Possible bug in LoggerContext.updateLoggers throwing ConcurrentModificationException

[pmk75]

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

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

logging-log4j2/log4j-core/src/main/java/org/apache/logging/log4j/core/LoggerContext.java

Line 812 in 3ffaf76

loggerRegistry.getLoggers().forEach(logger -> logger.updateConfiguration(config));

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 ?

[ppkarwasz]

Hi @pmk75,

This is a duplicate of #3234 and was fixed in release 2.24.3 (see release notes).

[pmk75]

Many thanks @ppkarwasz and sorry for disturbance

[ppkarwasz]

No problem!