JEC is a Java Expression Compiler library (a JAR) that allows bidirectional expression evaluation and assignation (like book.pages[selectedPage].text). It was designed for mapping HTTP parameters to class fields in the model for a Java web controller.
Have a look at the javadoc until more information is provided.
JEC is quite simple and direct when you understand it. In order to help you to achieve that understanding, here are the main classes involved in expression evaluation: