Explain the execution flow of a BeanShell script inside a JMeter BeanShell PreProcessor?

Before the sampler it's attached to actually executes, JMeter reaches the BeanShell PreProcessor in the test plan's tree order and prepares to run its configured script.

JMeter creates, or reuses depending on configuration, a BeanShell Interpreter instance for this execution, and populates its namespace with the standard implicit variables - vars, props, log, ctx, and, if applicable to this position in the tree, prev referencing the previous sampler's result - making JMeter's runtime state available to the script.

The Interpreter then interprets the PreProcessor's script text from top to bottom, executing whatever logic it contains - commonly modifying request parameters via vars, performing a calculation, or preparing data the upcoming sampler will need - with any errors during this interpretation surfacing as a script execution failure for this specific element.

Once the script finishes executing, control returns to JMeter's normal tree-walking flow, and the attached sampler executes next, now able to read any variables the PreProcessor's script set into vars during its run.

Because BeanShell interprets the script fresh, rather than reusing a cached compiled form, on each execution by default, this entire sequence - namespace setup, interpretation, execution - repeats on every single iteration of every thread that reaches this PreProcessor, which is the specific behavior underlying BeanShell's higher per-iteration overhead compared to a caching engine like JSR223.

flowchart TD
  A[PreProcessor reached in tree order] --> B[Interpreter created/reused]
  B --> C[Namespace populated: vars, props, log, ctx, prev]
  C --> D[Script interpreted top to bottom]
  D --> E{Error during interpretation?}
  E -- Yes --> F[Element execution fails]
  E -- No --> G[Control returns to tree, attached sampler executes next]

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