The HACL for Java is a set of classes and methods that allow application programmers to access host applications at the 3270 and 5250 data stream levels easily and quickly. HACL implements the core host access function in a complete class model that is independent of any graphical display and only requires a Java-enabled browser or comparable Java environment to operate.
see https://www-01.ibm.com/software/network/commserver/windows/library/clsvr30.htm