Recently we run into a serious problem with our JBoss 4.2.1 server. About an 
hour (sometimes shorter) after the server is started, many threads (hundreds of 
them) start frequent lockup, all waiting to acquire a "synchronized" lock on 
AppClassLoader. And the thread that holds the lock has the following stack 
trace:

java.lang.ClassLoader.findBootstrapClass(Native Method)
java.lang.ClassLoader.findBootstrapClass0(ClassLoader.java:891)
java.lang.ClassLoader.loadClass(ClassLoader.java:301)
java.lang.ClassLoader.loadClass(ClassLoader.java:299)
sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
java.lang.ClassLoader.loadClass(ClassLoader.java:251)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1273)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1204)
-- snip --

I did a little timing exercise. This simple findBootstrapClass() call, which 
usually takes a few milliseconds at most, would take 20 seconds to 
multi-minutes to complete. During this time, since AppClassLoader.loadClass() 
is synchronized, the entire application simply stops and the requests quickly 
stack up and crash the application as a result.

I'm not saying this is a JBoss or JVM bug (though could be), since this only 
happens after we recently deploys our new application code. Some change in our 
code triggered this behavior but we don't know what it is, and this behavior 
only shows up in one of our JBoss servers.

Any experience on this kind of lockup? This problem has jeopardized our release 
schedule and any help will be greatly appreciated. 

Ting Zhou 

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