Retrofit2源码分析

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本文基于compile 'com.squareup.retrofit2:retrofit:2.2.0'分析

1.概述

Retrofit应该是现在最火的网络请求框架，它是基于okhttp的封装，非常适合restful url格式的请求，可以使用注解实现很多功能。

2.基本用法

public interface IApi {
    @POST("/toutiao/index")
    Call getTopNews(@QueryMap Map params);
}

// 构造Retrofit
Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(Constants.BASE_URL)
        .client(new OkHttpClient())
        .build();
Map params = new HashMap<>();
params.put("type", "top");
params.put("key", "52880cdc1ede6a63d0be1f1816f136a7");
// 动态生成代理对象
IApi api = retrofit.create(IApi.class);
// 生成OkhttpCall的代理对象
retrofit2.Call call = api.getTopNews(params);
// 异步执行
call.enqueue(new retrofit2.Callback() {
    @Override
    public void onResponse(retrofit2.Call call, retrofit2.Response response) {
        if (response.isSuccessful()) {
            try {
                   Log.e(TAG, "onResponse: " + response.body().string());
                } catch (IOException e) {
                    e.printStackTrace();
                }
        }
    }

    @Override
    public void onFailure(retrofit2.Call call, Throwable t) {
        Log.e(TAG, "onFailure: " + t.getMessage());
    }
});

3.源码分析

1.构造者模式创建Retrofit对象

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

      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        // 就是MainThreadExecutor，执行在主线程
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List adapterFactories = new ArrayList<>(this.adapterFactories);
      // 这里加了一个默认的ExecutorCallAdapterFactory，platform对应的实现就是 Android
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List converterFactories = new ArrayList<>(this.converterFactories);

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

2.通过动态代理得到接口实例，这里涉及了动态代理，可以参考java的动态代理机制详解

public  T create(final Class 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();
          // 当动态代理对象调用方法时，这个方法的调用就会被转发为由      
          // InvocationHandler这个接口的 invoke 方法来进行调用
          @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);
            }
            // 将method包装成ServiceMethod
            ServiceMethod serviceMethod =
                (ServiceMethod) loadServiceMethod(method);
            // 构造OkHttpCall
            OkHttpCall