What is the difference between BeanShell and JSR223 in terms of thread safety?

Both BeanShell and JSR223 script engines can be used safely across multiple concurrent threads in JMeter, but the details of how each engine manages script state per thread differ, and getting this wrong in either can produce subtle cross-thread data bugs.

A key practical consideration is that if a script, in either engine, references a variable in a way that's actually shared across threads rather than properly thread-local, like a static field or an improperly scoped shared object, that variable can be read or modified by multiple threads simultaneously, causing race conditions regardless of which scripting engine is being used.

JMeter's implicit vars (thread-local) and props (JVM-wide) variables exist precisely to make this distinction explicit and manageable for script authors in either engine - using vars for anything that must stay isolated per thread, and being deliberate and careful about props, or any other genuinely shared object, specifically because it's accessible across every thread concurrently.