从这个版本开始，httpclient的api发生了一次重大调整。主要包括如下：
Release 4.3 Final
-------------------
This is the first stable (GA) release of HttpClient 4.3. The most notable enhancements included
in this release are:
* Support for Java 7 try-with-resources for resource management (connection release.)
* Added fluent Builder classes for HttpEntity, HttpRequest, HttpClient and SSLContext instances.
* Deprecation of preference and configuration API based on HttpParams interface in favor of
constructor injection and plain configuration objects.
* Reliance on object immutability instead of access synchronization for thread safety.
Several old classes whose instances can be shared by multiple request exchanges have
been replaced by immutable equivalents.
* DefaultHttpClient, DecompressingHttpClient, CachingHttpClient and similar classes are
deprecated in favor of builder classes that produce immutable HttpClient instances.
* HttpClient builders now dynamically construct a request execution pipeline tailored
specifically to the user configuration by physically excluding unnecessary protocol components.
* There is now an option to construct a minimal HttpClient implementation that can only execute
basic HTTP message exchanges without redirects, authentication, state management or proxy support.
This feature might be of particular use in web crawler development.
* There is now option to avoid strict URI syntax for request URIs by executing HTTP requests
with an explicitly specified target host. HttpClient will no longer attempt to parse the request
URI if it does not need to extract the target host from it.
