Using javaagent with javassist in WildFly

Question

I've written a simple javaagent that basically injects some logging in classes that are part of the JDK, such as javax/imageio/spi/ServiceRegistry , using javassist .

It works fine in a simple standalone application, but I cannot get it to work in WildFly 10. What I have tried:

• Added the agent JAR as a module to WildFly along with the corresponding module.xml which declares <module name="org.javassist"/> as dependency
• Appended the agent package and org.javassist to JBOSS_MODULES_SYSTEM_PKGS
• Appended -javaagent:/full/path/to/agent/jar/in/module to JAVA_OPTS

WildFly starts but in server.log I get:

java.lang.NoClassDefFoundError: javassist/ClassPool
      at net.luniks.agent.LoggerChanger.transform(LoggerChanger.java:48)
      at sun.instrument.TransformerManager.transform(TransformerManager.java:188)
      at sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:428)
      at java.lang.ClassLoader.findBootstrapClass(Native Method)
      at java.lang.ClassLoader.findBootstrapClassOrNull(ClassLoader.java:1015)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:413)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:411)
      at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
      at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
      at java.lang.Class.forName0(Native Method)
      at java.lang.Class.forName(Class.java:348)
      at org.jboss.modules.ClassLoaderLocalLoader.loadClassLocal(ClassLoaderLocalLoader.java:74)
      at org.jboss.modules.Module.loadModuleClass(Module.java:606)
      at org.jboss.modules.ModuleClassLoader.findClass(ModuleClassLoader.java:190)

It seems the module class loader is used but even though the agent module has a dependency to org.javassist , it still cannot load its classes.

What am I missing?

Answer 1

Eventually I got it to work by adding only the package of the agent to JBOSS_MODULES_SYSTEM_PKGS (not "javassist") and adding the org.javassist module JAR to the boot classpath:

JAVA_OPTS="$JAVA_OPTS -Xbootclasspath/p:/path/to/wildfly/modules/system/layers/base/org/javassist/main/javassist-3.18.1-GA.jar"

Now the agent works, it can load the Javassist classes. Certainly not the clean way but it is just for debugging purposes...