Well, I found a few problems.
1) User and Order should not be a Spring Bean, they are domain objects and hold state. Not good for a Spring Bean
2) You are creating an ApplicationContext object directly in your OrderService, it should be outside of your classes that you are defining as Beans. So one problem is everytime you call addOrder, it goes and creates a brand new ApplicationContext.
You should only have one of these for your entire application. ApplicationContext is a heavy-weight object that can take a little bit of time to instantiate, There is a whole bunch of steps in Initialization that occurs when creating an ApplicationContext.
3) This is not a Spring bug, but a Hibernate issue. You have from User to Order a cascade option, but you do not have a cascade option from Order to User. And since you save the Order, it will not do anything to User. And I would assume this is a new user and the database would have a FK constraint in Order table for the user id. So the User information must be in the User table before any Order can be inserted into the Order table for that User. But you are looking up a User, so if it exists in the database, then that wouldn't be a problem.
Please post your exception stack trace because we are missing that context information on what is you actual problem.
Mark