161117、使用spring声明式事务抛出 identifier of an instance of

今天项目组有成员使用spring声明式事务出现下面异常，这里跟大家分享学习下。
异常信息：
org.springframework.orm.hibernate3.HibernateSystemException: identifier of an instance of com.xxx.entity.Activity was altered from 33 to null; nested exception is org.hibernate.HibernateException: identifier of an instance of com.xxx.entity.Activity was altered from 33 to null
出现异常代码：（都是在service层中的一个方法里）
@Service
@Transactional
public class GroupServiceImpl extends CoreManagerImpl<Group> implements GroupService
省略	
@Override
	public Group updateGroup(Group group,Group old) throws Exception {
		Group doGroupUpdate = doGroupUpdate(group, old);
		List<SubGroup> subs = doSubGroupsUpdate(group.getSubGroups(), doGroupUpdate.getId());
		doGroupUpdate.setSubGroups(subs);
		Activity activity = updateActivity(doGroupUpdate);
		activity.setId(null);//发生异常的原因
		updateSeller(doGroupUpdate);
		doGroupUpdate.setActivity(activity);
		return doGroupUpdate;
	}
修复后代码：
	@Override
	public Group updateGroup(Group group,Group old) throws Exception {
		Group doGroupUpdate = doGroupUpdate(group, old);
		List<SubGroup> subs = doSubGroupsUpdate(group.getSubGroups(), doGroupUpdate.getId());
		doGroupUpdate.setSubGroups(subs);
		Activity activity = updateActivity(doGroupUpdate);
		//activity.setId(null);
		updateSeller(doGroupUpdate);
		doGroupUpdate.setActivity(activity);
		return doGroupUpdate;
	}
原因分析：
使用spring声明试事务，在整个事务（里面有多个子事务）没有提交成功，就将id主键设置成了null（主键不可以为null），所以会抛出上述异常。spring声明式事务是在所有子操作都完成一起提交事务的。