NoClassDefFoundError custom controller using external jars

[#1]

Hi,

I am running into a problem with deploying a custom controller service to NiFi (I am using the latest version of NIfi 1.12 as of writing this). Now what I am trying to do is to create a custom controller service that would allow me to access a Virtuoso DB instance (similarly to DBCPConnectionPool). This controller relies on 2 external jar files (provided by Virtuoso) that I have placed inside a /libs folder under the module that implements the interface of my service. I have pointed to these jars as dependencies in the pom.xml of that same module and maven is able to import them. Furthermore, I am able to successfully compile and run a JUnit test of my implementation of the service which consists of simply getting a connection and checking if it is open.

The problem arises when I compile my service into a NAR and then deploy that service in NiFi. NiFi is able to detect it but when I try to enable it, it throws a NoClassDefFoundError that is related to these external jars. Now I am considering doing similarly to DBCPConnectionPool and create a property to point to these jars and load them externally, however, I simply do not understand what is causing the problem and I am too curious to let it go, I am also concerned that this approach may not be viable for these jars. So i am curious if someone can point me in the right way.
Here is my folder structure:

Here is the code I am trying to execute @OnEnabled. which passes JUnit

@OnEnabled
    public void onEnabled(final ConfigurationContext context) {

        getLogger().info("Initializing Virtuoso Repository");

        try {
            ConnectionPoolDataSource dataSource = new VirtuosoConnectionPoolDataSource();
            ((VirtuosoConnectionPoolDataSource)dataSource).setCharset(context.getProperty(REPOSITORY_CHARSET).getValue());
            ((VirtuosoConnectionPoolDataSource)dataSource).setServerName(context.getProperty(REPOSITORY_SERVER).getValue());
            ((VirtuosoConnectionPoolDataSource)dataSource).setInitialPoolSize(Integer.parseInt(context.getProperty(REPOSITORY_CONNECTIONPOOLSIZE).getValue()));
            ((VirtuosoConnectionPoolDataSource)dataSource).setMaxPoolSize(Integer.parseInt(context.getProperty(REPOSITORY_MAXCONNECTIONPOOLSIZE).getValue()));
            repository = new VirtuosoRepository(dataSource, context.getProperty(REPOSITORY_DEFAULTGRAPH).getValue(), true);
            repository.initialize();
        }catch (Exception e){
            getLogger().error("Unable to Initialize Virtuoso Repository", e);

        }

    }

Here is the pom where/how I declare the jar dependencies (This is where I am assuming the error might be)

And this is the log of the error I am getting when I try to enable it from NiFi:

2020-08-31 13:50:03,567 ERROR [Timer-Driven Process Thread-4] o.a.n.c.s.StandardControllerServiceNode Failed to invoke @OnEnabled method of StandardMyService[id=43ef57e8-0174-1000-843d-d32a07ba02bb] due to java.lang.NoClassDefFoundError: virtuoso/jdbc4/VirtuosoConnectionPoolDataSource
2020-08-31 13:50:03,567 ERROR [Timer-Driven Process Thread-4] e.e.c.dct.Virtuoso.StandardMyService StandardMyService[id=43ef57e8-0174-1000-843d-d32a07ba02bb] Could not Shut Down Repository: java.lang.NullPointerException
java.lang.NullPointerException: null
	at eu.everis.cordis.dct.Virtuoso.StandardMyService.shutdown(StandardMyService.java:187)
	at jdk.internal.reflect.GeneratedMethodAccessor376.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at org.apache.nifi.util.ReflectionUtils.invokeMethodsWithAnnotations(ReflectionUtils.java:142)

Hope someone can explain where the problem comes from,
BR,
Tino

[#2 · Accepted solution]

An update since I managed to solve the issue. I followed the advice given in this StackOverflow post https://stackoverflow.com/questions/364114/can-i-add-jars-to-maven-2-build-classpath-without-install... by creating a static in-project repository so that maven can have better access to the jars. It seemed to do the trick.