Clinical Quality Language (CQL) is a high-level, domain-specific language focused on clinical quality and targeted at measure and decision support artifact authors. HAPI embeds a CQL engine allowing the evaluation of clinical knowledge artifacts that use CQL to describe their logic.
A more detailed description of CQL is available at the CQL Specification Implementation Guide
The FHIR Clinical Reasoning module defines a set of resources, profiles, operations, etc. that can be used to work with clinical knowledge within FHIR. HAPI provides implementation for some of those operations, described in more detail below.
A complete working example of HAPI CQL can be found in the JPA Server Starter project. You may wish to browse its source to see how it is set up.
HAPI provides implementations for some operations using CQL in DSTU3 and R4: