Initial support for talking to a Festival server from java is included from version 1.3.0 and initial JSAPI support is included from 1.4.0. At present the JSAPI talks to a Festival server elsewhere rather than as part of the Java process itself.
A simple (Pure) Java festival client is given festival/src/modules/java/cstr/festival/Client.java with a wraparound script in festival/bin/festival_client_java.
See the file festival/src/modules/java/cstr/festival/jsapi/ReadMe for requirements and a small example of using the JSAPI interface.