Retrofit2.5怎麼作到網絡請求的處理的？

時間  2019-11-24
標籤 retrofit2.5 retrofit 怎麼 作到 網絡 請求 處理 欄目 系統網絡 简体版
原文   原文鏈接

Retrofit對請求方法的解析

Retrofit對咱們在接口類中定義的方法是如何轉換成了請求，又如何處理響應。這一切都在它的loadServiceMethod方法中java

代碼基於Retrofit2.5.0json

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;
  }
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主要是調用ServiceMethod.parseAnnotations數組

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);//這裏就是對請求的解析，也就是咱們定義的接口方法

    Type returnType = method.getGenericReturnType();//獲取返回的類型
    //省略無關代碼

    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }
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RequestFactory.parseAnnotations這個方法就是對咱們定義的接口方法的解析，在裏面解析註解和使用反射獲得了關於請求的全部信息。網絡

響應的處理

在完成了對請求的處理後構建了RequestFactory類，這個類裏面包含了請求的全部信息：ide

final class RequestFactory {
  static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
    return new Builder(retrofit, method).build();
  }

  private final Method method;
  private final HttpUrl baseUrl;
  final String httpMethod;
  private final @Nullable String relativeUrl;
  private final @Nullable Headers headers;
  private final @Nullable MediaType contentType;
  private final boolean hasBody;
  private final boolean isFormEncoded;
  private final boolean isMultipart;
  private final ParameterHandler<?>[] parameterHandlers;
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接下來繼續來看ServiceMethod的parseAnnotations剩下的語句oop

ServiceMethod.parseAnnotationspost

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    //省略無關代碼

    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);//對響應的處理
  }
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HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)ui

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations( Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);//建立請求適配器
    Type responseType = callAdapter.responseType();
    //省略無關代碼
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);//建立響應數據轉換器（Gson）

    okhttp3.Call.Factory callFactory = retrofit.callFactory;//Okhttp請求客戶端
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
  }
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建立請求適配器CallAdapter

首先進入第一行 HttpServiceMethod.createCallAdapterthis

private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> createCallAdapter( Retrofit retrofit, Method method) {
    Type returnType = method.getGenericReturnType();//獲取返回的類型
    Annotation[] annotations = method.getAnnotations();//獲取註解
    try {
      //noinspection unchecked
      return (CallAdapter<ResponseT, ReturnT>) retrofit.callAdapter(returnType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create call adapter for %s", returnType);
    }
  }
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在獲取了方法的返回類型和註解以後，調用spa

retrofit.callAdapter

public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }
  
    public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
      Annotation[] annotations) {
      //空判斷
    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;
      }
    }
  }
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這裏面是一個循環獲取請求適配器工廠數組callAdapterFactories，在retrofit建立的時候，咱們能夠知道往這個數組裏面加的是Android平臺默認的ExecutorCallAdapterFactory

public Retrofit build() {
      List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));   
}
static class Android extends Platform {
    @Override List<? extends CallAdapter.Factory> defaultCallAdapterFactories(
        @Nullable Executor callbackExecutor) {
      if (callbackExecutor == null) throw new AssertionError();
      //這裏能夠看到返回的類
      ExecutorCallAdapterFactory executorFactory = new ExecutorCallAdapterFactory(callbackExecutor);
      return Build.VERSION.SDK_INT >= 24
        ? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
        : singletonList(executorFactory);
    }    
}
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在剛纔的循環裏，調用了它的get方法，實際就是調用

ExecutorCallAdapterFactory.get

@Override public @Nullable CallAdapter<?, ?> get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
      return null;
    }
    final Type responseType = Utils.getCallResponseType(returnType);//根據返回類型獲取泛型裏面的想要獲取的響應類型
    return new CallAdapter<Object, Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public Call<Object> adapt(Call<Object> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }
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咱們定義的返回類型通常是泛型的形式，如Call或者Observable，因此Utils.getCallResponseType就是取出泛型裏面的類型。最後將獲得的類型傳入新建的CallAdapter

return new CallAdapter<Object, Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public Call<Object> adapt(Call<Object> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
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自此就完成了CallAdapter的建立，這裏記住它的adapt方法，裏面是返回的ExecutorCallbackCall，這個後面會調用，再來繼續看。

