Keycloak java.lang.NoClassDefFoundError: java/security/acl/Group using Springboot

So we are building a Springboot back-end with Keycloak integration. But we get the following error when running the back-end on a docker swarm service:

2020-06-29 21:17:51.694 ERROR 1 --- [nio-3304-exec-1] o.a.c.c.C.[Tomcat].[localhost]           : Exception Processing /Suitless/latest/

java.lang.NoClassDefFoundError: java/security/acl/Group

    at org.keycloak.adapters.tomcat.KeycloakAuthenticatorValve.createPrincipalFactory(KeycloakAuthenticatorValve.java:96) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.keycloak.adapters.tomcat.AbstractKeycloakAuthenticatorValve.createSessionTokenStore(AbstractKeycloakAuthenticatorValve.java:262) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.keycloak.adapters.tomcat.AbstractKeycloakAuthenticatorValve.getTokenStore(AbstractKeycloakAuthenticatorValve.java:251) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.keycloak.adapters.tomcat.KeycloakAuthenticatorValve.getTokenStore(KeycloakAuthenticatorValve.java:106) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.keycloak.adapters.tomcat.AbstractKeycloakAuthenticatorValve.checkKeycloakSession(AbstractKeycloakAuthenticatorValve.java:228) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.keycloak.adapters.tomcat.AbstractKeycloakAuthenticatorValve.invoke(AbstractKeycloakAuthenticatorValve.java:180) ~[spring-boot-container-bundle-10.0.1.jar!/:10.0.1]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) ~[na:na]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.36.jar!/:9.0.36]
    at java.base/java.lang.Thread.run(Thread.java:832) ~[na:na]

Caused by: java.lang.ClassNotFoundException: java.security.acl.Group
    at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:435) ~[na:na]
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589) ~[na:na]
    at org.springframework.boot.loader.LaunchedURLClassLoader.loadClass(LaunchedURLClassLoader.java:151) ~[run.jar:na]
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522) ~[na:na]

    ... 19 common frames omitted

I tried it with JDK 8 and JDK 11 since the following post implies that the wrong JDK can cause a problem: Stackoverflow post. It still gives this error.

The following is our build.gradle setup:

    ext {
        springBootVersion = '2.3.1.RELEASE'
    }
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
    }
}

plugins {
    id 'org.springframework.boot' version '2.3.1.RELEASE'
    id 'io.spring.dependency-management' version '1.0.9.RELEASE'
    id 'java'
}

apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

version = '0.0.1-SNAPSHOT'

bootJar {
    version = '0.0.1-SNAPSHOT'
}

sourceCompatibility = '1.8'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-hateoas'
    implementation 'org.springframework.boot:spring-boot-starter-data-mongodb'
    implementation 'org.springframework.boot:spring-boot-starter-validation'

    implementation group: 'joda-time', name: 'joda-time', version: '2.3'
    implementation group: 'io.jsonwebtoken', name: 'jjwt', version: '0.9.1'
    implementation 'javax.xml.bind:jaxb-api:2.3.0'

    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-security'
    implementation group: 'org.keycloak', name: 'keycloak-spring-boot-starter', version: '10.0.1'

    testImplementation('org.springframework.boot:spring-boot-starter-test')
    annotationProcessor "org.springframework.boot:spring-boot-configuration-processor"
}

Thanks for the comments. The problem that causes this error for me was the fact that my Dockerfile always pulled the latest JDK version. This is not version 11 or 8 but 14. So I just added a specific version to it in order for Keycloak not to break.