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You need at least Java 7 to run ProM 6.4. Java 6 and earlier will not do.
In ProM 6.4, the OpenXES library is not longer part of the framework. Instead, it is now part of the Log package (see its lib folder). As a result, the ProM 6.4 framework is now unaware of event logs.
Furthermore, several third-party libraries that were used by different packages have been centralized, to avoid conflicting versions of these libraries. These library packages include ApacheUtils (Apache 2.0 license), BasicUtils (L-GPL license), EMF (Eclipse 1.0 license), and Saxon (Mozilla Public 1.0 license).
As a result of these changes, ProM 6.4 might not run the older packages (from 6.3 and earlier) properly and vice versa.
ProM 6.4 has discontinued the buffered implementation of XLog, due to possible deadlock problems with this implementation. The naive XLog implementation has been changed such that it now uses much less memory (thanks to String interning).
The new XESLite package also offers some other XLog implementations.