Android网络编程11之源码解析Retrofit

时间 2019-12-07

标签 android 网络编程源码解析 retrofit 栏目 Android 繁體版

原文原文链接

前言
最近文章的产出确实不多，由于我正在写一本Android进阶书籍，两头很难兼顾，可是每月也得至少发一篇博客。上一篇咱们介绍了Retrofit的使用方法，这一篇咱们照例来学习Retrofit的源码。php

1.Retrofit的建立过程

当咱们使用Retrofit请求网络时，首先要写请求接口：java

public interface IpService {
    @GET("getIpInfo.php?ip=59.108.54.37")
      Call<IpModel> getIpMsg();

接着咱们经过调用以下代码来建立Retrofit：android

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(url)
                .addConverterFactory(GsonConverterFactory.create())
                .build();

Retrofit 是经过建造者模式构建出来的，接下来查看Builder方法作了什么：json

public Builder() {
      this(Platform.get());
    }

很简短，查看Platform的get方法，以下所示。缓存

private static final Platform PLATFORM = findPlatform();
  static Platform get() {
    return PLATFORM;
  }
  private static Platform findPlatform() {
    try {
      Class.forName("android.os.Build");
      if (Build.VERSION.SDK_INT != 0) {
        return new Android();
      }
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("java.util.Optional");
      return new Java8();
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("org.robovm.apple.foundation.NSObject");
      return new IOS();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
  }

Platform的get方法最终调用的是findPlatform方法，根据不一样的运行平台来提供不一样的线程池。接下来查看build方法，代码以下所示。网络

public Retrofit build() {
      if (baseUrl == null) {//1
        throw new IllegalStateException("Base URL required.");
      }
      okhttp3.Call.Factory callFactory = this.callFactory;//2
      if (callFactory == null) {
        callFactory = new OkHttpClient();//3
      }
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();//4
      }
      List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);//5
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
      List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);//6
      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }

从注释1处能够看出baseUrl 是必须指定的。注释2处callFactory默认为this.callFactory，this.callFactory就是咱们在构建Retrofit时调用callFactory方法所传进来的，以下所示。app

public Builder callFactory(okhttp3.Call.Factory factory) {
      this.callFactory = checkNotNull(factory, "factory == null");
      return this;
    }

所以，若是须要对OkHttpClient进行设置，则能够构建OkHttpClient对象，而后调用callFactory方法将设置好的OkHttpClient传进去。注释3处，若是没有设置callFactory则直接建立OkHttpClient。注释4的callbackExecutor用来将回调传递到UI线程。注释5的adapterFactories主要用于存储对Call进行转化的对象，后面在Call的建立过程会再次提到它。注释6处的converterFactories主要用于存储转化数据对象，后面也会说起到。此前在例子中调用的addConverterFactory(GsonConverterFactory.create())，就是设置返回的数据支持转换为Gson对象。最终会返回配置好的Retrofit类。ide

2.Call的建立过程

紧接着咱们建立Retrofit实例并调用以下代码来生成接口的动态代理对象：函数

IpService ipService = retrofit.create(IpService.class);

接下来看Retrofit的create方法作了什么，代码以下所示。学习

public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override public Object invoke(Object proxy, Method method, 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);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            ServiceMethod serviceMethod = loadServiceMethod(method);//1
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });
  }

能够看到create方法返回了一个Proxy.newProxyInstance动态代理对象，当咱们调用IpService的getIpMsg方法最终会调用InvocationHandler的invoke 方法，它有3个参数，第一个是代理对象，第二个是调用的方法，第三个是方法的参数。注释1处的loadServiceMethod(method)中的method就是咱们定义的getIpMsg方法。接下来查看loadServiceMethod方法里作了什么：

private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();
 ServiceMethod loadServiceMethod(Method method) {
    ServiceMethod result;
    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = new ServiceMethod.Builder(this, method).build();
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }

首先会从serviceMethodCache查询传入的方法是否有缓存，若是有就用缓存的ServiceMethod，若是没有就建立一个，并加入serviceMethodCache缓存起来。接下来看ServiceMethod是如何构建的，代码以下所示。

public ServiceMethod build() {
      callAdapter = createCallAdapter();//1
      responseType = callAdapter.responseType();//2
      if (responseType == Response.class || responseType == okhttp3.Response.class) {
        throw methodError("'"
            + Utils.getRawType(responseType).getName()
            + "' is not a valid response body type. Did you mean ResponseBody?");
      }
      responseConverter = createResponseConverter();//3
      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);//4
      }
     ...
      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0; p < parameterCount; p++) {
        Type parameterType = parameterTypes[p];
        if (Utils.hasUnresolvableType(parameterType)) {
          throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
              parameterType);
        }
        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];//5
        if (parameterAnnotations == null) {
          throw parameterError(p, "No Retrofit annotation found.");
        }
        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }
      ...
      return new ServiceMethod<>(this);
    }

