Why does Apache JMeter print a banner telling you not to load test in GUI mode?

## What the banner actually says `JMeterGuiLauncher.startGui` prints eight lines to standard output before the splash screen even appears: ``` ================================================================================ Don't use GUI mode for load testing !, only for Test creation and Test debugging. For load testing, use CLI Mode (was NON GUI): jmeter -n -t [jmx file] -l [results file] -e -o [Path to web report folder] & increase Java Heap to meet your test requirements: Modify current env variable HEAP="-Xms1g -Xmx1g -XX:MaxMetaspaceSize=256m" in the jmeter batch file Check : https://jmeter.apache.org/usermanual/best-practices.html ================================================================================ ``` It is unconditional. Every GUI launch prints it, whether you go on to run anything or not, and there is no property that turns it off. The equivalent note is repeated in the manual: *"GUI mode should only be used for creating the test script, CLI mode (NON GUI) must be used for load testing."* ## Why JMeter draws the line where it does The banner is a statement about the process, not a style preference: - In GUI mode the load threads and the Swing event thread live in **one JVM**. Repainting trees, tables and graphs is work that JVM does instead of driving requests. - Every listener you can see is a listener that is holding results. `View Results Tree` and `View Results in Table` keep sample data in memory so they can render it; the manual names both explicitly as things to use only while scripting. - The banner's own second half is about heap for exactly that reason — a GUI session with listeners open needs headroom that has nothing to do with the test. The general question of how you tell load-generator overhead apart from slowness in the system under test is performance-testing groundwork, not a JMeter feature, and is deliberately not answered here. JMeter's own contribution is narrower and concrete: use `-n`. ## What `-n` changes in the process - The launcher inspects the raw argument array for `-n`, `-s` or `-g`. If one is present and `java.awt.headless` has not already been set, it sets it to `true`, so no display is needed. - `JMeterGuiLauncher` is never reached, so no `MainFrame`, no splash screen, no look-and-feel setup, and no banner. - The system property `JMeter.NonGui` is set to `"true"`, which elements can read. - The built-in Summariser takes over reporting on the console, since `bin/jmeter.properties` ships `summariser.name=summary` uncommented. ## What the GUI is still for The banner says "Test creation and Test debugging", and that is the whole of it: 1. Building and editing the plan tree, with the element help and the Function Helper Dialog. 2. A one- or two-thread smoke run with `View Results Tree` open, to see request and response bodies while you get extractors and assertions right. 3. Opening a finished JTL in a listener to inspect a handful of rows. ## The mistake the banner is trying to prevent The failure it is aimed at is not a crash. A GUI run finishes, the listeners fill up, and the numbers on screen look like a result. Nothing marks them as untrustworthy. That is why the project chose a loud, unconditional console banner over a log line: by the time you would notice, you already have a chart. Two practical consequences follow: - Treat any measurement produced from a GUI session as scripting output, not as a run. - Once the plan is right, delete or disable the GUI listeners and let `-l` write the JTL, which is what the banner's own suggested command line does.