Concurrency issue in ProxyFactory --------------------------------- Key: JASSIST-259 URL: https://issues.jboss.org/browse/JASSIST-259 Project: Javassist Issue Type: Bug Affects Versions: 3.12.0.GA Environment: AIX, IBM J9 VM (build 2.8, JRE 1.8.0 AIX ppc64-64 Compressed References 20150722_258693 (JIT enabled, AOT enabled) Reporter: Christian Beikov Assignee: Shigeru Chiba Priority: Critical Fix For: 3.21.0-GA Look at the following line to understand the issue https://github.com/jboss-javassist/javassist/blob/master/src/main/javassi... If there are 2 threads T1 and T2 at this line, T1 pauses and T2 runs through, on some systems T1 may encounter thisClass to be null after it wakes up again. This leads to Hibernate getting a null class in JavassistProxyFactory and trying to create an instance of that which fails with a NPE. Can anyone please confirm this issue? Maybe there is something in the JMM that I am not aware of that makes this code safe, but as far as I can tell, this looks like a real bug that just happens to uncover itself on an environment like J9 on AIX.