Appendix A. Simple, the expression language

 

Camel offers a powerful expression language, which back in the earlier days wasn’t as powerful and was labeled Simple. It has evolved to become much more since then, but don’t worry: it’s still simple to use.

The Simple language is provided out of the box in the camel-core JAR file, which means you don’t have to add any JARs on the classpath to use it.

A.1. Introducing Simple

In a nutshell, the Simple expression language evaluates an expression on the current instance of Exchange that is under processing. The Simple language can be used for both expressions and predicates, which makes it a perfect match to be used in your Camel routes.

For example, the Content-Based Router EIP can leverage the Simple language to define predicates in the when clauses, as shown here: 

from("activemq:queue:quotes")
    .choice()
        .when(simple("${body} contains 'Camel'")).to("activemq:camel")
        .when(simple("${header.amount} > 1000")).to("activemq:bigspender")
        .otherwise().to("activemq:queue:other");

The equivalent Spring XML example would be as follows:

A.2. Syntax

A.3. Built-in variables

A.4. Built-in functions

A.5. Built-in file variables

A.6. Built-in operators

A.7. The OGNL feature

A.8. Using Simple from custom Java code

A.9. Summary