JMeter BeanShell PreProcessor

Pre-Processors execute before the sampler runs. They allow you to dynamically manipulate variables, set up preconditions, or modify requests on the fly.

BeanShell PreProcessor

This pre-processor uses BeanShell scripting (a Java-like language) to run code. It has access to JMeter variables (vars), properties, and logging functions (log).

An example of a JMeter Pre-Processor and a breakdown of how it works. This example uses a BeanShell PreProcessor to modify a variable before the sampler runs.

• Setup:

  • Create a new Thread Group.
  • Add a Sampler (e.g., HTTP Request) under the Thread Group.
  • Right-click on the HTTP Request and add a Pre-Processor → BeanShell PreProcessor.

• Script Example:

  <span class="hljs-comment">// Example: Append a timestamp to a variable for uniqueness</span>

  <span class="hljs-type">String</span> <span class="hljs-variable">currentTime</span> <span class="hljs-operator">=</span> String.valueOf(System.currentTimeMillis());


  <span class="hljs-comment">// Set a JMeter variable 'uniqueId'</span>

  vars.put(<span class="hljs-string">"uniqueId"</span>, <span class="hljs-string">"ID_"</span> + currentTime);


  <span class="hljs-comment">// Print to JMeter log (optional)</span>

  log.info(<span class="hljs-string">"Generated uniqueId: "</span> + vars.get(<span class="hljs-string">"uniqueId"</span>));

  

• Explanation:

  • Purpose: This pre-processor script generates a unique identifier by appending the current timestamp to the prefix “ID_”. This can be useful for ensuring each request has unique data, such as for avoiding caching issues.
  • vars.put(...): Saves the computed value into a JMeter variable named uniqueId that can be referenced later in your HTTP Request (e.g., in parameters or headers) using ${uniqueId}.
  • log.info(...): Logs the generated ID to the JMeter log file for debugging purposes.

 

Example Use Case

Imagine you need to send a unique value in every HTTP request to avoid server caching issues. Instead of manually updating this value, you can use a pre-processor to automatically generate and set it. Later in your HTTP Request, you can refer to the variable as ${uniqueId}, and each request will have a unique identifier based on the current time.