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<List<String>>
}

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<List<String>> {
            override fun onResponse(call: Call<List<String>>, response: Response<List<String>>) {
                TODO("Not yet implemented")
            }
            override fun onFailure(call: Call<List<String>>, 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<Converter.Factory> converterFactories = new ArrayList<>();
    // 网络请求适配器工厂的集合，默认是ExecutorCallAdapterFactory
    private final List<CallAdapter.Factory> 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<String> 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<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
      // Make a defensive copy of the converters.
      List<Converter.Factory> 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> T create(final Class<T> 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<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.");
    }
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }
  abstract @Nullable T invoke(Object[] args);
}

执行请求过程

  public void enqueue(final Callback<T> 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<T> 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<T> 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。

到此这篇关于Android Retrofit原理深入探索的文章就介绍到这了,更多相关Android Retrofit内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！