源码的分析将从基本的使用方法入手，分析retrofit的实现方案，以及其中涉及到的一些有趣的技巧。

‘Android技术交流群653583088，欢迎大家加入交流，畅谈！本群有免费学习资料视频’并且免费分享retrofit源码解析视频

简单使用

定义HTTP API

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

创建Retrofit并生成API的实现

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

调用API方法，生成Call

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

Retrofit的创建

retrofit实例的创建，使用了builder模式，从下面的源码中可以看出

public static final class Builder {
    Builder(Platform platform) {
        this.platform = platform;
        converterFactories.add(new BuiltInConverters());
    }
    public Builder() {
        // Platform.get()方法可以用于判断当前的环境
        this(Platform.get());
    }
    public Builder baseUrl(String baseUrl) {
      checkNotNull(baseUrl, "baseUrl == null");
      HttpUrl httpUrl = HttpUrl.parse(baseUrl);
      if (httpUrl == null) {
        throw new IllegalArgumentException("Illegal URL: " + baseUrl);
      }
      return baseUrl(httpUrl);
    }
    
    public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();// 新建Client，留到之后newCall什么的
      }
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }
      // Make a defensive copy of the adapters and add the default Call adapter.
      List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
      // Make a defensive copy of the converters.
      List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);
      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }
}

retrofit.create

好玩的地方开始了，我们先来看看这个方法

public <T> T create(final Class<T> service) {
  Utils.validateServiceInterface(service);
  if (validateEagerly) {
    eagerlyValidateMethods(service);
  }
  // 动态代理，啦啦啦
  return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
      new InvocationHandler() {
        // platform 可以分辨出你是在android，还是java8，又或者别的
        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.
          // 这里的invoke，Object方法都走这里，比如equals、toString、hashCode什么的
          if (method.getDeclaringClass() == Object.class) {
            return method.invoke(this, args);
          }
          // java8默认方法，1.8的新特性
          if (platform.isDefaultMethod(method)) {
            return platform.invokeDefaultMethod(method, service, proxy, args);
          }
          // 这里是核心代码了
          ServiceMethod<Object, Object> serviceMethod =
              (ServiceMethod<Object, Object>) loadServiceMethod(method);
          OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
          return serviceMethod.callAdapter.adapt(okHttpCall);
        }
      });
}

可以看出创建API使用了动态代理，根据接口动态生成的代理类，将接口的都转发给了负责连接代理类和委托类的InvocationHandler实例，接口方法也都通过其invoke方法来处理。
在invoke方法中，首先会通过Platform.get()方法判断出当前代码的执行环境，之后会先把Object和Java8的默认方法进行一个处理，也是在进行后续处理之前进行去噪。其中的关键代码其实就是最后三句，这也是这篇文章将要分析的

创建ServiceMethod

erviceMethod<?, ?> loadServiceMethod(Method method) {
  // 从缓存里面取出，如果有的话，直接返回好了
  ServiceMethod<?, ?> result = serviceMethodCache.get(method);
  if (result != null) return result;
  synchronized (serviceMethodCache) {
    result = serviceMethodCache.get(method);
    if (result == null) {
      // 为null的话，解析方法的注解和返回类型、参数的注解he参数类型，新建一个ServiceMethod
      result = new ServiceMethod.Builder<>(this, method).build();// ->
      // 新建的ServiceMethod加到缓存列表里面
      serviceMethodCache.put(method, result);
    }
  }
  return result;
}

注解的解析

CallAdapter和Converter等到后面再分析，这里先看看parseMethodAnnotation(annotation)，功能和其名字一样，其对方法注解进行了解析

/**
 * 解析方法注解，呜啦啦
 * 通过判断注解类型来解析
 * @param annotation
 */
private void parseMethodAnnotation(Annotation annotation) {
  if (annotation instanceof DELETE) {
    parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);
  } else if (annotation instanceof GET) {
    parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);
  } 
  // 其他的一些方法注解的解析
  ...
}
private void parseHttpMethodAndPath(String httpMethod, String value, boolean hasBody) {
  if (this.httpMethod != null) {// 已经赋值过了
    throw methodError("Only one HTTP method is allowed. Found: %s and %s.",
        this.httpMethod, httpMethod);
  }
  this.httpMethod = httpMethod;
  this.hasBody = hasBody;
  // value为设置注解方法时候，设置的值，官方例子中的users/{user}/repos or user
  if (value.isEmpty()) {
    return;
  }
  // 查询条件的一些判断
    ...
  this.relativeUrl = value;
  this.relativeUrlParamNames = parsePathParameters(value);
}
`

在解析注解时，先通过instanceof判断出注解的类型，之后调用parseHttpMethodAndPath方法解析注解参数值，并设置httpMethod、relativeUrl、relativeUrlParamNames等属性。

上面说了API中方法注解的解析，现在来看看方法参数注解的解析，这是通过调用parseParameterAnnotation方法生成ParameterHandler实例来实现的，代码比较多，这里挑选@Query来看看。

else if (annotation instanceof Query) {
Query query = (Query) annotation;
String name = query.value();
boolean encoded = query.encoded();
Class<?> rawParameterType = Utils.getRawType(type);// 返回基础的类
gotQuery = true;
// 可以迭代，Collection
if (Iterable.class.isAssignableFrom(rawParameterType)) {
  if (!(type instanceof ParameterizedType)) {
    throw parameterError(p, rawParameterType.getSimpleName()
        + " must include generic type (e.g., "
        + rawParameterType.getSimpleName()
        + "<String>)");
  }
  ParameterizedType parameterizedType = (ParameterizedType) type;
  Type iterableType = Utils.getParameterUpperBound(0, parameterizedType);// 返回基本类型
  Converter<?, String> converter =
      retrofit.stringConverter(iterableType, annotations);
  return new ParameterHandler.Query<>(name, converter, encoded).iterable();
} else if (rawParameterType.isArray()) {// Array
  Class<?> arrayComponentType = boxIfPrimitive(rawParameterType.getComponentType());// 如果是基本类型，自动装箱
  Converter<?, String> converter =
      retrofit.stringConverter(arrayComponentType, annotations);
  return new ParameterHandler.Query<>(name, converter, encoded).array();
} else {// Other
  Converter<?, String> converter =
      retrofit.stringConverter(type, annotations);
  return new ParameterHandler.Query<>(name, converter, encoded);
}

