A customer reports (https://na1.salesforce.com/50030000000ZG7i) Tomcat http session replication throws NPE under load. Here is the example trace:

15:59:10,424 ERROR [JBossCacheManager] processExpires: failed with exception: java.lang.NullPointerException
15:59:10,425 INFO [STDOUT] java.lang.NullPointerException
15:59:10,426 INFO [STDOUT] at org.jboss.web.tomcat.tc5.session.JBossCacheManager.processExpires(JBossCacheManager.java:436)
15:59:10,426 INFO [STDOUT] at org.jboss.web.tomcat.tc5.session.JBossManager.backgroundProcess(JBossManager.java:644)
15:59:10,427 INFO [STDOUT] at org.apache.catalina.core.StandardContext.backgroundProcess(StandardContext.java:4647)
15:59:10,427 INFO [STDOUT] at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1619)
15:59:10,427 INFO [STDOUT] at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1628)
15:59:10,428 INFO [STDOUT] at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1628)
15:59:10,428 INFO [STDOUT] at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1608)
15:59:10,428 INFO [STDOUT] at java.lang.Thread.run(Thread.java:534)