March 1, 2021
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Java helps bring old code to the Web

  • By Rick Grehan
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Vendors are giving their Java tools legacy chops
The Java world is making a play for becoming the legacy system conversion tool of choice. For example, Java became available on the AS/400 in February '98, and IBM is already promoting it as the "RPG of the future for that platform." Ironically, Java and the AS/400 share some architectural similarities, such as being built on a two-layer architecture. For an executing Java app, the upper layer is the bytecode representation of the program. Bytecodes are executed within a JVM, which contains all the target-specific (hardware dependent) logic. Meanwhile, apps on the AS/400 execute in the upper layer-called OS/400-which communicates to a lower-level, hardware-dependent layer, called SLIC (System Licensed Internal Code).

Some impedance mismatches intrude, however. For example, it took two years to bring Java to the AS/400, the main stumbling block being that OS/400 lacked thread support. In addition, once the engineers on the porting project decided-for performance reasons-to implement the AS/400 JVM in the lower-level SLIC layer, rather than running the JVM under OS/400, they were faced with the fact that the Javasoft JVM was written in C, while SLIC was written in C++. So the engineers had to make several enhancements to implement a SLIC-level JVM.

Some legacy upgrades require no more than access to the mainframe database. In such cases, IBM's VisualAge for Java can help. VisualAge provides support through IBM's NCF (network computing framework) for accessing a DB2 database through a JDBC/DB2 interface. VisualAge's Enterprise Access Builder will query a DB2 database, then query the tables within that database at the user's selection. VisualAge can generate access beans for tables selected within the database. You can then use VisualAge's Visual Composition Editor to construct applets that access the DB2 database.

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This article was originally published on August 1, 1999

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