Android小知識-剖析Retrofit中ServiceMethod相關參數以及建立過程

時間 2019-11-24

標籤 android 知識剖析 retrofit servicemethod 相關參數以及建立過程欄目 Android 简体版

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在上一節《Android小知識-剖析Retrofit中的網絡請求接口》介紹了在Retrofit中經過動態代理獲取網絡請求接口的代理類，在執行網絡請求時會執行InvocationHandler的invoke方法執行網絡請求的相關操做。web

public <T> T create(final Class<T> 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, @Nullable Object[] args) throws Throwable {
                        //標記1
                        ServiceMethod<Object, Object> serviceMethod =
                                (ServiceMethod<Object, Object>) loadServiceMethod(method);
                        
                        OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
                        return serviceMethod.adapt(okHttpCall);
                    }
                });
    }
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在標記1處調用了loadServiceMethod方法，這個方法在上一小節已經介紹過，從緩存集合serviceMethodCache獲取對應方法的ServiceMethod，緩存集合中存在就直接返回，不然建立ServiceMethod對象，這個ServiceMethod對象對應的是網絡請求接口中的一個方法相關參數的封裝。json

進入ServiceMethod類：數組

final class ServiceMethod<R, T> {
  ...

  private final okhttp3.Call.Factory callFactory;
  private final CallAdapter<R, T> callAdapter;

  private final HttpUrl baseUrl;
  private final Converter<ResponseBody, R> responseConverter;
  private final String httpMethod;
  private final String relativeUrl;
  private final Headers headers;
  private final MediaType contentType;
  ...
  private final ParameterHandler<?>[] parameterHandlers;

  ...
}
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這個ServiceMethod是個泛型類，內部定義了不少成員變量，callFactory是咱們的網絡請求工廠，用於生產網絡請求的Call對象，這個Call在Retrofit中是對OkHttp Call的封裝；callAdapter是網絡請求適配器，主要將網絡請求適用於不一樣的平臺，好比RxJava；baseUrl是網絡基地址；responseConverter是數據轉換器，將服務器返回的數據，好比json格式或者是xml格式等等，轉換成Java對象；httpMethod是網絡請求的方法，好比GET、POST等等；relativeUrl是網絡請求的相對地址，好比網絡請求接口中註解GET後的http請求地址；headers表示Http的請求頭；contentType表示網絡請求的body類型；parameterHandlers是方法參數的處理器。緩存

ServiceMethod的實例是經過Builder來建立，進入ServiceMethod的Builder類：服務器

Builder(Retrofit retrofit, Method method) {
      this.retrofit = retrofit;
      this.method = method;
      this.methodAnnotations = method.getAnnotations();
      this.parameterTypes = method.getGenericParameterTypes();
      this.parameterAnnotationsArray = method.getParameterAnnotations();
    }
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Builder構造函數中對成員變量進行賦值，傳入retrofit和method分別賦值給Builder的成員變量retrofit和method，method表示網絡請求方式，好比GET、POST等等，接着獲取方法當中的註解賦值給methodAnnotations，parameterTypes獲取的是網絡請求接口中方法參數的類型，parameterAnnotationsArray獲取的是網絡請求接口方法中註解的內容。微信

Builder的相關初始化完畢後，經過build方法建立ServiceMethod對象：網絡

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      ...
    }
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在build方法中經過createCallAdapter方法建立CallAdapter對象，createCallAdapter方法根據網絡請求接口方法返回值和它的註解類型，從Retrofit中獲取對應網絡請求適配器。ide

createCallAdapter方法：函數

private CallAdapter<T, R> createCallAdapter() {
      Type returnType = method.getGenericReturnType();
      if (Utils.hasUnresolvableType(returnType)) {
        throw methodError(
            "Method return type must not include a type variable or wildcard: %s", returnType);
      }
      if (returnType == void.class) {
        throw methodError("Service methods cannot return void.");
      }
      //標記1
      Annotation[] annotations = method.getAnnotations();
      try {
        //noinspection unchecked
        return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
      } catch (RuntimeException e) { // Wide exception range because factories are user code.
        throw methodError(e, "Unable to create call adapter for %s", returnType);
      }
    }
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經過method.getGenericReturnType()獲取網絡請求接口方法返回的類型，以後在標記2處，經過method.getAnnotations()獲取網絡請求接口方法中的註解，最後調用retrofit的callAdater方法返回CallAdapter對象，傳入的就是前面獲取的網絡請求接口方法的返回類型和方法內的註解類型。

進入Retrofit的callAdapter方法：

public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }
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繼續進入nextCallAdapter方法：

public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
      Annotation[] annotations) {
    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;
      }
    }

    ...
    throw new IllegalArgumentException(builder.toString());
  }
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方法內前兩行作非空判斷，接着經過for循環遍歷callAdapterFactories網絡請求適配器工廠集合，看有沒有合適的網絡請求適配器工廠，若是沒有拋出異常。

回到ServiceMethod的Builder類的build方法：

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      ...
    }
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根據前面獲取到的網絡請求適配器以後，經過responseType方法獲取網絡適配器返回的數據類型。

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      ...
      responseConverter = createResponseConverter();
      ...
    }
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接着經過createResponseConverter方法獲取數據解析器。

進入createResponseConverter方法：

private Converter<ResponseBody, T> createResponseConverter() {
      Annotation[] annotations = method.getAnnotations();
      try {
        return retrofit.responseBodyConverter(responseType, annotations);
      } catch (RuntimeException e) { // Wide exception range because factories are user code.
        throw methodError(e, "Unable to create converter for %s", responseType);
      }
    }
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經過method的getAnnotations方法獲取網絡請求接口中方法的註解，接着經過retrofit的responseBodyConverter方法獲取Converter對象。

responseBodyConverter方法：

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

public <T> Converter<ResponseBody, T> nextResponseBodyConverter( @Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
    ...
    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) {
        return (Converter<ResponseBody, T>) converter;
      }
    }

    ...
    throw new IllegalArgumentException(builder.toString());
  }
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nextResponseBodyConverter方法內部遍歷數據轉換器工廠集合，從中獲取合適的數據轉換器，這裏默認獲取的是Gson的數據轉換器。

回到ServiceMethod的Builder類的build方法：

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      ...
      responseConverter = createResponseConverter();
      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }
      ...
    }
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拿到數據轉換器後，經過遍歷網絡請求接口方法的註解，parseMethodAnnotation方法主要是對網絡請求接口方法上面的註解類型進行判斷，同時根據註解類型對Builder的相關成員變量進行賦值。

回到ServiceMethod的Builder類的build方法：

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      ...
      responseConverter = createResponseConverter();
      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }
      ...
      //標記1
      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0; p < parameterCount; p++) {
        ...
        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
        ...

        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }
      ...
      return new ServiceMethod<>(this);
    }
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