Introducing the Host Access Class Library

The Host Access Class Library (HACL) for Java provides a set of classes and methods that allow the development of platform independent applications that can access host information at the data stream level. HACL implements the core host access function in a complete class model that is independent of any graphical display and requires only a Java-enabled browser or comparable Java™ environment to operate. The class library represents an object-oriented abstraction of a host connection that includes reading and writing the host presentation space, enumerating the fields in the presentation space, reading the operator information area (OIA) for status information, transferring files, host printing, and performing asynchronous notification of significant events.

With HACL, application developers can write Java applets that manipulate data from the data stream presentation space (such as 3270, 5250, and Virtual Terminal) without requiring the users to have the applets residing on their machines. The presentation space represents a virtual terminal screen that contains both data and associated attributes presented by host applications. HACL Java applets can open a session to the host, wait for incoming host data, get specific strings from the imaginary screen, get associated attributes of the strings, set new string values, send data stream function keys back to the host, and wait for the next host response. After an interaction is complete, the applet can switch to other tasks or simply close the session. The entire operation can be done without ever showing host screens.