Android網絡編程11之源碼解析Retrofit

時間 2019-12-07

標籤 android 網絡編程源碼解析 retrofit 欄目 Android 简体版

原文原文鏈接

前言
最近文章的產出確實不多，由於我正在寫一本Android進階書籍，兩頭很難兼顧，可是每月也得至少發一篇博客。上一篇咱們介紹了Retrofit的使用方法，這一篇咱們照例來學習Retrofit的源碼。php

1.Retrofit的建立過程

當咱們使用Retrofit請求網絡時，首先要寫請求接口：java

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

接着咱們經過調用以下代碼來建立Retrofit：android

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

Retrofit 是經過建造者模式構建出來的，接下來查看Builder方法作了什麼：json

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

很簡短，查看Platform的get方法，以下所示。緩存

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方法最終調用的是findPlatform方法，根據不一樣的運行平臺來提供不一樣的線程池。接下來查看build方法，代碼以下所示。網絡

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);
    }

從註釋1處能夠看出baseUrl 是必須指定的。註釋2處callFactory默認爲this.callFactory，this.callFactory就是咱們在構建Retrofit時調用callFactory方法所傳進來的，以下所示。app

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

所以，若是須要對OkHttpClient進行設置，則能夠構建OkHttpClient對象，而後調用callFactory方法將設置好的OkHttpClient傳進去。註釋3處，若是沒有設置callFactory則直接建立OkHttpClient。註釋4的callbackExecutor用來將回調傳遞到UI線程。註釋5的adapterFactories主要用於存儲對Call進行轉化的對象，後面在Call的建立過程會再次提到它。註釋6處的converterFactories主要用於存儲轉化數據對象，後面也會說起到。此前在例子中調用的addConverterFactory(GsonConverterFactory.create())，就是設置返回的數據支持轉換爲Gson對象。最終會返回配置好的Retrofit類。ide

2.Call的建立過程

緊接着咱們建立Retrofit實例並調用以下代碼來生成接口的動態代理對象：函數

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

接下來看Retrofit的create方法作了什麼，代碼以下所示。學習

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);
          }
        });
  }

能夠看到create方法返回了一個Proxy.newProxyInstance動態代理對象，當咱們調用IpService的getIpMsg方法最終會調用InvocationHandler的invoke 方法，它有3個參數，第一個是代理對象，第二個是調用的方法，第三個是方法的參數。註釋1處的loadServiceMethod(method)中的method就是咱們定義的getIpMsg方法。接下來查看loadServiceMethod方法裏作了什麼：

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;
  }

首先會從serviceMethodCache查詢傳入的方法是否有緩存，若是有就用緩存的ServiceMethod，若是沒有就建立一個，並加入serviceMethodCache緩存起來。接下來看ServiceMethod是如何構建的，代碼以下所示。

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);
    }

註釋1處調用了createCallAdapter方法，它最終會獲得咱們在構建Retrofit調用build方法時adapterFactories添加的對象的get方法，Retrofit的build方法部分代碼：

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

adapterFactories列表默認會添加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方法會獲得CallAdapter對象，它的responseType方法會返回數據的真實類型，好比 Call<IpModel>，它就會返回IpModel。adapt方法會建立ExecutorCallbackCall，它會將call的回調轉發至UI線程。
接着回到ServiceMethod的 build方法，註釋2處調用CallAdapter的responseType獲得的是返回數據的真實類型。
註釋3處調用createResponseConverter方法來遍歷converterFactories列表中存儲的Converter.Factory，並返回一個合適的Converter用來轉換對象。此前咱們在構建Retrofit 調用了addConverterFactory(GsonConverterFactory.create())將GsonConverterFactory（Converter.Factory的子類）添加到converterFactories列表中，表示返回的數據支持轉換爲Json對象。
註釋4處遍歷parseMethodAnnotation方法來對請求方式（好比GET、POST）和請求地址進行解析。註釋5處對方法中的參數註解進行解析（好比@Query、@Part）。最後建立ServiceMethod類並返回。
接下來回過頭來查看Retrofit的create方法，在調用了loadServiceMethod方法後會建立OkHttpCall，OkHttpCall的構造函數只是進行了賦值操做。緊接着調用serviceMethod.callAdapter.adapt(okHttpCall)，callAdapter的adapt方法前面講過，它會建立ExecutorCallbackCall，ExecutorCallbackCall的部分代碼以下所示。

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);
            }
          });
        }
      });
    }

能夠看出ExecutorCallbackCall是對Call的封裝，它主要添加了經過callbackExecutor將請求回調到UI線程。
當咱們獲得Call對象後會調用它的enqueue方法，其實調用的是ExecutorCallbackCall的enqueue方法，而從註釋1處能夠看出ExecutorCallbackCall的enqueue方法最終調用的是delegate的enqueue方法。delegate從Retrofit的create方法的代碼中咱們知道它其實就是OkHttpCall。

3.Call的enqueue方法

接下來咱們就來查看OkHttpCall的enqueue方法，代碼以下所示。

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);
      }
      ...
  }

註釋1處調用了okhttp3.Call的enqueue方法。註釋2處調用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;
    }
  }

根據返回的不一樣的狀態碼code值來作不一樣的操做，若是順利則會調用註釋2處的代碼，接下來看toResponse方法裏作了什麼：

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

這個responseConverter就是此前講過在ServiceMethod的build方法調用createResponseConverter方法返回的Converter，在此前的例子中咱們傳入的是GsonConverterFactory，所以能夠查看GsonConverterFactory的代碼，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 中有一個方法responseBodyConverter，它最終會建立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方法裏會將回調的數據轉換爲Json格式。所以咱們也知道了此前調用responseConverter.convert是爲了轉換爲特定的數據格式。Call的enqueue方法主要作的就是用OKHttp來請求網絡並將返回的Response進行數據轉換並回調給UI線程。至此，Retrofit的源碼就講到這裏。