Expression and EL

Work with the session with the Expression Language

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, but 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:

"${foo.size()}"             // expects `foo` to point to a Scala or Java collection attribute and returns its size
"${foo.random()}"           // expects `foo` to point to an indexed collection and returns a random element
"${foo.exists()}"           // returns true if the session contains a `foo` attribute, false otherwise
"${foo.isUndefined()}"      // returns true if the session doesn't contains a `foo` attribute, false otherwise
"${foo(5)}"                 // expects `foo` to point to an indexed collection or a Tuple and returns the 5-th element
"${foo(n)}"                 // expects `foo` to point to an indexed collection or a Tuple and `n` to point to an Int and returns the n-th element
"${}"                // expects foo to point to a Map and returns the value associated with key `bar`
"${foo._2}"                 // expects `foo` points to a Tuple and returns the second element (identical to idiomatic Scala Tuple syntax, 1 based index)
"${foo.jsonStringify()}"    // properly formats into a JSON value (wrap Strings with double quotes, deal with null)
"${currentTimeMillis()}"    // System.currentTimeMillis
"${currentDate(<pattern>)}" // new Date() formatted with a java.text.SimpleDateFormat pattern

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.

Escaping ${

To prevent "${" from being interpreted by the EL compiler, add a $ before it. "$${foo}" will be turned into "${foo}".

If you want a $ before the placeholder, add another $. Assuming the session attribute foo holds "FOO", "$$${foo}" will be turned into "$FOO".

This can go on and on. In general, if there are 2n-1 $ characters before ${ – an even number of $ characters totally – there will be n $ before { in the final string; if there are 2n $ before ${ – an odd number totally – there will be n $ before the placeholder.


Most Gatling DSL methods actually take Expression[T] parameters, which is a type alias for Session => Validation[T]. This way, one can pass functions to generate parameters, possibly based on the Session’s content.

exec(http("Inline").get(session => "/inline/" + session("param").as[String].toLowerCase))

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.

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