Hi,
We had an issue in our project where large hql expressions holding on lock to
ChangeAwareClassloader in weblogic.
We had an hql query which was like
select * from ClassA a where a.identifier = 'id1' or a.identifier='id2' or
..... a.identifier='id1000'.
This was certainly bad to have this big or expression, but an issue this caused
in weblogic is something interesting.
Hibernate query engine processes each identifier in a query to see if its a
Java Class. So for each one of ClassA, a.identifier etc it calls Class.forName,
if NoClassDefFound exception is thrown, ReflectHelper.getConstantValue returns
null.
Now the problem is, if the expression is huge, recursive calls to
/NodeTraverser.visitDepthFirst create huge stack trace. While building
ClassNotFoundException, system class loader gets stuck to build all this stack
trace. Because weblogic/utils/classloaders/ChangeAwareClassLoader.loadClass
method is synchronized, any other thread trying to load any other class is
blocked. If the stack trace is huge and taking consideratble amount of time to
create Exception object, the lock is held for that much time, blocking every
other thread trying to any the class. It brought down our servers in test
environment.
Is this a known issue in weblogic and hibernate? I know that building such a
large expression is bad(and we have already changed the code which was doing
that), but the way its blocking the classloader in weblogic is an issue I
think.
Following is the thread dump.
"[STUCK] ExecuteThread: '9' for queue: 'weblogic.kernel.Default (self-tuning)'"
id=614 idx=0x3f8 tid=15385 prio=1 alive, in native, daemon
at pthread_cond_wait@@GLIBC_2.3.2+170(:0)@0x318c008a7a
at vmtWaitUntilNotSoftSuspended+66(:0)@0x2a955e35c2
at find_codeinfo+138(:0)@0x2a955b032e
at cmFindStackCodeInfoWithHint+44(:0)@0x2a955b040e
at frameIterFindCodeInfo+19(:0)@0x2a9571b11f
at frameIterGetCurrentAndStep+130(:0)@0x2a9571b489
at jniFillInStackTrace+368(:0)@0x2a95630d25
at JVM_FillInStackTrace+9(:0)@0x2a95647661
at RJNI_jrockit_vm_Reflect_fillInStackTrace0+9(:0)@0x2a956f5983
at jrockit/vm/Reflect.fillInStackTrace0(Ljava/lang/Throwable;)V(Native
Method)
at java/lang/Throwable.fillInStackTrace()Ljava/lang/Throwable;(Native
Method)[optimized]
at java/lang/Throwable.<init>(Throwable.java:218)[optimized]
at java/lang/Exception.<init>(Exception.java:59)[inlined]
at
java/lang/ClassNotFoundException.<init>(ClassNotFoundException.java:65)[optimized]
at java/net/URLClassLoader$1.run(URLClassLoader.java:200)[inlined]
at
jrockit/vm/AccessController.doPrivileged(AccessController.java:255)[inlined]
at java/net/URLClassLoader.findClass(URLClassLoader.java:188)[optimized]
at java/lang/ClassLoader.loadClass(ClassLoader.java:306)[optimized]
^-- Holding lock: java/net/urlclassloa...@0x4993580[thin lock]
at java/lang/ClassLoader.loadClass(ClassLoader.java:299)[optimized]
^-- Holding lock:
weblogic/utils/classloaders/genericclassloa...@0x469e900[thin lock]
at java/lang/ClassLoader.loadClass(ClassLoader.java:251)[inlined]
at
weblogic/utils/classloaders/GenericClassLoader.loadClass(GenericClassLoader.java:158)[optimized]
at
weblogic/utils/classloaders/FilteringClassLoader.findClass(FilteringClassLoader.java:83)[inlined]
at
weblogic/utils/classloaders/FilteringClassLoader.loadClass(FilteringClassLoader.java:68)[optimized]
