Host Access Class Library for Java

Introduction to the Host Access Class Library for Java

The Host Access Class Library (HACL) for Java provides a core 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 only requires 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, 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.

HACL is a significant improvement over traditional emulator programming interfaces, like EHLLAPI, in several ways. These improvements are:

• EHLLAPI requires significant preparation before and interpretation after function calls.
• The one EHLLAPI entry point function is confusing and without context.
• EHLLAPI has non-intuitive mnemonic key values. For example, in EHLLAPI the mnemonic for Enter is @E and in HACL it is [enter].
• HACL is an object-oriented API, with all the well known benefits of the object-oriented programming paradigm.

• It is platform independent.
• It operates directly on the data stream rather than on the interpreted emulator screen. This eliminates the overhead of interpreting and displaying the data stream in a visual window.
• It does not require emulator software running on the local workstation, reducing dependencies on platform-specific screen formats and keyboard layouts.
• It is downloadable and executable on client workstations using standard Web and Java technology. This provides significant maintenance and resource savings.

See Host Access Class Library Java Objects for details on specific classes.

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