今天我們主要聊一聊 Retrofit 的源碼窘哈，他是怎樣對 OkHttp 包裝的脆贵。

此次分析源碼的版本 Retrofit 2.5.0

首先我們先了解一下 Retrofit 用法牍汹。

public interface GitHubService {
  @GET("users/{user}/repos")
  Call<List<Repo>> listRepos(@Path("user") String user);
}

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("https://api.github.com/")
        .build();
         
GitHubService service = retrofit.create(GitHubService.class);

Call<List<Repo>> call = service.listRepos("octocat");

//發(fā)送網(wǎng)絡請求(異步)
call.enqueue(new Callback<List<Repo>>() {
    @Override
    public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {

    }

    @Override
    public void onFailure(Call<List<Repo>> call, Throwable t) {

    }
});

// 發(fā)送網(wǎng)絡請求（同步）
Response<List<Repo>> response = call.execute();

Retrofit 的 Builder 建造者模式

首先我們看到 Retrofit 的 Builder 建造者模式西疤，

public Retrofit build() {
    // 配置 baseUrl
    if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
    }
    
    // 配置 OkHttpClient
    okhttp3.Call.Factory callFactory = this.callFactory;
    if (callFactory == null) {
        callFactory = new OkHttpClient();
    }

    // 執(zhí)行 callback 的線程
    Executor callbackExecutor = this.callbackExecutor;
    if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
    }

    // Call 適配器，比如 RxJava、Java8 適配器
    List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
    callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));

    // 數(shù)據(jù)轉(zhuǎn)換器，比如 Gson、Jackson 等轉(zhuǎn)換器
    List<Converter.Factory> converterFactories = new ArrayList<>(
      1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
        converterFactories.add(new BuiltInConverters());
        converterFactories.addAll(this.converterFactories);
        converterFactories.addAll(platform.defaultConverterFactories());

        return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
    }
}

根據(jù)接口方法獲取 Service

接下來我們再往下看 retrofit.create() 方法

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();
          private final Object[] emptyArgs = new Object[0];

          @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
              throws Throwable {
            // 處理默認方法
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            // 處理接口方法
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
          }
        });
    }

create() 本身就是一個動態(tài)代理醋界。

調(diào)用 Service 的方法就會走動態(tài)代理

先看 loadServiceMethod() 方法，等會再看 invoke();

ServiceMethod<?> loadServiceMethod(Method method) {
    // 查詢方法緩存
    ServiceMethod<?> result = serviceMethodCache.get(method);
    if (result != null) return result;
    
    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        // 解析方法的注解以及返回類型確定各項參數(shù)以及 Call 適配器提完、數(shù)據(jù)轉(zhuǎn)換器形纺。
        result = ServiceMethod.parseAnnotations(this, method);
        serviceMethodCache.put(method, result);
      }
    }
    return result;
}

繼續(xù)往下看 ServiceMethod.parseAnnotations() 方法。

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    // 解析方法的注解徒欣、參數(shù)注解逐样、請求頭等等，具體解析方式自己看 parseAnnotations() 方法的代碼
    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.");
    }
    // 解析并創(chuàng)建 Call 適配器打肝、數(shù)據(jù)轉(zhuǎn)換器
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

繼續(xù)查看 HttpServiceMethod.parseAnnotations() 方法脂新。

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    // 根據(jù)返回類型創(chuàng)建并獲取 CallAdapter
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
    Type responseType = callAdapter.responseType();
    if (responseType == Response.class || responseType == okhttp3.Response.class) {
      throw methodError(method, "'"
          + Utils.getRawType(responseType).getName()
          + "' is not a valid response body type. Did you mean ResponseBody?");
    }
    if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) {
      throw methodError(method, "HEAD method must use Void as response type.");
    }
    
    // 根據(jù) Response 類型創(chuàng)建并獲取 Converter
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
}

繼續(xù)回去看 invoke() 方法

@Override ReturnT invoke(Object[] args) {
    // 轉(zhuǎn)換成相應的類型
    return callAdapter.adapt(
        new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}

// 以 Retrofit 默認適配器為例，假設 SDK version 小于 24
@Override public Call<Object> adapt(Call<Object> call) {
    return new ExecutorCallbackCall<>(callbackExecutor, call);
}

發(fā)送異步網(wǎng)絡請求

@Override public void enqueue(final Callback<T> callback) {
      checkNotNull(callback, "callback == null");
      // 這里 delegate 是 OkHttpCall 對象
      // 大家可以自己看一看 OkHttpCall 的 enqueue() 方法粗梭，它本身就是對 okhttp3.Call 的包裝争便。
      delegate.enqueue(new Callback<T>() {
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          // callbackExecutor 最初配置的線程運行環(huán)境
          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);
            }
          });
        }
      });
}

發(fā)送同步網(wǎng)絡請求

@Override public Response<T> execute() throws IOException {
      // 這里 delegate 是 OkHttpCall 對象
      return delegate.execute();
}