at java/lang/ClassLoader.loadClass(ClassLoader.java:299)[optimized]
^-- Holding lock:
weblogic/utils/classloaders/genericclassloa...@0x4969138[thin lock]
at java/lang/ClassLoader.loadClass(ClassLoader.java:299)[inlined]
at java/lang/ClassLoader.loadClass(ClassLoader.java:251)[inlined]
at
weblogic/utils/classloaders/GenericClassLoader.loadClass(GenericClassLoader.java:158)[inlined]
at
weblogic/utils/classloaders/ChangeAwareClassLoader.loadClass(ChangeAwareClassLoader.java:35)[optimized]
^-- Holding lock:
weblogic/utils/classloaders/changeawareclassloa...@0x4915178[recursive]
at jrockit/vm/Classes.loadClassInternal(Classes.java:75)[optimized]
^-- Holding lock:
weblogic/utils/classloaders/changeawareclassloa...@0x4915178[fat lock]
at jrockit/vm/RNI.c2java(JJJJJ)V(Native Method)
at
jrockit/vm/Classes.forName0(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;(Native
Method)
at jrockit/vm/Classes.forName(Classes.java:125)[inlined]
at java/lang/Class.forName(Class.java:164)[inlined]
at
org/hibernate/util/ReflectHelper.classForName(ReflectHelper.java:100)[inlined]
at
org/hibernate/util/ReflectHelper.getConstantValue(ReflectHelper.java:122)[inlined]
at
org/hibernate/hql/ast/QueryTranslatorImpl$JavaConstantConverter.handleDotStructure(QueryTranslatorImpl.java:569)[inlined]
at
org/hibernate/hql/ast/QueryTranslatorImpl$JavaConstantConverter.visit(QueryTranslatorImpl.java:564)[optimized]
at
org/hibernate/hql/ast/util/NodeTraverser.visitDepthFirst(NodeTraverser.java:40)[optimized]
Following is part of thread dump which shows a thread waiting for Classloader
lock.
"[ACTIVE] ExecuteThread: '51' for queue: 'weblogic.kernel.Default
(self-tuning)'" id=7096 idx=0x6f8 tid=7459 prio=5 alive, in native, blocked,
daemon
-- Blocked trying to get lock:
weblogic/utils/classloaders/changeawareclassloa...@0x4915178[fat lock]
at pthread_cond_wait@@GLIBC_2.3.2+170(:0)@0x318c008a7a
at tsiWaitForSignalForever+58(:0)@0x2a957035b5
at tsiWaitForSignal+39(:0)@0x2a95703610
at tsiWaitForLockSignal+39(:0)@0x2a957036f6
at tsWaitForJavaLockSignal+31(:0)@0x2a957037a5
at RJNI_jrockit_vm_Threads_waitForUnblockSignal+14(:0)@0x2a956f7620
at jrockit/vm/Threads.waitForUnblockSignal()V(Native Method)
at jrockit/vm/Locks.fatLockBlockOrSpin(Locks.java:1674)[optimized]
at jrockit/vm/Locks.lockFat(Locks.java:1775)[optimized]
at jrockit/vm/Locks.monitorEnterSecondStageHard(Locks.java:1311)[optimized]
at jrockit/vm/Locks.monitorEnterSecondStage(Locks.java:1258)[optimized]
at
weblogic/utils/classloaders/ChangeAwareClassLoader.loadClass(ChangeAwareClassLoader.java:35)[optimized]
at
com/sun/org/apache/xerces/internal/impl/dv/ObjectFactory.findProviderClass(ObjectFactory.java:403)[inlined]
at
com/sun/org/apache/xerces/internal/impl/dv/ObjectFactory.newInstance(ObjectFactory.java:354)[inlined]
at
com/sun/org/apache/xerces/internal/impl/dv/DTDDVFactory.getInstance(DTDDVFactory.java:98)[optimized]
^-- Holding lock: java/lang/cl...@0x4fba6b8[recursive]
at
com/sun/org/apache/xerces/internal/impl/dv/DTDDVFactory.getInstance(DTDDVFactory.java:83)[inlined]
at
com/sun/org/apache/xerces/internal/parsers/XML11Configuration.<init>(XML11Configuration.java:565)[inlined]
Thanks,
Unmesh
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