hibernate, to be lazy or not to be lazy? [closed]

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Closed last year.

I have entity A, which has a many-to-many relation to entity B.

So the table layout is : A, AB(mapping table), B

To get an object of entity A: I call A.getById() which does getHibernateTemplate().get(A.class, id) using spring and hibernate.

The problem is, sometimes ensuing code will just need A, sometimes ensuing code will continue to access the associated B's, so we'd like to use lazy loading in some cases and eager in some other cases. but the problem is that all database access is provided through the same single ADao.java, so there is only one method getById().

Should I create two versions of method getById()?

But then for more complex cases, if A is also attached to C through many-to-many, then there could be variants of lazy-loading-C and eager-loading-C,so the required getById() variants quickly grows exponentially.

what is your opinion on this choice?

Thanks

回答1:

For general considerations take a look at the Hibernate 3.6 docs about fetching strategies. The default fetching strategy is defined in mapping annotations or in a hbm.xml file. There are three ways to dynamically switch from a default lazy-loading strategy to an eager-loading strategy. The first two require separate implementations of DAO methods for lazy-loading and eager-loading use cases:

1. Criteria.setFetchMode() in a Hibernate Criteria query
2. FETCH keyword in a HQL query
3. Since Hibernate 3.5 (not quite sure now, maybe 3.6) there is the third option of using fetch profiles to dynamically switch from lazy-loading to eager-loading.

A fetch profile is enabled/disabled on a session scope. So, provided the desired fetch profile is set in the current session, you can use the same DAO method for a lazy-loading as well as for an eager-loading use case.

The important thing to be aware of here is that you can only switch from a lazy-loading strategy defined in annotations or in a hbm.xml file to an eager-loading strategy and not vice versa. This restriction is independent of the method used for switching the fetching strategy.

回答2:

Well, you described the problem correctly. It's just a tradeoff between simplicity (just one method) and performance (two methods, each returning exactly what is needed). If the response time is correct by just using the single method and lazy-loading the Bs, then don't touch anything. If the response time is too long, and you have measured that eager-loading it would make it correct, then introduce a new method.

Keep things simple, and optimize only if needed. Lazy-loading associations is fast, because it just make a query on a foreign key, whcih should be indexed in the database.

Also not that loading two toMany associations is pretty rare: it's common to show all the products of a category on a page, but it's uncommon to show all the puchases of all the products of a category.

来源：https://stackoverflow.com/questions/8493087/hibernate-to-be-lazy-or-not-to-be-lazy