JMeter Beanshell Server
The Beanshell Interpreter in JMeter can act as a server which as stated in the JMeter documentation is accessible by telnet or http.
So what, I hear you say, well this can be useful and we will explore some of these uses in this Blog Post.
Before we move on the definition of a Beanshell can be found on the official Web Site.
But at a high level:
You can use BeanShell interactively for Java experimentation and debugging as well as to extend your applications in new ways. Scripting Java lends itself to a wide variety of applications including rapid prototyping, user scripting extension, rules engines, configuration, testing, dynamic deployment, embedded systems, and even Java education.
In short, BeanShell is dynamically interpreted Java, plus a scripting language and flexible environment all rolled into one clean package.
So effectively the Beanshell can dynamically change properties or variables while the test is running.