HttpAsyncClient 是一个异步的 HTTP 客户端开发包，基于 HttpCore NIO 和 HttpClient 组件。
HttpAsyncClient 的出现并不是为了替换 HttpClient，而是作为一个补充用于需要大量并发连接，对性能要求非常高的基于HTTP的原生数据通信，而且提供了事件驱动的 API。
HttpComponents HttpAsyncClient 4.1-beta1 发布，此版本现已提供下载，值得关注的更新内容如下：
* Support for pipelined request execution
* Enhanced redesigned and rewritten default SSL hostname verifier with
improved RFC 2818 compliance
* Default SSL hostname verifier and default cookie policy now validate
certificate identity and cookie domain of origin against the public
suffix list maintained by Mozilla.org <https://publicsuffix.org/list>
* Authentication cache thread-safety: authentication caches used by
HttpAsyncClient is now thread-safe and can be shared by multiple
contexts in order to re-use authentication state for subsequent requests