HttpServiceMethod的parseAnnotations第一行代碼到此就完了，繼續往下執行：

建立響應數據轉換器（如Gson）和請求客戶端

HttpServiceMethod.parseAnnotations

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations( Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
    Type responseType = callAdapter.responseType();//獲取具體的響應類型
    //省略無關代碼
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
  }
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注意這裏傳給responseConverter的responseType是咱們剛纔獲取Call泛型裏面的具體類型，也就是咱們想要拿到的實體類類型。

這個過程跟剛纔建立CallAdapter的過程很像，就連方法名都很像，因此來看看createResponseConverter方法：

HttpServiceMethod.createResponseConverter

private static <ResponseT> Converter<ResponseBody, ResponseT> createResponseConverter( Retrofit retrofit, Method method, Type responseType) {
    Annotation[] annotations = method.getAnnotations();
    try {
      return retrofit.responseBodyConverter(responseType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create converter for %s", responseType);
    }
  }
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經過method的getAnnotations拿到註解列表，而後跟返回類型傳入responseBodyConverter方法：

retrofit.responseBodyConverter

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

    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) {
        //noinspection unchecked
        return (Converter<ResponseBody, T>) converter;
      }
    }
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這個過程簡直跟剛纔建立CallAdapter的過程如出一轍，也是循環從converterFactories取出Converter，根據以前Retrofit的建立時咱們傳入的是GsonConverterFactory，因此是調用的它的responseBodyConverter方法：

GsonConverterFactory.responseBodyConverter

public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }
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這裏的代碼就很面熟了，經過TypeToken獲取類型，以後返回一個Gson解析器，它的convert就是調用Gson進行泛型的解析。

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 {
      T result = adapter.read(jsonReader);
      if (jsonReader.peek() != JsonToken.END_DOCUMENT) {
        throw new JsonIOException("JSON document was not fully consumed.");
      }
      return result;
    } finally {
      value.close();
    }
  }
}
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到這裏就完成了響應字符串的解析器的建立，接下來會執行

okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
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建立okhttp請求的客戶端，這裏命名爲callFactory，在Retrofit建立的時候咱們傳入的okhttpClient就是賦值給了callFactory，最後將建立好的對象傳入HttpServiceMethod返回。

到此爲止就執行完了HttpServiceMethod的parseAnnotations方法，在這個方法裏面，建立了幾個重要的東西

  • 對請求和響應作具體處理的CallAdapter類，具體爲ExecutorCallAdapterFactory，這裏要注意它的adapt方法返回ExecutorCallbackCall類
  • 響應數據解析工廠Converter，具體爲GsonResponseBodyConverter
  • okhttp請求客戶端

發送請求

終於到了這一步了，在作好了全部的準備工做以後，最終Retrofit怎麼來發送請求呢，首先一步步回到最開始的地方： 在HttpServiceMethod.parseAnnotations方法是在ServiceMethod的parseAnnotations調用的

ServiceMethod.parseAnnotations

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.");
    }

    //在這裏調用並返回
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

  abstract T invoke(Object[] args);
}
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而後ServiceMethod.parseAnnotations方法是在Retrofit的loadServiceMethod調用的

而後loadServiceMethod是在Retrofit的create方法裏面建立動態代理的invoke調用的

Retrofit.create

public <T> T create(final Class<T> service) {
    //省略無關代碼
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          //省略無關代碼

          @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args) throws Throwable {
            //省略無關代碼
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
          }
        });
  }
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能夠看到，在loadServiceMethod以後調用了invoke方法，也就是ServiceMethod的invoke方法，點進去以後發現它是個ServiceMethod是個抽象類，它的invoke須要實現類來重寫：

abstract class ServiceMethod<T> {
    abstract T invoke(Object[] args);
}
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ServiceMethod類只有一個實現類，也就是咱們剛纔看到過的HttpServiceMethod，因此實際上調用的是它的invoke方法：