注释1处调用了createCallAdapter方法，它最终会获得咱们在构建Retrofit调用build方法时adapterFactories添加的对象的get方法，Retrofit的build方法部分代码：

List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

adapterFactories列表默认会添加defaultCallAdapterFactory，defaultCallAdapterFactory指的是ExecutorCallAdapterFactory，ExecutorCallAdapterFactory的get方法以下所示。

public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
      return null;
    }
    final Type responseType = Utils.getCallResponseType(returnType);
    return new CallAdapter<Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }
      @Override public <R> Call<R> adapt(Call<R> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }

get方法会获得CallAdapter对象，它的responseType方法会返回数据的真实类型，好比 Call<IpModel>，它就会返回IpModel。adapt方法会建立ExecutorCallbackCall，它会将call的回调转发至UI线程。
接着回到ServiceMethod的 build方法，注释2处调用CallAdapter的responseType获得的是返回数据的真实类型。
注释3处调用createResponseConverter方法来遍历converterFactories列表中存储的Converter.Factory，并返回一个合适的Converter用来转换对象。此前咱们在构建Retrofit 调用了addConverterFactory(GsonConverterFactory.create())将GsonConverterFactory（Converter.Factory的子类）添加到converterFactories列表中，表示返回的数据支持转换为Json对象。
注释4处遍历parseMethodAnnotation方法来对请求方式（好比GET、POST）和请求地址进行解析。注释5处对方法中的参数注解进行解析（好比@Query、@Part）。最后建立ServiceMethod类并返回。
接下来回过头来查看Retrofit的create方法，在调用了loadServiceMethod方法后会建立OkHttpCall，OkHttpCall的构造函数只是进行了赋值操做。紧接着调用serviceMethod.callAdapter.adapt(okHttpCall)，callAdapter的adapt方法前面讲过，它会建立ExecutorCallbackCall，ExecutorCallbackCall的部分代码以下所示。

ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }
    @Override public void enqueue(final Callback<T> callback) {
      if (callback == null) throw new NullPointerException("callback == null");
      delegate.enqueue(new Callback<T>() {//1
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                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);
            }
          });
        }
      });
    }

能够看出ExecutorCallbackCall是对Call的封装，它主要添加了经过callbackExecutor将请求回调到UI线程。
当咱们获得Call对象后会调用它的enqueue方法，其实调用的是ExecutorCallbackCall的enqueue方法，而从注释1处能够看出ExecutorCallbackCall的enqueue方法最终调用的是delegate的enqueue方法。delegate从Retrofit的create方法的代码中咱们知道它其实就是OkHttpCall。

3.Call的enqueue方法

接下来咱们就来查看OkHttpCall的enqueue方法，代码以下所示。

public void enqueue(final Callback<T> callback) {
    if (callback == null) throw new NullPointerException("callback == null");
    okhttp3.Call call;
   ...
    call.enqueue(new okhttp3.Callback() {//1
      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
          throws IOException {
        Response<T> response;
        try {
          response = parseResponse(rawResponse);//2
        } catch (Throwable e) {
          callFailure(e);
          return;
        }
        callSuccess(response);
      }
      ...
  }

注释1处调用了okhttp3.Call的enqueue方法。注释2处调用parseResponse方法：

Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();
   ...
    int code = rawResponse.code();
    if (code < 200 || code >= 300) {
      try {
        ResponseBody bufferedBody = Utils.buffer(rawBody);
        return Response.error(bufferedBody, rawResponse);
      } finally {
        rawBody.close();
      }
    }
    if (code == 204 || code == 205) {
      return Response.success(null, rawResponse);
    }
    ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
    try {
      T body = serviceMethod.toResponse(catchingBody);//2
      return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
      catchingBody.throwIfCaught();
      throw e;
    }
  }

根据返回的不一样的状态码code值来作不一样的操做，若是顺利则会调用注释2处的代码，接下来看toResponse方法里作了什么：

T toResponse(ResponseBody body) throws IOException {
    return responseConverter.convert(body);
  }

这个responseConverter就是此前讲过在ServiceMethod的build方法调用createResponseConverter方法返回的Converter，在此前的例子中咱们传入的是GsonConverterFactory，所以能够查看GsonConverterFactory的代码，以下所示。

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

在GsonConverterFactory 中有一个方法responseBodyConverter，它最终会建立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 {
      return adapter.read(jsonReader);
    } finally {
      value.close();
    }
  }
}

在GsonResponseBodyConverter的convert方法里会将回调的数据转换为Json格式。所以咱们也知道了此前调用responseConverter.convert是为了转换为特定的数据格式。Call的enqueue方法主要作的就是用OKHttp来请求网络并将返回的Response进行数据转换并回调给UI线程。至此，Retrofit的源码就讲到这里。