https://bz.apache.org/bugzilla/show_bug.cgi?id=69615
Bug ID: 69615
Summary: Potential Regression in Tomcat 9.0.97+(Including
Tomcat 10 and Tomcat 11) — NoClassDefFoundError for
Dynamically Loaded Classes
Product: Tomcat 9
Version: 9.0.97
Hardware: PC
OS: All
Status: NEW
Severity: regression
Priority: P2
Component: Catalina
Assignee: dev@tomcat.apache.org
Reporter: dushyant.shinde...@gmail.com
Target Milestone: -----
We would like to report a potential regression issue observed in Tomcat
9.0.97+, including Tomcat 10 and Tomcat 11, that appears to break expected
behaviour regarding dynamically loaded JARs.
## Issue Description:
Our application is a WAR-based deployment running on Tomcat 11 with Java 21.
The application allows importing extensions (JARs) at runtime, which are
dynamically loaded using URLClassLoader.
In Tomcat 9.0.96 and earlier, dynamically loaded classes were immediately
recognized without issues. However, starting from Tomcat 9.0.97, we encountered
the following error when attempting to access classes from these dynamically
loaded JARs:
java.lang.NoClassDefFoundError: org/apache/logging/log4j/spi/ExtendedLogger
## Steps to Reproduce:
1. Deploy a WAR-based application on Tomcat 9.0.97+.
2. Dynamically load a JAR (e.g. log4j-api-2.17.1.jar) using the following code:
URLClassLoader webappClassLoader = (URLClassLoader)
Thread.currentThread().getContextClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(webappClassLoader, new Object[]{jarUrl});
3. Attempt to access a class from the dynamically loaded JAR, such as
org.apache.logging.log4j.spi.ExtendedLogger.
4. Observe the NoClassDefFoundError exception despite the JAR being
successfully loaded.
## Findings:
• Debugging confirmed that the JAR was successfully loaded by the classloader.
• Classes like org.apache.logging.log4j.LogManager and
org.apache.logging.log4j.Logger were accessible, but
org.apache.logging.log4j.spi.ExtendedLogger still resulted in a
NoClassDefFoundError.
• Restarting Tomcat resolves the issue, confirming it’s related to Tomcat's
internal caching.
## Root Cause Analysis:
This issue appears to be related to the notFoundClassResourceCacheSize caching
mechanism introduced in Tomcat 9.0.97.
• If a class is requested before its JAR is loaded, Tomcat caches it as "not
found."
• Even after dynamically loading the JAR, Tomcat does not reattempt the class
lookup, causing a NoClassDefFoundError.
• This behaviour differs from Tomcat 9.0.96 and earlier, where dynamically
loaded JARs were recognized immediately without requiring a server restart.
## Impact:
This caching behaviour is problematic for applications that rely on runtime JAR
loading and introduces unexpected behaviour that breaks compatibility with
previous Tomcat versions. Given that Tomcat has historically maintained
backward compatibility, this change could be considered a regression.
## Request:
We believe this behaviour deviates from expected functionality and may impact
other applications that rely on dynamic class loading. We kindly request that
the Tomcat team review this behaviour and consider addressing it in future
releases to restore compatibility.
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