Retrofit網絡請求源碼解析

1.使用者的retrofit建立

/**
     * 初始化Retrofit
     */
    public static void init() {
        okHttpClient = HttpsUtils.getOKHttpClient();
        //設置Retrofit
        Retrofit client = new Retrofit.Builder()
                .baseUrl(HOST)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .client(okHttpClient)
                .build();
        //建立業務請求對象
        api = client.create(RequestApi.class);
    }
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2. Retrofit:build()方法組裝數據以及添加適配和轉化器

public Retrofit build() {
      //校驗baseUrl
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
      //獲取回調工廠，默認使用okhttp
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }
      //添加線程池
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // 調用相關的適配器
      List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // 執行相關的轉換器
      List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);

      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }
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buid方法主要是構建一個真正的retrofit對象，加載適配器，轉化器，callbackExecutor線程池，baseUrl等基本數據

3.建立的入口(Retrofit create())

public <T> T create(final Class<T> service) {
    //檢測是不是對應的接口
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      // 接口方法的緩存
      eagerlyValidateMethods(service);
    }
    //核心方法，動態代碼相關方法
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          //獲取對應的支持的平臺 android java
          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);
            }
            //真正執行的方法
            ServiceMethod<Object, Object> serviceMethod =
                (ServiceMethod<Object, Object>) loadServiceMethod(method);
            OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });
  }
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動態代理的核心方法，都會調用invoke，前面會加一些方法校驗，並且retrofit一樣支持android和java,因此也對平臺作了封裝，同時也加map緩存對應的代理對象

4.loadServiceMethod加載服務方法

ServiceMethod<?, ?> loadServiceMethod(Method method) {
    ServiceMethod<?, ?> result = serviceMethodCache.get(method);
    if (result != null) return result;
    //加一個異步鎖
    synchronized (serviceMethodCache) {
      //從的map中去拿
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = new ServiceMethod.Builder<>(this, method).build();
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }
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加了一個同步鎖，防止多線程的數據混亂問題，map緩存增長執行速度，構建以後一樣加入map中去

5.查看ServiceMethod對象的建立方法Builder

public ServiceMethod build() {
      //構建響應Call的適配器，能夠使用者經過addCallAdapterFactory() 工廠類添加
      callAdapter = createCallAdapter();
      //.....響應適配器的返回類型校驗
      // 建立響應的Converter變換器 可經過addConverterFactory 工廠類添加
      responseConverter = createResponseConverter();

      //一系列的請求註解校驗，具體看源碼

      return new ServiceMethod<>(this);
    }
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構建響應的適配器和轉換器，這些均可以被使用者經過工廠類從外部傳進來

6. Retrofit nextCallAdapter() 建立適配器

ServiceMethod類：
private CallAdapter<T, R> createCallAdapter() {
      //獲取接口指定的返回類型，Call或者是Observatble
      Type returnType = method.getGenericReturnType();
     //省略部分校驗
      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);
      }
    }
   //Retrofit的callAdapter方法
  public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }

  public CallAdapter<?, ?> nextCallAdapter(CallAdapter.Factory skipPast, Type returnType,
      Annotation[] annotations) {
    checkNotNull(returnType, "returnType == null");
    checkNotNull(annotations, "annotations == null");

    int start = adapterFactories.indexOf(skipPast) + 1;
    for (int i = start, count = adapterFactories.size(); i < count; i++) {
      CallAdapter<?, ?> adapter = adapterFactories.get(i).get(returnType, annotations, this);
      if (adapter != null) {
        return adapter;
      }
    }

   //省略
  }
    
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createCallAdapter校驗返回值類型 以後會回調Retrofit的callAdapter方法以後執行適配器list中的適配器get方法

7.ExecutorCallAdapterFactory的get方法，以後調用線程池的建立方法

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

      @Override public Call<Object> adapt(Call<Object> call) {
      //建立線程池
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }

 
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8.call.enqueue(new Callback)開始真正請求數據，調用ExecutorCallbackCall的enqueue方法

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) {
      if (callback == null) throw new NullPointerException("callback == null");
      調用okhtt.Call方法裏面的enqueue
      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()) {
                
                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);
            }
          });
        }
      });
    }

   //省略
  }
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進入了okhttp的請求中

9.把子線程請求的數據回調給主線程

這裏感受比較巧妙，基本都是使用handler，在Platform類中定義android平臺時能夠看到以下一段代碼恰好和上面的ExecutorCallbackCall代理類delegate.enqueue(new Callback())呼應

static class Android extends Platform {
    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

    static class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());
     //執行execute用handler的post方法到主線程
      @Override public void execute(Runnable r) {
        handler.post(r);
      }
    }
  }
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利用主線程的handler的post方法在執行execute的時候把數據返回主線程

最後加上流程圖