在进行grails开发中出现了一个郁闷的问题，代码如下

AController(){
   def aService	
   def insertSome(){
	aService.insert()
   }
}

AService(){
    
    def insert(){
	for(po in poListFromRequest){
	   po.save()	
	}
	for(po2 in po2ListFromRequest){
	   po2.save()	
	}		
    }	
}



class Po1Job{       
    //定时的变量设置
    def timeout = 10000l
		
    //delete 操作
    def delFuntion(){
	def poList = Po.list()

      //将 po 表中的数据全部转移到 polog 表中
	for(po in poList){           //假设po表中无重复数据
	    def polog = new PoLog()
	    polog.properties= po.properties
	    polog.save()
             com.mysite.EmailUtil.send();    //发送email
	    po.delete()
         }
    } 	
}


//同 Po1Job
class Po2Job{
    //定时的变量设置
    def timeout = 10000l
		
    //delete 操作
    def delFuntion(){
	def po2List = Po2.list()	
         //将po2 表中的数据全部转移到 po2log 表中
	for(po2 in po2List){
	    def po2log = new Po2Log()
	    po2log.properties= po2.properties
 	    po2log.save()
             com.mysite.EmailUtil.send();  //发送email
	    po2.delete();	
         }
    } 	
}

结果出现了下面错误

org.quartz.SchedulerException: JobListener 'sessionBinderListener' threw exceptiwas incorrect): [po#14] [See nested exception: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction

org.hibernate.StaleObjectStateException: Row was updated or deleted by another t
ransaction (or unsaved-value mapping was incorrect): [po#14]
at org.hibernate.persister.entity.AbstractEntityPersister.check(Abstract
EntityPersister.java:1765)
at org.hibernate.persister.entity.AbstractEntityPersister.delete(Abstrac
tEntityPersister.java:2523)
at org.hibernate.persister.entity.AbstractEntityPersister.delete(Abstrac
tEntityPersister.java:2697)
at org.hibernate.action.EntityDeleteAction.execute(EntityDeleteAction.ja
va:74)
at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:250)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:234)

at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:146)

at org.hibernate.event.def.AbstractFlushingEventListener.performExecutio
ns(AbstractFlushingEventListener.java:298)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlus
hEventListener.java:27)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000)
at org.codehaus.groovy.grails.plugins.quartz.listeners.SessionBinderJobL
istener.jobWasExecuted(SessionBinderJobListener.java:55)
at org.quartz.core.QuartzScheduler.notifyJobListenersWasExecuted(QuartzS
cheduler.java:1659)
at org.quartz.core.JobRunShell.notifyJobListenersComplete(JobRunShell.ja
va:352)
at org.quartz.core.JobRunShell.run(JobRunShell.java:227)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.j
ava:520)

哪位达人能解释一下为什么出现乐观锁异常么？