While Controller

Part of the Logic Controllers category. Also documented in context in the full Component Reference.

TL;DR: the While Controller repeats its children as long as a condition stays true (or forever, if left blank, until the enclosing loop/thread stops) — the standard element for polling patterns like “keep checking job status until it’s done.”

The While Controller runs its children until the condition is “false”.

Note

JMeter will expose the looping index as a variable named __jm__<Name of your element>__idx. So for example, if your While Controller is named WC, then you can access the looping index through \${__jm__WC__idx}. Index starts at 0

Possible condition values:

• blank - exit loop when last sample in loop fails
• LAST - exit loop when last sample in loop fails. If the last sample just before the loop failed, don’t enter loop.
• Otherwise - exit (or don’t enter) the loop when the condition is equal to the string “false”

Note

The condition can be any variable or function that eventually evaluates to the string “false”. This allows the use of [__jexl3](/functions/jexl3/), [__groovy](/functions/groovy/) function, properties or variables as needed.

Note

Note that the condition is evaluated twice, once before starting sampling children and once at end of children sampling, so putting non idempotent functions in Condition (like [__counter](/functions/counter/)) can introduce issues.

For example:

• \${VAR} - where VAR is set to false by some other test element
• \${__jexl3(\${C}==10)}
• \${__jexl3("\${VAR2}"=="abcd")}
• \${_P(property)} - where property is set to “false” somewhere else

Name	Required	Description
Name	No	Descriptive name for this controller that is shown in the tree, and used to name the transaction.
Condition	No	blank, LAST, or variable/function

Common gotchas

• A blank condition means loop forever (until the thread is told to stop) — a very common way to accidentally hang a test plan; always set an explicit exit condition or a max-iteration guard variable.
• The condition is re-evaluated after each pass through the children, so the loop always runs at least once — model that if your logic assumes a “check first, then maybe run” pattern.
• Same JavaScript-expression syntax as If Controller ("${done}" != "true"), including the same quoting pitfalls.
• For polling with a bounded wait, pair with a Constant Timer inside the loop (see Timers, Think Time & Pacing) so you’re not hammering the endpoint every iteration.

Related

• Logic Controllers & Flow Control
• Full Component Reference
• Functions and Variables