If the drools-core or drools-compiler jar is missing in the runtime classpath, the error message should mention that -------------------------------------------------------------------------------------------------------------------- Key: DROOLS-1761 URL: https://issues.jboss.org/browse/DROOLS-1761 Project: Drools Issue Type: Enhancement Components: core engine Reporter: Geoffrey De Smet Assignee: Mario Fusco Instead of doing these behaviors: - Exception "Child services have no parent" {code} Caused by: java.lang.RuntimeException: Child services [org.kie.api.internal.assembler.KieAssemblers] have no parent at org.kie.api.internal.utils.ServiceDiscoveryImpl.buildMap(ServiceDiscoveryImpl.java:186) {code} -- https://stackoverflow.com/questions/46690139/exception-child-services-hav... - Return of null {code} Exception in thread "main" java.lang.NullPointerException: null at org.optaplanner.core.config.score.director.ScoreDirectorFactoryConfig.buildDroolsScoreDirectorFactory(ScoreDirectorFactoryConfig.java:461) {code} -- https://stackoverflow.com/questions/46349199/optaplanner-nullpointerexcep... -- https://stackoverflow.com/questions/46199392/optaplanner-nullpointerexcep... Instead of that, the kie-api code should have an error message that gives the user a clue that the drools-core.jar (etc) isn't in the classpath.