Apache HttpComponents Client
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Apache HttpComponents Core (blocking I/O)
Async Http Client library purpose is to allow Java applications to easily execute HTTP requests and asynchronously process the HTTP responses.
Apache HttpComponents Client fluent API
Apache HttpComponents AsyncClient
Jetty :: Asynchronous HTTP Client
Google HTTP Client Library for Java. Functionality that works on all supported Java platforms, including Java 5 (or higher) desktop (SE) and web (EE), Android, and Google App Engine.