Instrumentation Module not loaded when the extension is built as fat jar with Maven shade plugin

[mfridljand]

It seems that I am observing a very strange issue when splitting my extension into multiple modules and building it as a fat JAR using the Maven Shade Plugin.
Suppose my extension otel-test-ext_v1 provides instrumentation for two libraries, A and B. Initially, it contains two instrumentation modules in separate packages:

   com.my.otel.alib.ALibInstrumentationModule
   com.my.otel.blib.BLibInstrumentationModule 

In this setup, everything works fine. The agent log contains:

...
[otel.javaagent 2026-05-18 14:05:59:079 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.instrumentation.InstrumentationLoader - Loading instrumentation my-alib [class com.my.otel.alib.ALibInstrumentationModule]
[otel.javaagent 2026-05-18 14:05:59:080 +0200] [main] INFO com.my.otel.alib.ALibInstrumentation - Loading ALibInstrumentation
[otel.javaagent 2026-05-18 14:05:59:080 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.field.VirtualFieldImplementationInstallerFactory - Found InstrumentationModule which does not implement InstrumentationModuleMuzzle: com.my.otel.alib.ALibInstrumentationModule@499683c4
[otel.javaagent 2026-05-18 14:05:59:081 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.instrumentation.InstrumentationLoader - Loading instrumentation my-blib [class com.my.otel.blib.BLibInstrumentationModule]
[otel.javaagent 2026-05-18 14:05:59:082 +0200] [main] INFO com.my.otel.blib.BLibInstrumentation - Loading BLibInstrumentation
[otel.javaagent 2026-05-18 14:05:59:082 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.field.VirtualFieldImplementationInstallerFactory - Found InstrumentationModule which does not implement InstrumentationModuleMuzzle: com.my.otel.blib.BLibInstrumentationModule@e48bf9a
..
[otel.javaagent 2026-05-18 14:05:59:089 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.instrumentation.InstrumentationLoader - Installed 289 instrumentation(s)
[otel.javaagent 2026-05-18 14:05:59:089 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.AgentInstaller - Installed 1 extension(s)

For better modularity, I then split the single instrumentation project into separate projects, each containing instrumentation for only one library. So instead of a single Java project otel-test-ext, I now have two projects:

xotel-test-ext-a  
 Contains com.my.otel.alib.ALibInstrumentationModule  , instrumentation my-alib 
                  com.my.otel.alib.ALibInstrumentation  

xotel-test-ext-b
 Contains com.my.otel.blib.BLibInstrumentationModule
                  com.my.otel.alib.BLibInstrumentation

The extension otel-test-ext_v2 is then built as a fat JAR using the Maven Shade Plugin, so that it contains all classes from both projects A and B.
In this setup, it seems only the first instrumentation module is being loaded. The agent log contains:

…
[otel.javaagent 2026-05-18 13:24:47:731 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.instrumentation.InstrumentationLoader - Loading instrumentation my-alib [class com.my.otel.alib.ALibInstrumentationModule]
[otel.javaagent 2026-05-18 13:24:47:732 +0200] [main] INFO com.my.otel.alib.ALibInstrumentation - Loading ALibInstrumentation
[otel.javaagent 2026-05-18 13:24:47:732 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.field.VirtualFieldImplementationInstallerFactory - Found InstrumentationModule which does not implement InstrumentationModuleMuzzle: com.my.otel.alib.ALibInstrumentationModule@46d567cb
…
[otel.javaagent 2026-05-18 13:24:47:738 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.instrumentation.InstrumentationLoader - Installed 288 instrumentation(s)
[otel.javaagent 2026-05-18 13:24:47:738 +0200] [main] DEBUG io.opentelemetry.javaagent.tooling.AgentInstaller - Installed 1 extension(s)
..

There is no “Loading instrumentation my-blib” log message. The com.my.otel.blib.BLibInstrumentationModule seems not being loaded.

There are no apparent differences between otel-test-ext_v1.jar and otel-test-ext_v2.jar. The both files are of almost the same size, and have apparently the same .class.

otel_agent_test_v1.log
otel_agent_test_v2.log
xotel-test-ext_v2.zip
xotel-test-ext_v1.zip

[laurit]

META-INF/services/io.opentelemetry.javaagent.extension.instrumentation.InstrumentationModule is different between the two jars. If you are using gradle shadow plugin you can use mergeServiceFiles to merge the service files.

[mfridljand]

Thanks,
So, the agent relies on the META-INF/services/io.opentelemetry.javaagent.extension.instrumentation.InstrumentationModule?
All modules have to be listed there?

[laurit]

yes, we use https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html