Retrofit2 源码解析

1. Retrofit是什么

来自Retrofit官网的介绍：

A type-safe HTTP client for Android and Java

一个基于OkHttp的RESTFUL Api请求工具

2. Retrofit的用法

• 创建Retrofit对象

public static final String API_URL = "https://api.github.com";

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl(API_URL)
    .addConverterFactory(GsonConverterFactory.create())
    .build();

• api新建一个Java接口，用Java注解来描述这个api

public interface GitHubApiService {
@GET("/users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
// you can add some other meathod
}

• 这个retrofit对象创建一个GitHubService对象：

GitHubApiService gitHubApiService=retrofit.create(GitHubApiService.class);//获取API接口的实现类的实例对象

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

• 异步回调结果

// 请求数据，并且处理response
call.enqueue(new Callback<List<Repo>>() {
    @Override
    public void onResponse(Response<List<Repo>> list) {
        Log.d("list： " + list);
    }
    @Override
    public void onFailure(Throwable t) {
    }
});

• 同步回调结果

 List<Repo> list = call.execute();

3. Retrofit的原理

Retrofit整个工作流程：将java接口，转换成一个个Http请求，交由Okhttp进行发送。

Retrofit库依赖了Okhttp库和okio库,核心库分为Http包和其他核心类。

Http包里面包含各种定义的注解，帮忙开发者定义请求方式和请求参数。

如何将一个请求放到Okhttp里面去?

答案是： Retrofit利用了java的动态代理技术。

动态代理代码如下：

/** Create an implementation of the API defined by the {@code service} interface. */
public <T> T create(final Class<T> service) {
  Utils.validateServiceInterface(service);
  if (validateEagerly) {
     eagerlyValidateMethods(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, 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);
        }
        ServiceMethod serviceMethod = loadServiceMethod(method);
        OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
        return serviceMethod.callAdapter.adapt(okHttpCall);
      }
    });

首先看动态代理方法，这里是j接口动态代理的标准写法

 Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
    new InvocationHandler(){});

InvocationHandler类中的插入方法

      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);
        }
        ServiceMethod serviceMethod = loadServiceMethod(method);
        OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
        return serviceMethod.callAdapter.adapt(okHttpCall);
      }

api对象其实是一个动态代理对象,类型是接口类型，真正的对象是被动态代理拦截，然后调用Proxy.newProxyInstance方法中的InvocationHandler对象。

其中invoke方法：

• Object proxy: 代理对象

• Method method：调用的方法

• Object... args：方法的参数

然后Retrofit就会用Java反射获取到方法的注解信息，配合args参数，创建一个ServiceMethod对象。

ServiceMethod就像是一个处理器，传入Retrofit对象和Method对象，调用各个接口和解析器，最终生成一个Request，包含api 的域名、path、http请求方法、请求头、是否有body、是否是multipart等等。最后返回一个Call对象，Retrofit2中Call接口的默认实现是OkHttpCall，它默认使用OkHttp3作为底层http请求client

翻译请求方式和参数，同时构建出ServiceMethod对象

 public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      if (responseType == Response.class || responseType == okhttp3.Response.class) {
        throw methodError("'"
            + Utils.getRawType(responseType).getName()
            + "' is not a valid response body type. Did you mean ResponseBody?");
      }
      responseConverter = createResponseConverter();

      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }

      if (httpMethod == null) {
        throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
      }

      if (!hasBody) {
        if (isMultipart) {
          throw methodError(
              "Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
        }
        if (isFormEncoded) {
          throw methodError("FormUrlEncoded can only be specified on HTTP methods with "
              + "request body (e.g., @POST).");
        }
      }

      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0; p < parameterCount; p++) {
        Type parameterType = parameterTypes[p];
        if (Utils.hasUnresolvableType(parameterType)) {
          throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
              parameterType);
        }

        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
        if (parameterAnnotations == null) {
          throw parameterError(p, "No Retrofit annotation found.");
        }

        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }

      if (relativeUrl == null && !gotUrl) {
        throw methodError("Missing either @%s URL or @Url parameter.", httpMethod);
      }
      if (!isFormEncoded && !isMultipart && !hasBody && gotBody) {
        throw methodError("Non-body HTTP method cannot contain @Body.");
      }
      if (isFormEncoded && !gotField) {
        throw methodError("Form-encoded method must contain at least one @Field.");
      }
      if (isMultipart && !gotPart) {
        throw methodError("Multipart method must contain at least one @Part.");
      }

      return new ServiceMethod<>(this);
    }

总结来说：使用Java动态代理的目的就要拦截被调用的Java方法，然后解析这个Java方法的注解，最后生成Request由OkHttp发送。