Why are 2 classes in an example application loaded by 2 different moduleclassloaders?

Question

For studying purposes I have put dependencies in the pom of an application called jaxb_study,

<dependency>
        <groupId>org.jdom</groupId>
        <artifactId>jdom2</artifactId>
        <version>2.0.6.1</version>
    </dependency>

     <dependency>
            <groupId>javax.transaction</groupId>
            <artifactId>javax.transaction-api</artifactId>
            <version>1.3</version>
        </dependency>

In the debugger I look at a few moduleclassloaders and it's classpath (I hope I determined the classpath correctly).

In my application there is a class Book in WEB-INF/classes. It's moduleclassloader is the one of the application module, jaxb_study.main. When I look at a part of it's classpath, I see,

cl = {ModuleClassLoader@22107} "ModuleClassLoader for Module "deployment.jaxb_study.war" from Service Module Loader"
 paths = {AtomicReference@22126} "org.jboss.modules.Paths@1e58e53e"
  value = {Paths@22148} 
   allPaths = {Collections$UnmodifiableMap@22150}  size = 130
                
    "org/jdom2/output" -> {Collections$UnmodifiableRandomAccessList@22292}  size = 1
         value = {Collections$UnmodifiableRandomAccessList@22292}  size = 1
          0 = {VFSResourceLoader@22394} 
           root = {VirtualFile@22395} ""/D:/work/bvv-22979/eap-7.4.23/content/jaxb_study.war/WEB-INF/lib/jdom2-2.0.6.1.jar""
           rootName = "jdom2-2.0.6.1.jar"
            
    "javax/transaction" -> {Collections$UnmodifiableRandomAccessList@22310}  size = 1
     value = {Collections$UnmodifiableRandomAccessList@22310}  size = 1
      0 = {VFSResourceLoader@22371} 
       root = {VirtualFile@22372} ""/D:/work/bvv-22979/eap-7.4.23/content/jaxb_study.war/WEB-INF/lib/javax.transaction-api-1.3.jar""

When I later look at the moduleclassloaders of org.jdom2.Document I see the moduleclassloader of the application as above,

cl = ModuleClassLoader for Module "deployment.jaxb_study.war" from Service Module Loader

and when I look at the moduleclassloader of javax.transaction.RollbackException I see

cl = {ModuleClassLoader@22702} "ModuleClassLoader for Module "javax.transaction.api" version 2.0.0.Final-redhat-00005 from local module loader @75ed9710 (finder: local module finder @4fc5e095 (roots: D:\work\bvv-22979\eap-7.4.23\modules,D:\work\bvv-22979\eap-7.4.23\modules\system\layers\base\.overlays\layer-base-jboss-eap-7.4.23.CP,D:\work\bvv-22979\eap-7.4.23\modules\system\layers\base))"
 paths = {AtomicReference@22716} "org.jboss.modules.Paths@4087c386"
  value = {Paths@22735} 
   allPaths = {Collections$UnmodifiableMap@22737}  size = 7
    "javax/transaction" -> {Collections$UnmodifiableRandomAccessList@22747}  size = 1
     value = {Collections$UnmodifiableRandomAccessList@22747}  size = 1
      0 = {JarFileResourceLoader@22755} 
       jarFile = {JarFile@22756} 
       rootName = "jboss-transaction-api_1.3_spec-2.0.0.Final-redhat-00005.jar"
       rootUrl = {URL@22689} "jar:file:/D:/work/bvv-22979/eap-7.4.23/modules/system/layers/base/javax/transaction/api/main/jboss-transaction-api_1.3_spec-2.0.0.Final-redhat-00005.jar!/"

jdom2 and transactions are on the classpath of the application moduleclassloader with jars from WEB-INF/lib. The Document class is loaded with this moduleclassloader from the jar in WEB-INF/lib. But RollbackException is loaded with the moduleclassloader of the transactions module, from another jar then the one from WEB-INF/lib. So why is Document not loaded by the moduleclassloader of jdom2? Or does it not exist?

ehsavoie · Accepted Answer · 2025-10-02 07:21:20Z

1

That's expected, JBoss already provide the javax.transaction API as part of Java EE, so it will be provided to your application as well as the one you have added to your application. That's why it is a bad practice to add JavaEE jars in your application. It should be in the provided scope of maven.

answered Oct 2 at 7:21

ehsavoie

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