Expressions and predicates are built-in types in Camel that you’ve seen used throughout this book. They’re very versatile and are used in different places, but they’re most noticeable in the Camel routes. For example, predicates are used in the Message Filter and Content-Based Router EIPs. Expressions are used for computing correlation keys for the Aggregator EIP, and appendix A covered the Simple expression language, which is another testament to the versatility of expressions.
A Camel expression (org.apache.camel.Expression) is evaluated at runtime on the instance of Exchange that is under processing. You can use either standard or custom expressions.