https://bz.apache.org/bugzilla/show_bug.cgi?id=64506
Bug ID: 64506
Summary: NullPointerException when loading webapp class
Product: Tomcat 9
Version: 9.0.27
Hardware: All
OS: Linux
Status: NEW
Severity: major
Priority: P2
Component: Catalina
Assignee: dev@tomcat.apache.org
Reporter: arvind.tal...@veeva.com
Target Milestone: -----
We have upgraded our Tomcat version to 9.0.27 from 8.0.42, since then we have
been pretty frequently running into the below NullPointerException when our
application classes get loaded, we believe this is due a concurrency issue
(more details below).
java.lang.NullPointerException
at
org.apache.catalina.webresources.CachedResource.getURL(CachedResource.java:317)
at
org.apache.catalina.webresources.FileResource.getCodeBase(FileResource.java:277)
at
org.apache.catalina.loader.WebappClassLoaderBase.findClassInternal(WebappClassLoaderBase.java:2350)
at
org.apache.catalina.loader.WebappClassLoaderBase.findClass(WebappClassLoaderBase.java:865)
at
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1334)
at
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1188)
Our Environment:
Tomcat Version: 9.0.27. Our application classes exploded to WEB-INF/classes.
Our Investigation:
To troubleshoot the issue, we have looked at code in
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java
and
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/CachedResource.java#L70,
and it appears that we would run into this error if CachedResource is used when
org.apache.catalina.webresources.CachedResource#webResource isn't initialized
(i.e. stays null).
Upon further debugging and viewing code in
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L59,
it seems like it is possible that CachedResource could be used when its
#webResource isn't initialized when two threads concurrently ask for the same
resource but each with a different value for the boolean
useClassLoaderResources.
Consider this for example with 2 threads calling into Cache#getResource(String
path, boolean useClassLoaderResources) for the same resource but with two
different values for useClassLoaderResources and the resource is not in cache,
both threads end up at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L82,
then:
Thread 1:
useClassLoaderResources=true
A new CachedResource is created at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L77
and put into cache at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L82
but CachedResource#validateResource (where CachedResource#webResource is
initialized) is not called yet, this would happen at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L87.
Thread 2:
useClassLoaderResources=false
A new CachedResource is created at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L77
but finds the CachedResource in the cache (put in by the above thread) at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L82
and calls CachedResource#validateResource at line
https://github.com/apache/tomcat/blob/9.0.27/java/org/apache/catalina/webresources/Cache.java#L112,
but #validateResource doesn't do anything and returns because
useClassLoaderResources is false, and so CachedResource#webResource remains
uninitialized.
Assuming that Thread 1 hasn't initialized #webResource yet, when this thread
(Thread 2) calls into CachedResource#getURL we would run into this error.
Looks like the changes in the revision
https://svn.apache.org/viewvc?view=revision&revision=1831828 are related. This
seems like a concurrency issue, and we haven't seen this addressed in newer
versions (from the changelog here
https://ci.apache.org/projects/tomcat/tomcat9/docs/changelog.html)
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