Compare and Contrast JOLAP and XML for Analysis and Intelligent Business Strategies: OLAP in the Database. I've covered some of this previously. Especially, JMI.
"JOLAP is a J2EE objected-oriented application programming interface (API) designed specifically to addresses the programming needs of Java developers by providing a standard set of object classes and methods for BI."
"XMLA (www.xmla.org) is a linguistic interface with no preference for programming language or object model. This linguistic interface is implemented as a web services and also defines a standard query language (mdXML) for BI."
"Hyperion views JOLAP and XMLA as complementary rather than competing standards. Although you can implement XMLA without using JOLAP, the JOLAP specification supports the web services architecture that depends on J2EE application servers, XML, and SOAP messages.
In fact, Hyperion’s implementation of XMLA uses our Java API (which was developed based on our JOLAP specification work) to communicate with the Essbase Analytic Services (OLAP Server). Our XMLA web service accepts a SOAP message, takes the mdXML statement contained in the SOAP message, and passes it to the Analytic Services engine for processing through the Java API. The result set is passed back to the XMLA web service through the Java API, where it is wrapped in a SOAP message and sent to the requesting client."