javax.persistence.TransactionRequiredException: no transaction is in progress
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I've a service that store many objects into DB with HQL. But I've this exception

    javax.persistence.TransactionRequiredException: no transaction is in progress
	at org.hibernate.ejb.AbstractEntityManagerImpl.flush(AbstractEntityManagerImpl.java:789)
	at sun.reflect.GeneratedMethodAccessor14.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:365)
	at $Proxy42.flush(Unknown Source)
	at sun.reflect.GeneratedMethodAccessor14.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:240)
	at $Proxy42.flush(Unknown Source)
	at org.skyway.spring.util.dao.AbstractJpaDao.flush(AbstractJpaDao.java:90)
	at sun.reflect.GeneratedMethodAccessor16.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:318)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:110)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202)
	at $Proxy52.flush(Unknown Source).........

May be the problem is a lot of number of concurrent operations active on DB.
Any suggestions?