Expressions
Expressions can be used to access variables and calculate values dynamically.
This is particularly useful when automating a process using BPMN and orchestrating human tasks.
Some attributes of BPMN elements require an expression, for example, a sequence flow condition on an exclusive gateway. Other attributes can define an expression optionally as an alternative to a static value, for example, a timer definition of a timer catch event.
Expressions vs. static values​
Some attributes of BPMN elements, like the timer definition of a timer catch event, can be defined in one of two ways:
- As an expression (e.g.
= remainingTime) - As a static value (e.g.
PT2H)
Expressions always start with an equals sign (=). For example, = order.amount > 100. The text following the equal sign is the actual expression. For example, order.amount > 100 checks if the amount of the order is greater than 100.
If the element does not start with the prefix, it is used as a static value. A static value is used either as a string (e.g. job type) or as a number (e.g. job retries). A string value must not be enclosed in quotes.
An expression can also define a static value by using literals (e.g. = "foo", = 21, = true, = [1,2,3], = {x: 22}, etc.)
The expression language​
An expression is written in Friendly Enough Expression Language (FEEL). FEEL is part of the OMG's Decision Model and Notation (DMN) specification. It is designed to have the following properties:
- Free of side effects
- Simple data model with JSON-like object types: numbers, dates, strings, lists, and contexts
- Syntax designed for business professionals and developers
- Three-valued logic (true, false, null)
Camunda 8 integrates the FEEL Scala engine to evaluate FEEL expressions.
Next steps​
Read more about FEEL expressions and how to use them on the following pages: