Re: [JBoss-dev] Possible Synchronization Issue in UnifiedLoaderRepository2

Wed, 09 Oct 2002 21:06:44 -0700

Title: Possible Synchronization Issue in UnifiedLoaderRepository2
I have ported the recent 3.2 class loader synch fixes to 3.0. In general class loading
cannot synch on the loader as it leads deadlock. Resource lookups could probably
synch on the resource map as I don't think it can induce class loading, but for now
it synchs on the repository class.
 
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Scott Stark
Chief Technology Officer
JBoss Group, LLC
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----- Original Message -----
Sent: Wednesday, October 09, 2002 1:29 PM
Subject: [JBoss-dev] Possible Synchronization Issue in UnifiedLoaderRepository2

Hi folks,

I am running the Jakarta James mail server in Jboss_3.0.2 and I'm frequently seeing ConcurrentModificationExceptions in the UnifiedLoaderRepository2 while Javamail is attempting to load it's protocol provider and address map properties files.  The full exception is:

    java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:762)
        at java.util.HashMap$KeyIterator.next(HashMap.java:798)
        at org.jboss.mx.loading.UnifiedLoaderRepository2.getResourceFromRepository(UnifiedLoaderRepository2.java:344)
        at org.jboss.mx.loading.UnifiedLoaderRepository2.getResource(UnifiedLoaderRepository2.java:295)
        at org.jboss.mx.loading.UnifiedClassLoader.getResource(UnifiedClassLoader.java:296)
        at java.lang.ClassLoader.getResourceAsStream(ClassLoader.java:941)
        at java.lang.Class.getResourceAsStream(Class.java:1298)
        at javax.mail.Session.loadProviders(Session.java:771)
        at javax.mail.Session.<init>(Session.java:81)
        at javax.mail.Session.getInstance(Session.java:103)
        at org.apache.james.transport.mailets.RemoteDelivery.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:536

I've taken a quick look at the UnifiedLoaderRepository2.java source file and it looks like there's something amiss with the synchronization around the classloader HashSets and their iterators.  Here's what the key methods do:

  - loadClassFromRepository has no synchronization.
  - getResourceFromRepository has no synchronization.
  - getURLs synchronizes on the object instance (this).
  - addClassLoader synchronizes at the class level (UnifiedLoaderRepository2.class)
  - removeClassLoader synchronizes at the class level (UnifiedLoaderRepository2.class)
  - findClass synchronizes on the object instance (this).

When I change them all to synchronize on the object instance, everything seems to work OK.

Steve

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