在Servlet会话中缓存Hibernate实体

Question

Let's say, I cache the current user in a servlet session (or alike) and get a request like "get all books of the current user". Naively, I'd go for

hibernateSession
.createCriteria(Book.class)
.add(Restrictions.eq("user", currentUser))
.list();

and this works, but in more complicated cases (which I can't reproduce now), I get a

TransientObjectException: object references an unsaved transient instance

So I thought, I should attach the current user to the session somehow. I tried

currentUser = (User) session.merge(currentUser);

only to find out that it issues a database query, thus making the caching no better than storing the id only.

Can such a caching be done efficiently?

Answer 1

I would also recommend caching only the ID, rather than the entire entity. However, you are looking for the load(...) method intead of merge(...) or get(...). The advantage of the load(...) method is that it simply generates an entity that is attached to your session without making a query to the database. Assuming that nobody else can come along and delete the record so that you know that your record still exists then the drawbacks to the load method (which are certainly there) are not relevant for your use-case.

https://docs.jboss.org/hibernate/orm/3.5/javadocs/org/hibernate/Session.html#load(java.lang.Class, java.io.Serializable)

This will give you an object that you can use to query as in your example

hibernateSession
.createCriteria(Book.class)
.add(Restrictions.eq("user", currentUser))
.list();

Or else that you can also use to set in other entities while persisting

book.setUser(currentUser);

In the version of hibernate I tested with, accessing the hashCode(), toString() or equals() method caused the entity to be initialized, including a database query. But provided you only need to use it to query or set your foreign key references while persisting other entities, the load(Class, Serializable) method is what you are looking for.

Edited - What happens if record does not exist You get an ObjectNotFoundException. It extends UnresolvableObjectException which extends HibernateException, which is of course, a RuntimeException. This means that your code exits rather ungracefully (unless you want to catch this exception everywhere - but of course that it is not a reasonable solution)

org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.yourcode.YourEntity#id that does not exist]
at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:419)
at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:154)
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:143)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:174)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
at com.yourcode.YourEntity_$$_javassist_143.getNonIdentifierMethod(YourEntity_$$_javassist_143.java)
at com.yourcode.YourBusinessLogic.method(YourBusinessLogic.java:56)

Answer 2

Why not cache the id only? For the Books query, you could simply do this:

hibernateSession
.createCriteria(Book.class)
.add(Restrictions.eq("user.id", currentUserId))
.list();

If you need the cached user object for other reasons, you could simply use currentUser.id , rather than currentUserId .