ValidationException

Changing log4j to version 1.2.15 clarifies the exception as follow:
Caused by: org.hibernate.HibernateException: Error applying BeanValidation relational constraints
	at org.hibernate.cfg.beanvalidation.BeanValidationIntegrator.applyRelationalConstraints(BeanValidationIntegrator.java:219)
	at org.hibernate.cfg.beanvalidation.BeanValidationIntegrator.integrate(BeanValidationIntegrator.java:126)
	at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:303)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1750)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1788)
	at org.springframework.orm.hibernate4.LocalSessionFactoryBuilder.buildSessionFactory(LocalSessionFactoryBuilder.java:242)
	at org.springframework.orm.hibernate4.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:372)
	at org.springframework.orm.hibernate4.LocalSessionFactoryBean.afterPropertiesSet(LocalSessionFactoryBean.java:357)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1514)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1452)
	... 61 more
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.hibernate.cfg.beanvalidation.BeanValidationIntegrator.applyRelationalConstraints(BeanValidationIntegrator.java:208)
	... 70 more
Caused by: org.hibernate.HibernateException: Unable to build the default ValidatorFactory
	at org.hibernate.cfg.beanvalidation.TypeSafeActivator.getValidatorFactory(TypeSafeActivator.java:524)
	at org.hibernate.cfg.beanvalidation.TypeSafeActivator.applyDDL(TypeSafeActivator.java:119)
	... 75 more
Caused by: javax.validation.ValidationException: Unable to find a default provider
	at javax.validation.Validation$GenericBootstrapImpl.configure(Validation.java:264)
	at javax.validation.Validation.buildDefaultValidatorFactory(Validation.java:111)
	at org.hibernate.cfg.beanvalidation.TypeSafeActivator.getValidatorFactory(TypeSafeActivator.java:521)