pmk75 opened a new issue, #3551:
URL: https://github.com/apache/logging-log4j2/issues/3551
## Description
LoggerContext.updateLoggers throws a ConcurrentModificationException
randomly when called by javamelody filter initialisation during tomcat startup.
Result is that tomcat is incorrectly started, process is there but does not
serve any request
## Configuration
**Version:** 2.24.2
**Operating system:** Ubuntu 24.04
**JDK:** amazon-corretto-17.0.10.8.1-linux-x64
## Logs
```java
19-Mar-2025 05:35:40.941 SEVERE [main]
org.apache.catalina.core.StandardContext.filterStart Exception starting filter
[javamelody]
java.util.ConcurrentModificationException
at
java.base/java.util.HashMap$ValueSpliterator.forEachRemaining(HashMap.java:1784)
at
java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
at
java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:276)
at
java.base/java.util.WeakHashMap$ValueSpliterator.forEachRemaining(WeakHashMap.java:1217)
at
java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
at
java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
at
java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
at
java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
at
java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at
java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
at
org.apache.logging.log4j.core.LoggerContext.updateLoggers(LoggerContext.java:776)
at
org.apache.logging.log4j.core.LoggerContext.updateLoggers(LoggerContext.java:766)
at
net.bull.javamelody.Log4J2Appender.register(Log4J2Appender.java:86)
at
net.bull.javamelody.FilterContext.initLogs(FilterContext.java:333)
at
net.bull.javamelody.FilterContext.<init>(FilterContext.java:98)
at
net.bull.javamelody.MonitoringFilter.init(MonitoringFilter.java:148)
at
org.apache.catalina.core.ApplicationFilterConfig.initFilter(ApplicationFilterConfig.java:244)
```
## Reproduction
Randomly appears at startup (every 5 to 10 starts)
## Discussion
The related javamelody code is here
https://github.com/javamelody/javamelody/blob/6bb2b275b7dc77cded89ac661352094e10449e16/javamelody-core/src/main/java/net/bull/javamelody/Log4J2Appender.java#L86
And the exception is thrown here
https://github.com/apache/logging-log4j2/blob/3ffaf76fbd893055d8b60e0d1c319676aa58437e/log4j-core/src/main/java/org/apache/logging/log4j/core/LoggerContext.java#L812
May be it would be safer to wrap the loggerRegistry.getLoggers()
collection, in a separate one such as
```
new ArrayList<>(loggerRegistry.getLoggers()).forEach(....
```
to avoid potential concurrent modification.
Not really sure of the cause and therefore the potential solution.
Did anybody else noticed this issue ?
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