArrayList<>(
              1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
      // 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());
      converterFactories.addAll(this.converterFactories);
      converterFactories.addAll(platform.defaultConverterFactories());
      return new Retrofit(
          callFactory,
          baseUrl,
          unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories),
          callbackExecutor,
          validateEagerly);
    }

创建网络请求接口实例过程

使用动态代理的方式拿到所有注解配置后，创建网络请求接口实例。

  public  T create(final Class service) {
    validateServiceInterface(service);
    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 @Nullable 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);
                }
                args = args != null ? args : emptyArgs;
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args);
              }
            });
  }

跟踪 loadServiceMethod，parseAnnotations解析注解配置得到ServiceMethod，然后加入到serviceMethodCache缓存中，是一个ConcurrentHashMap。

  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;
  }

abstract class ServiceMethod {
  static  ServiceMethod 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.");
    }
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }
  abstract @Nullable T invoke(Object[] args);
}

执行请求过程

  public void enqueue(final Callback callback) {
    Objects.requireNonNull(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 {
          // 创建一个OkHttp的Request对象请求
          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 response;
            try {
              // 解析网络请求返回的数据
              response = parseResponse(rawResponse);
            } catch (Throwable e) {
              throwIfFatal(e);
              callFailure(e);
              return;
            }
            try {
              callback.onResponse(OkHttpCall.this, response);
            } catch (Throwable t) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }
          @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) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }
        });
  }

  Response parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();
    // Remove the body's source (the only stateful object) so we can pass the response along.
    rawResponse =
        rawResponse
            .newBuilder()
            .body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
            .build();
    int code = rawResponse.code();
    // 根据响应返回的状态码进行处理 
    if (code < 200 || code >= 300) {
      try {
        // Buffer the entire body to avoid future I/O.
        ResponseBody bufferedBody = Utils.buffer(rawBody);
        return Response.error(bufferedBody, rawResponse);
      } finally {
        rawBody.close();
      }
    }
    if (code == 204 || code == 205) {
      rawBody.close();
      return Response.success(null, rawResponse);
    }
    ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
    try {
      // 将响应体转为Java对象
      T body = responseConverter.convert(catchingBody);
      return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
      // If the underlying source threw an exception, propagate that rather than indicating it was
      // a runtime exception.
      catchingBody.throwIfCaught();
      throw e;
    }
  }

总结

首先，使用建造者模式通过Builder构建一个Retrofit实例，Builder类中的参数对象都配置到Retrofit对象中，然后使用动态代理的方式拿到所有注解配置后，创建网络请求接口实例，生成OkHttp请求，通过callAdapterFactory找到对应的执行器，比如RxJava2CallAdapterFactory，最后通过ConverterFactory将返回数据解析成JavaBean，使用者只需关心请求参数，内部实现由Retrofit封装完成，底层请求还是基于okhttp实现的。

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