Unable to resolve persistence unit root URL in Java 9 multi module environment - spring

Question

I'm trying to make a Spring Boot Maven multi-module app in Java 9. It works without adding Spring Data support and #Entity classes, but now it won't start.
The problem is the following exception:
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is javax.persistence.PersistenceException: Unable to resolve persistence unit root URL
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1699) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:573) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.context#5.0.9.RELEASE/org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1089) ~[spring-context-5.0.9.RELEASE.jar:na]
at spring.context#5.0.9.RELEASE/org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:859) ~[spring-context-5.0.9.RELEASE.jar:na]
at spring.context#5.0.9.RELEASE/org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.0.9.RELEASE.jar:na]
at spring.boot#2.0.5.RELEASE/org.springframework.boot.SpringApplication.refresh(SpringApplication.java:780) [spring-boot-2.0.5.RELEASE.jar:na]
at spring.boot#2.0.5.RELEASE/org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:412) [spring-boot-2.0.5.RELEASE.jar:na]
at spring.boot#2.0.5.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:333) [spring-boot-2.0.5.RELEASE.jar:na]
at spring.boot#2.0.5.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:1277) [spring-boot-2.0.5.RELEASE.jar:na]
at spring.boot#2.0.5.RELEASE/org.springframework.boot.SpringApplication.run(SpringApplication.java:1265) [spring-boot-2.0.5.RELEASE.jar:na]
at hu.kleatech.jigsaw/hu.kleatech.jigsaw.MainApplication.main(MainApplication.java:18) [classes/:na]
Caused by: javax.persistence.PersistenceException: Unable to resolve persistence unit root URL
at spring.orm#5.0.9.RELEASE/org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.determineDefaultPersistenceUnitRootUrl(DefaultPersistenceUnitManager.java:637) ~[spring-orm-5.0.9.RELEASE.jar:na]
at spring.orm#5.0.9.RELEASE/org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.preparePersistenceUnitInfos(DefaultPersistenceUnitManager.java:459) ~[spring-orm-5.0.9.RELEASE.jar:na]
at spring.orm#5.0.9.RELEASE/org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.afterPropertiesSet(DefaultPersistenceUnitManager.java:440) ~[spring-orm-5.0.9.RELEASE.jar:na]
at spring.orm#5.0.9.RELEASE/org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.afterPropertiesSet(LocalContainerEntityManagerFactoryBean.java:328) ~[spring-orm-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1758) ~[spring-beans-5.0.9.RELEASE.jar:na]
at spring.beans#5.0.9.RELEASE/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1695) ~[spring-beans-5.0.9.RELEASE.jar:na]
... 15 common frames omitted
Caused by: java.io.FileNotFoundException: class path resource [] cannot be resolved to URL because it does not exist
at spring.core#5.0.9.RELEASE/org.springframework.core.io.ClassPathResource.getURL(ClassPathResource.java:195) ~[spring-core-5.0.9.RELEASE.jar:na]
at spring.orm#5.0.9.RELEASE/org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager.determineDefaultPersistenceUnitRootUrl(DefaultPersistenceUnitManager.java:633) ~[spring-orm-5.0.9.RELEASE.jar:na]
... 20 common frames omitted
There are a few examples of this exception on the internet, but none of them is relevant for me. I believe that the multi-module Java 9 environment causes the problem somehow, so this question is not a duplicate of the others.
I tried every possible solutions I could find and think of, I even tried to manually configure everything, none of them helped or got me closer to the cause. But there's one thing I found interesting in those solutions: it seems that this exception is thrown when Spring Boot cannot find any #Entity class. So I'm thinking the #Entity classes are somehow not visible through the module system.
Since the problem seems to be dependent of the environment, here's the whole project: (Note that it's in very early state, most of it is incomplete, but should at least run)
https://github.com/KleaTech/szakdolgozat
Any help is appreciated, I'm out of ideas.

Answer 1

You need to add these dependencies to pom.xml (or provide them in a different way, if you do not use maven):
<dependency>
<groupId>javax.xml.bind</groupId>
<artifactId>jaxb-api</artifactId>
<version>2.3.0</version>
</dependency>
<dependency>
<groupId>net.bytebuddy</groupId>
<artifactId>byte-buddy</artifactId>
<version>1.9.3</version>
</dependency>
and include them in module-info.java
open module org.demo {
...
requires java.xml.bind;
requires net.bytebuddy;
}