Expression and EL

Expression Language

Most Gatling DSL methods can be passed Expression Language Strings.

This is a very convenient feature to write dynamic values.

Gatling parses Strings parameter values and turn them into functions that will compute a result based on the data stored into the Session when they will be evaluated.

The Gatling EL is not a dynamic language, bug just placeholders with a few additional helpers.

If you need a full blown templating engine, you can use our Pebble support.

Gatling EL uses a ${attributeName} syntax, where attributeName is the name of an attribute in the Session.

For example:


Moreover, Gatling EL provide the following built-in functions:

"${foo.size()}"          // returns the size of `foo` if `foo` points to a Scala or Java collection attribute
"${foo.random()}"        // returns a random element of `foo` if `foo` points to an indexed collection
"${foo.exists()}"        // returns true if the session contains a `foo` attribute, false otherwise
"${foo.isUndefined()}"   // returns true if the session doesn't contains a `foo` attribute, false otherwise
"${foo(5)}"              // returns the 5th element of `foo` if `foo` points to an indexed collection
"${foo(n)}"              // returns the n-th element of `foo` if `n` points to an Int and `foo` to an indexed collection
"${}"             // returns the value associated with key `bar` if `foo` points to a map
"${foo._2}"              // returns the second element if `foo` points to a Tuple object
"${foo.jsonStringify()}" // properly formats into a JSON value (wrap Strings with double quotes, deal with null)

You can also combine different Gatling EL builtin functions. For example if foo is a List of Lists ${foo(0)(0)} will return first element of the first list in foo. ${foo.list.random()} will return random element from an indexed collection associated with key list in a map foo.

Gatling EL supports the following indexed collections: java.util.List, Seq and Array. It also supports both Scala and Java maps. Function .size supports any Scala or Java collection.


This Expression Language only works on String values being passed to Gatling DSL methods. Such Strings are parsed only once, when the Gatling simulation is being instanciated.

For example queryParam("latitude", session => "${latitude}") wouldn’t work because the parameter is not a String, but a function that returns a String.

Also, queryParam("latitude", "${latitude}".toInt) wouldn’t because the toInt would happen before passing the parameter to the queryParam method.

The solution here would be to pass a function:

session => session("latitude").validate[Int].


By default, IntelliJ will automatically prepend your String with an s as soon as you start typing ${ because it thinks you want to use Scala’s String interpolation. You need to remove this s to use Gatling EL.


Most Gatling DSL methods actually takes Expression[T] parameters, which is a type alias for Session => Validation[T].

How is it that one can also pass Strings and other values then?

The reason is that there are implicit conversions:

  • when passing a String, it gets automagically parsed turn them into Expressions thanks to Gatling EL compiler.
  • when passing a value of another type, it gets automagically wrapped into an Expression that will always return this static value.


Implicit conversions are only triggered when expected type and passed parameter type don’t match, for example trying to pass a String to a method that expects an Expression instead. Those implicit conversions are triggered at compile time.


For more information about Validation, please check out the Validation reference.