====== AsyncHttpClient ======
  * https://github.com/AsyncHttpClient/async-http-client
  * async, non-blocking IO Java HTTP Client

===== Retrofit 2 지원 =====
  * https://github.com/AsyncHttpClient/async-http-client/tree/master/extras/retrofit2
  * [[java:retrofit|retrofit]]
  * ''Call.Factory'' 객체를 한 번만 생성해서 공유해서 사용할 것. 매번 생성시 리소스 고갈 됨.
<code java>
// instantiate async-http-client
AsyncHttpClient httpClient = ...

// instantiate async-http-client call factory
Call.Factory callFactory = AsyncHttpClientCallFactory.builder()
    .httpClient(httpClient)                 // required
    .onRequestStart(onRequestStart)         // optional
    .onRequestFailure(onRequestFailure)     // optional
    .onRequestSuccess(onRequestSuccess)     // optional
    .requestCustomizer(requestCustomizer)   // optional
    .build();

// instantiate retrofit
Retrofit retrofit = new Retrofit.Builder()
    .callFactory(callFactory) // use our own call factory
    .addConverterFactory(ScalarsConverterFactory.create())
    .addConverterFactory(JacksonConverterFactory.create())
    // ... add other converter factories
    // .addCallAdapterFactory(RxJavaCallAdapterFactory.createAsync())
    .validateEagerly(true) // highly recommended!!!
    .baseUrl("https://api.github.com/");

// time to instantiate service
GitHub github = retrofit.create(Github.class);

// enjoy your type-safe github service api! :-)
</code>