HttpServiceMethod.invoke

@Override ReturnT invoke(Object[] args) {
    return callAdapter.adapt(
        new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
  }
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在這裏只有一行代碼，其實是調用callAdapter.adapt，那這個callAdapter是什麼呢？剛纔咱們已經知道它是ExecutorCallAdapterFactory建立的，而後在建立的callAdapter的adapt方法返回ExecutorCallbackCall類，因此來看看這個類：

ExecutorCallbackCall是ExecutorCallAdapterFactory的靜態內部類

ExecutorCallbackCall

static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override public void enqueue(final Callback<T> callback) {
      checkNotNull(callback, "callback == null");

      delegate.enqueue(new Callback<T>() {
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
                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);
            }
          });
        }
      });
    }

    @Override public boolean isExecuted() {
      return delegate.isExecuted();
    }

    @Override public Response<T> execute() throws IOException {
      return delegate.execute();
    }

    @Override public void cancel() {
      delegate.cancel();
    }

    @Override public boolean isCanceled() {
      return delegate.isCanceled();
    }

    @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
    @Override public Call<T> clone() {
      return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
    }

    @Override public Request request() {
      return delegate.request();
    }
  }
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在這個類裏面，咱們就看到了不少很熟悉的方法，enqueue方法發送請求，實際的請求和處理就是在這裏面，可是還有疑惑就是具體的處理咱們仍是沒看到，callbackExecutor和delegate這兩個熟悉纔是關鍵所在：

callbackExecutor

這個是在ExecutorCallAdapterFactory的構造方法中傳入的，它的構造方法又是在Platform的defaultCallAdapterFactories方法中調用的，看到Platform，天然能夠想到它的Android平臺類，因此這個callbackExecutor其實是MainThreadExecutor

@Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }
    
    static class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

      @Override public void execute(Runnable r) {
        handler.post(r);
      }
    }    
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其實代碼很簡單，使用了咱們最經常使用的Handler，讓代碼在主線程中執行，也就是咱們一般作的在子線程請求網絡，主線程中更新UI。

delegate

delegate實際是在HttpServiceMethod的invoke方法裏傳入：

@Override ReturnT invoke(Object[] args) {
    return callAdapter.adapt(
        new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
  }
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其實是OkHttpCall這個類，這個類裏面就是發送請求和處理響應的具體處理，代碼不少，咱們只看看構造方法和enqueue

OkHttpCall(RequestFactory requestFactory, Object[] args,
      okhttp3.Call.Factory callFactory, Converter<ResponseBody, T> responseConverter) {
    this.requestFactory = requestFactory;//請求工廠，裏面包含了請求的全部信息
    this.args = args;//參數
    this.callFactory = callFactory;//請求客戶端，okhttp
    this.responseConverter = responseConverter;//響應的json解析器（Gson）
  }
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OkHttpCall.enqueue

@Override public void enqueue(final Callback<T> callback) {
    checkNotNull(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 {
          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 {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          throwIfFatal(e);
          callFailure(e);
          return;
        }

        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      @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) {
          t.printStackTrace();
        }
      }
    });
  }
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代碼其實很簡單，就是使用okhttp請求，而後在響應方法onResponse作相應的解析和處理，最後完成網絡請求。

總結

Retrofit對響應的解析大概可分爲

  1. 建立處理適配器CallAdapter，具體實現類爲ExecutorCallbackCall，在這裏面進行發送請求和處理響應等相關的操做，最終是調用okhttp進行網絡請求
  2. 建立響應數據轉換器Converter，具體實現類爲GsonResponseBodyConverter，用於對返回json數據的解析，這個是咱們本身傳入的Gson解析工廠，Retrofit也支持其餘的第三方解析庫
  3. 發送請求，具體的操做類是OkHttpCall來發送請求和處理響應，MainThreadExecutor來作回調的處理，在主線程中處理已經轉化好的實體類。
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