There are two different test elements used to define the samplers:
Java
uses the HTTP implementation provided by the JVM. This has some limitations in comparison with the HttpClient implementations - see below.
HTTPClient4
uses Apache HttpComponents HttpClient 4.x.
Blank Value
does not set implementation on HTTP Samplers, so relies on HTTP Request Defaults if present or on jmeter.httpsampler property defined in jmeter.properties
The Java HTTP implementation has some limitations:
There is no control over how connections are re-used. When a connection is released by JMeter, it may or may not be re-used by the same thread.
The API is best suited to single-threaded usage - various settings are defined via system properties, and therefore apply to all connections.
No support of Kerberos authentication
It does not support client based certificate testing with Keystore Config.
Better control of Retry mechanism
It does not support virtual hosts.
It supports only the following methods: GET, POST, HEAD, OPTIONS, PUT, DELETE and TRACE
Better control on DNS Caching with DNS Cache Manager
