Chapter 19. Best Practices - Hibernate reference 2.1.8 ga 正式版英文参考手册

Use an Address class to encapsulate street, suburb, state, postcode. This encourages code reuse and simplifies refactoring.

Hibernate makes identifier properties optional. There are all sorts of reasons why you should use them. We recommend that identifiers be 'synthetic' (generated, with no business meaning) and of a non-primitive type. For maximum flexibility, use java.lang.Long or java.lang.String.

Don't use a single monolithic mapping document. Map com.eg.Foo in the file com/eg/Foo.hbm.xml. This makes particularly good sense in a team environment.

Deploy the mappings along with the classes they map.

This is a good practice if your queries call non-ANSI-standard SQL functions. Externalising the query strings to mapping files will make the application more portable.

As in JDBC, always replace non-constant values by "?". Never use string manipulation to bind a non-constant value in a query! Even better, consider using named parameters in queries.

Hibernate lets the application manage JDBC connections. This approach should be considered a last-resort. If you can't use the built-in connections providers, consider providing your own implementation of net.sf.hibernate.connection.ConnectionProvider.

Suppose you have a Java type, say from some library, that needs to be persisted but doesn't provide the accessors needed to map it as a component. You should consider implementing net.sf.hibernate.UserType. This approach frees the application code from implementing transformations to / from a Hibernate type.

In performance-critical areas of the system, some kinds of operations (eg. mass update / delete) might benefit from direct JDBC. But please, wait until you know something is a bottleneck. And don't assume that direct JDBC is necessarily faster. If need to use direct JDBC, it might be worth opening a Hibernate Session and using that SQL connection. That way you can still use the same transaction strategy and underlying connection provider.

From time to time the Session synchronizes its persistent state with the database. Performance will be affected if this process occurs too often. You may sometimes minimize unnecessary flushing by disabling automatic flushing or even by changing the order of queries and other operations within a particular transaction.

When using a servlet / session bean architecture, you could pass persistent objects loaded in the session bean to and from the servlet / JSP layer. Use a new session to service each request. Use Session.update() or Session.saveOrUpdate() to update the persistent state of an object.

Database Transactions have to be as short as possible for best scalability. However, it is often neccessary to implement long running Application Transactions, a single unit-of-work from the point of view of a user. This Application Transaction might span several client requests and response cycles. Either use Detached Objects or, in two tiered architectures, simply disconnect the Hibernate Session from the JDBC connection and reconnect it for each subsequent request. Never use a single Session for more than one Application Transaction usecase, otherwise, you will run into stale data.

This is more of a necessary practice than a "best" practice. When an exception occurs, roll back the Transaction and close the Session. If you don't, Hibernate can't guarantee that in-memory state accurately represents persistent state. As a special case of this, do not use Session.load() to determine if an instance with the given identifier exists on the database; use find() instead.

Use eager (outer-join) fetching sparingly. Use proxies and/or lazy collections for most associations to classes that are not cached at the JVM-level. For associations to cached classes, where there is a high probability of a cache hit, explicitly disable eager fetching using outer-join="false". When an outer-join fetch is appropriate to a particular use case, use a query with a left join fetch.

Hide (Hibernate) data-access code behind an interface. Combine the DAO and Thread Local Session patterns. You can even have some classes persisted by handcoded JDBC, associated to Hibernate via a UserType. (This advice is intended for "sufficiently large" applications; it is not appropriate for an application with five tables!)

If you compare objects outside of the Session scope, you have to implement equals() and hashCode(). Inside the Session scope, Java object identity is guaranteed. If you implement these methods, never ever use the database identifier! A transient object doesn't have an identifier value and Hibernate would assign a value when the object is saved. If the object is in a Set while being saved, the hash code changes, breaking the contract. To implement equals() and hashCode(), use a unique business key, that is, compare a unique combination of class properties. Remember that this key has to be stable and unique only while the object is in a Set, not for the whole lifetime (not as stable as a database primary key). Never use collections in the equals() comparison (lazy loading) and be careful with other associated classes that might be proxied.

Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.