The noclassdeffound is due to the missing Scala library so adding it to web-inf/lib is correct. Adding a library to the classpath won't lead to out of memory errors though, so it seems to be related to your app.

What is the class ExecutionEngine? What's the stacktrace (although that won't necessarily help). Are you sure your webapp is doing the same as your java app which has no problems? How are you testing your webapp? Could it simply be that many concurrent threads (requests) cause the OOME, but one thread works fine (like the plain java app)?

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