Online Workshops
Seize the deal!
Caching Best Practices
2.5 hours
High-Performance SQL
4 hours
High-Performance SQL
12 hours
JVM Boolean Options
Last modified:
Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly. Hypersistence Optimizer is that tool!
Introduction
While trying to generate a Java Heap Dump, I remembered there is one JVM option I could use for this purpose. Since I can’t always remember these options’ names, I went to the Oracle documentation.
The problem
So I could extract the following arguments:
-XX:-HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/logs/jvm/dumps/
After limiting my Java Heap size to a value that I know it’s too low, I was expecting the Heap Dump to be generated whenever I got an OutOfMemoryError. But no dump got generated. I googled the issue, checked for JVM bugs but the only reported issue was a miss-usage when you give the JVM options after the Java Main class, but that wasn’t my case.
The fix
Then I stumbled on a slightly different version of my original setting (the one that I copy-pasted from the Oracle site):
-XX:+HeapDumpOnOutOfMemoryError
Then, I remember I once read about Boolean JVM options, and the very same Oracle site details this usage:
“Boolean options are turned on with -XX:+ and turned off with -XX:-.”
I think the Oracle JVM options table should show the “+” version, since that’s usually what you are looking for, especially because it’s disabled by default (so the “-” version behaves like not giving it at all).
Online Workshops
If you enjoyed this article, I bet you are going to love my upcoming Online Workshops!
- Caching Best Practices with JPA and Hibernate (2.5 hours) on the 30th of September
- High-Performance SQL (4 hours) on the 6th of October in collaboration with Voxxed Days Ticino
- High-Performance SQL (12 hours) starting on the 28th of October in collaboration with Bouvet
Conclusion
The HeapDumpPath should point to a folder, but if your setting is something like /logs/jvm/dumps/, and your OS only contains /logs/jvm, then you won’t get a “java_pid.hprof” file within the /logs/jvm/dumps/ folder, but a dump file in /logs/jvm/, as the JVM doesn’t create the missing folders (a.k.a mkdirs).
