High CPU due to multiple Java threads accessing HashMap simultaneously

Environment

• Java
• JBoss Enterprise Application Platform (EAP)

Issue

• High cpu and threads consuming the CPU have HashMap access at the top of the stack trace. For example:

"RMI TCP Connection(4468)-160.57.110.76" daemon prio=6 tid=0x5a436400 nid=0x9e4 runnable [0x6068d000]
   java.lang.Thread.State: RUNNABLE
     at java.util.HashMap.get(HashMap.java:303)
     at com.example.service.getWidget(Component.java:796)
...

"WorkManager(3)-122" daemon prio=6 tid=0x5b496000 nid=0x18fc runnable [0x70b2e000]
   java.lang.Thread.State: RUNNABLE
     at java.util.HashMap.get(HashMap.java:303)
     at com.example.service.getWidget(Component.java:796)
...

Resolution

• Replace HashMap with ConcurrentHashMap.

Root Cause

• HashMap is not a thread-safe structure.
• Accessing a HashMap from multiple threads simultaneously can result in an infinite loop, causing high cpu. See Content from bugs.sun.com is not included.Content from bugs.sun.com is not included.http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6423457. for more.
• Users of JDK 1.6 should ensure to upgrade to u33 or higher, due to this bug Content from bugs.java.com is not included.JDK-7027300 : Unsynchronized HashMap access causes endless loop and related backports.
• High CPU due to Richfaces HashMap.get() infinite loop
• High CPU in concurrent access to the JSSESupport keySizeCache map
• High cpu in beanutils WeakHashMap
• High CPU due to unsafe HashMap access in TomcatInjectionContainer.getEncInjectionsForClass
• High CPU in org.apache.axis.utils.JavaUtils.isEnumClass
• High CPU due to SunLayoutEngine.getEngine HashMap loops
• High CPU due to unsafe HashMap access in org.apache.felix.framework.util.WeakZipFileFactory$WeakZipFile
• Threads looping in WeakHashMap.get called from com.sun.facelets.tag.MetaRulesetImpl.getMetadataTarget on JBoss

Diagnostic Steps

• See Java application high CPU.

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