JSR223 PreProcessor

Part of the Pre Processors category. Also documented in context in the full Component Reference.

TL;DR: the JSR223 PreProcessor runs Groovy code immediately before its parent sampler fires — the standard place to build a signed request, compute a timestamp-based header, or mutate a variable right before it’s used.

The JSR223 PreProcessor allows JSR223 script code to be applied before taking a sample.

Name	Required	Description
Name	No	Descriptive name for this element that is shown in the tree.
Language	Yes	The JSR223 language to be used
Parameters	No	Parameters to pass to the script. The parameters are stored in the following variables: - Parameters - string containing the parameters as a single variable - args - String array containing parameters, split on white-space
Script file	No	A file containing the script to run, if a relative file path is used, then it will be relative to directory referenced by “user.dir” System property
Script compilation caching	No	Unique String across Test Plan that JMeter will use to cache result of Script compilation if language used supports [Compilable](https://docs.oracle.com/javase/8/docs/api/javax/script/Compilable.html) interface (Groovy is one of these, java, beanshell and javascript are not) :::note See note in JSR223 Sampler Java System property if you’re using Groovy without checking this option :::
Script	Yes (unless script file is provided)	The script to run.

The following JSR223 variables are set up for use by the script:

• log - (Logger) - can be used to write to the log file
• Label - the String Label
• FileName - the script file name (if any)
• Parameters - the parameters (as a String)
• args - the parameters as a String array (split on whitespace)
• ctx - (JMeterContext) - gives access to the context
• vars - (JMeterVariables) - gives read/write access to variables: vars.get(key); vars.put(key,val); vars.putObject("OBJ1",new Object()); vars.getObject("OBJ2");
• props - (JMeterProperties - class java.util.Properties) - e.g. props.get("START.HMS"); props.put("PROP1","1234");
• sampler - (Sampler)- gives access to the current sampler
• OUT - System.out - e.g. OUT.println("message")

For details of all the methods available on each of the above variables, please check the Javadoc

Common gotchas

• It runs before the sampler’s request is built from the JMeter GUI fields, but variable substitution in those fields happens at request-build time — so setting a JMeter variable here (vars.put(...)) and referencing it as ${myVar} in the sampler works correctly.
• Scope matters: a PreProcessor attached directly to a sampler only runs for that sampler; one attached to a Thread Group or Controller runs before every sampler underneath it.
• Prefer Groovy over BeanShell for the same reason as the JSR223 Sampler — BeanShell’s per-iteration interpretation cost adds up fast across thousands of iterations.
• Common use: computing an HMAC signature or JWT right before an HTTP Request that needs it in a header.

Related

• Correlation & Dynamic Values
• Regex Extractor Builder
• JSR223 Groovy Scripting Guide
• Full Component Reference
• Functions and Variables