eBook > The JMeter Playbook: Build, Scale, and Optimize Performance Tests

Variables & Functions

Apart from loading data from CSV files, JMeter offers several other ways for creating and manipulating variables in the test design or run time.

Back to top

User Defined Variables

The simplest way to set a variable is through the User Defined Variables config element (or the Test Plan’s built-in variable table).

Right-click Test Plan → Add → Config Element → User Defined Variables.

NameValue
baseUrlhttps://demoblaze.com
timeout5000

Reference them as ${baseUrl} and ${timeout} where required.

Note: User Defined Variables are evaluated once at test start. They do not change between iterations. For per-iteration values, use CSV Data Set Config or functions.

Back to top

JMeter Functions

JMeter includes a rich library of built-in functions that could be embedded in any field using the syntax ${__functionName(args)}. You can explore and test all available functions via Tools → Function Helper Dialog in the GUI.

Commonly Used Functions

FunctionPurposeExampleOutput
__RandomRandom integer in given range${__Random(1,100,)}42
__RandomStringRandom string of given length from the characters provided${__RandomString(8,abcdef0123456789,)}a3f0c1b9
__timeCurrent timestamp in different formats${__time(yyyy-MM-dd,)}2026-04-04
__UUIDRandom GUID-like structure${__UUID()}550e8400-e29b-...
__threadNumCurrent thread (virtual user) number${__threadNum}5
__iterationNumCurrent Thread Group iteration (loop) number${__iterationNum}3
__P / __propertyRead a JMeter property${__P(host,localhost)}localhost (default)
__setPropertySet a global property${__setProperty(token,abc123,)} 
__VEvaluate a nested variable${__V(user_${index})}value of user_3
__groovyInline Groovy expression${__groovy(vars.get("count").toInteger()+1,)}6

Example: Unique Email per Thread

${__threadNum}+user_${__RandomString(5,abcdefghijklmnop,)}@test.com

This produces something like 5+user_kfmab@test.com ; random enough for registration tests if the load is not too high.

Back to top

Setting Variables at Runtime

JSR223 PreProcessor / PostProcessor

For complex logic, use a JSR223 Test Elements with Groovy (recommended for performance):

import java.time.Instant

// Set a variable scoped to the current thread
vars.put("requestId", UUID.randomUUID().toString())
vars.put("timestamp", Instant.now().toString())

Then reference ${requestId} and ${timestamp} in your sampler.

BeanShell vs. Groovy

LanguagePerformanceRecommendation
GroovyCompiled, fast✅ Always prefer Groovy
BeanShellInterpreted, slow❌ Avoid in load tests
JavaScript (Nashorn)ModerateRemoved in Java 15+

Best practice: Always select groovy in the Language dropdown of JSR223 test elements and check the “Cache compiled script if available” box.

Back to top

Variables vs. Properties: When to Use Which

NeedUse
Data unique to each thread (usernames, tokens local to thread/virtual user)Variable - vars.put() / ${varName}
Data shared across thread groups (i.e. global token from setUp Thread Group)Property - props.put() / ${__P(name)}
Value passed from the command lineProperty - -Jname=value

Passing Properties from the Command Line

jmeter -n -t test.jmx -Jhost=api.staging.example.com -Jthreads=100

In the test plan:

Server Name       : ${__P(host,localhost)}
Number of Threads : ${__P(threads,1)}
Ramp-up period    : ${__P(ramp-up,1)}
Loop Count        : ${__P(loops,1)}
Duration          : ${__P(duration,60)}

The second argument is a default value used if the property is not supplied. This way, you can parameterize your test from the command line without having to mofify the test plan file itself.

Back to top

Summary

  • User Defined Variables are evaluated once at test start - good for constants and to avoid duplicate data.
  • Functions (__Random, __time, __UUID, etc.) generate dynamic values inline.
  • JSR223 + Groovy gives you full combined programming power of JDK, JMeter API and Groody SDK for complex variable logic.
  • Use Properties for global or command-line-driven values; use Variables for thread-local data.

Not let’s see how JMeter Post-Processors (Extractors) can extract data from responses and save it into variables for later re-use.

Back to top