Java™ class files are upward compatible (JDK 1.1.x to 1.2.x to 1.3.x to 1.4.x to 1.5.x) as long as they do not make use of a feature for which Sun has dropped or changed support.
See The Source for Java Technology java.sun.com for information on release-to-release availability.
When Java programs on an iSeries™ server are optimized using Create Java Program (CRTJVAPGM) command, a Java Program (JVAPGM) is attached to the class file. The internal structure of these JVAPGMs changed on V4R4. This means that JVAPGMs created before V4R4 are not valid on V4R4 and later releases. You must recreate the JVAPGMs or the system automatically creates a JVAPGM at the same optimization level as before. It is, however, recommended that you manually perform a CRTJVAPGM, especially with JAR or ZIP files. This produces the best optimization with the smallest program size.
For best performance at optimization level 40, it is recommended to preform CRTJVAPGM on each i5/OS™ release or JDK version change. This is especially true if the JDKVER facility is used on CRTJVAPGM, as this results in methods from the Sun JDK being inlined into your JVAPGM. This can be a great advantage to performance. If, however, changes are made in the JDK on subsequent releases that invalidate those inlines, the programs may actually run slower than at lower optimizations. This is because special case code must be run to get proper operation.
See Java runtime performance for more detailed performance information.