As we will see later, most of the EIP need a rule definition to apply a routing logic to a message. The rule is described using an expression.

It means that we have to define expressions or predicates in the Enterprise Integration Patterns. An expression returns any kind of value, whereas a predicate returns true or false only.

Camel supports a lot of different languages to declare expressions or predicates. It doesn't force you to use one, it allows you to use the most appropriate one.

For instance, Camel supports xpath, mvel, ognl, python, ruby, PHP, JavaScript, SpEL (Spring Expression Language), Groovy, and so on as expression languages. It also provides native Camel prebuilt functions and languages that are easy to use such as header, constant, or simple languages.