在@Query中，将分成Collection、array、other三种情况处理参数，之后根据这些参数，调用ParameterHandler中的Query静态类，创建出一个ParameterHandler实例。这样循环直到解析了所有的参数注解，组合成为全局变量parameterHandlers，之后构建请求时会用到

OkHttpCall

ServiceMethod创建完成之后，我们来看看下一行代码中的OkHttpCall类，里面的包含了请求的执行和响应处理，我们来看看异步请求的做法

OkHttpCall(ServiceMethod<T, ?> serviceMethod, Object[] args) {
  this.serviceMethod = serviceMethod;
  this.args = args;
}
@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();// 创建OkHttp3.Call
    } catch (Throwable 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)
      throws IOException {
    Response<T> response;
    try {
      response = parseResponse(rawResponse);// ->
    } catch (Throwable e) {
      callFailure(e);
      return;
    }
    callSuccess(response);
  }
  @Override public void onFailure(okhttp3.Call call, IOException e) {
    try {
      callback.onFailure(OkHttpCall.this, e);
    } catch (Throwable t) {
      t.printStackTrace();
    }
  }
  private void callFailure(Throwable e) {
    try {
      callback.onFailure(OkHttpCall.this, e);
    } catch (Throwable t) {
      t.printStackTrace();
    }
  }
  private void callSuccess(Response<T> response) {
    try {
      callback.onResponse(OkHttpCall.this, response);
    } catch (Throwable t) {
      t.printStackTrace();
    }
  }
});
}
private okhttp3.Call createRawCall() throws IOException {
  Request request = serviceMethod.toRequest(args);// 根据ParameterHandler组装Request.Builder，生成Request
  okhttp3.Call call = serviceMethod.callFactory.newCall(request);// Retrofit中创建的new OkHttpClient().newCall(request)
  ...
  return call;
}

CallAdapter

现在来看看enqueue传入的参数callback,这个参数可能和很多人心中想的并不一样，它并不是用户在使用时传入的那个Callback对象。那么他是从哪里来的呢？不知道你还记不记得我之前在Retrofit.Builder.build()方法中提到过一句代码Platform.get()。在不使用addCallAdapterFactory的情况下。将会使用Platform的一种内部类，在Android环境下将会使用到Android类（这其实是个策略模式）

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 {
    // Looper.getMainLooper()就是为嘛响应会在主线程的原因
    private final Handler handler = new Handler(Looper.getMainLooper());
    @Override public void execute(Runnable r) {
      handler.post(r);
    }
  }
}

上面的代码先稍微放一下，我们继续看retrofit.Bulider.build,其中有几句比较关键的代码

callFactory = new OkHttpClient();
callbackExecutor = platform.defaultCallbackExecutor();
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

结合Android类中的代码可以看出，其最后生成了ExecutorCallAdapterFactory类。虽然看到了CallAdapter.Factory，但是到底是哪里执行了enqueue方法呢？现在我们来看看retrofit.create的最后一句代码serviceMethod.callAdapter.adapt(okHttpCall)。

Converter

现在回到OkhttpCall.enqueue方法中，在其中还有一句重要的代码没有看，那就是response = parseResponse(rawResponse);,我们来看看这其中做了什么。

Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException
  ResponseBody rawBody = rawResponse.body();
  // Remove the body's source (the only stateful object) so we can pass th
  rawResponse = rawResponse.newBuilder()
      .body(new NoContentResponseBody(rawBody.contentType(), rawBody.conte
      .build();
  ...
  ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
  try {
    T body = serviceMethod.toResponse(catchingBody);// 解析body，比如Gson解析
    return Response.success(body, rawResponse);
  } catch (RuntimeException e) {
    // If the underlying source threw an exception, propagate that rather 
    // a runtime exception.
    catchingBody.throwIfCaught();
    throw e;
  }
}
### ServiceMethod
R toResponse(ResponseBody body) throws IOException {
  return responseConverter.convert(body);
}

可以看出parseResponse最终调用了Converter.convert方法。这里以常用的GsonConverterFactory为例。

# GsonConverterFactory
@Override
public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
    Retrofit retrofit) {
  TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
  return new GsonResponseBodyConverter<>(gson, adapter);
}
# 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();
    }
  }
}

responseBodyConverter方法中用到的type参数就是之前我在CallAdapter中提到的responseType方法的返回值。生成adapter方法，用于convert方法使用。OkHttpCall在这之后的代码就比较简单了，通过回调将转换后得响应数据发送出去即可

本文分析了Retrofit的执行流程，其实包含了Retrofit、ServiceMethod、OkHttpCall、CallAdapter、Converter等方面。Retrofit的代码相对是比较少，也比较容易理解的，不过却是很好的架构实例。

‘Android技术交流群653583088，欢迎大家加入交流，畅谈！本群有免费学习资料视频’并且免费分享retrofit源码解析视频