Mingyang.net：No identifier specified for entity

org.hibernate.AnnotationException: No identifier specified for entity: net.mingyang.modules.system.ConfigGroup
    org.hibernate.cfg.InheritanceState.determineDefaultAccessType(InheritanceState.java:277)
    org.hibernate.cfg.InheritanceState.getElementsToProcess(InheritanceState.java:224)
    org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:775)
    org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:3788)
    org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3742)
    org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1410)
    org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)
    org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)
    ......