When using JDK 9.0.1 & hibernate-core 5.2.11.Final I get this exception when trying to create an EntityManagerFactory:

Caused by: java.io.IOException: invalid constant type: 19 at 5
at javassist.bytecode.ConstPool.readOne(ConstPool.java:1241) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at javassist.bytecode.ConstPool.read(ConstPool.java:1172) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at javassist.bytecode.ConstPool.<init>(ConstPool.java:185) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at javassist.bytecode.ClassFile.read(ClassFile.java:807) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at javassist.bytecode.ClassFile.<init>(ClassFile.java:148) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.archive.scan.spi.ClassFileArchiveEntryHandler.toClassFile(ClassFileArchiveEntryHandler.java:61) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.archive.scan.spi.ClassFileArchiveEntryHandler.handleEntry(ClassFileArchiveEntryHandler.java:47) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.archive.internal.JarFileBasedArchiveDescriptor.visitArchive(JarFileBasedArchiveDescriptor.java:147) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.archive.scan.spi.AbstractScannerImpl.scan(AbstractScannerImpl.java:47) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.model.process.internal.ScanningCoordinator.coordinateScan(ScanningCoordinator.java:75) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.prepare(MetadataBuildingProcess.java:98) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.<init>(EntityManagerFactoryBuilderImpl.java:228) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.<init>(EntityManagerFactoryBuilderImpl.java:170) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.boot.spi.Bootstrap.getEntityManagerFactoryBuilder(Bootstrap.java:76) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.HibernatePersistenceProvider.getEntityManagerFactoryBuilder(HibernatePersistenceProvider.java:181) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.HibernatePersistenceProvider.getEntityManagerFactoryBuilderOrNull(HibernatePersistenceProvider.java:129) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.HibernatePersistenceProvider.getEntityManagerFactoryBuilderOrNull(HibernatePersistenceProvider.java:71) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at org.hibernate.jpa.HibernatePersistenceProvider.createEntityManagerFactory(HibernatePersistenceProvider.java:52) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:79) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at be.demmel.games.gw2.gui.MainGUI.init(MainGUI.java:72) ~[gw2tools-1.0.0_RC1-SNAPSHOT-jar-with-dependencies.jar:?]
at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:897) ~[javafx.graphics:?]
... 2 more

Upgrading the javassist Maven transitive dependency (from 3.20.0-GA) to 3.22.0-GA fixed the issue.
This javassist issue was tracked here: https://github.com/jboss-javassist/javassist/issues/147.