Hibernate.orgCommunity Documentation
<component>:
Use an Address class to encapsulate street,
suburb, state, postcode.
This encourages code reuse and simplifies refactoring.
Hibernate makes identifier properties optional. There are a range of reasons why you should use them. We recommend that identifiers be 'synthetic', that is, generated with no business meaning.
Identify natural keys for all entities, and map them using
<natural-id>. Implement equals() and
hashCode() to compare the properties that make up the natural key.
Do not use a single monolithic mapping document. Map com.eg.Foo in
the file com/eg/Foo.hbm.xml. This makes sense, particularly in
a team environment.
Deploy the mappings along with the classes they map.
This is recommended if your queries call non-ANSI-standard SQL functions. Externalizing the query strings to mapping files will make the application more portable.
As in JDBC, always replace non-constant values by "?". Do not use string manipulation to bind a non-constant value in a query. You should also consider using named parameters in queries.
Hibernate allows the application to manage JDBC connections, but his approach should be considered
a last-resort. If you cannot use the built-in connection providers, consider providing your
own implementation of org.hibernate.connection.ConnectionProvider.
Suppose you have a Java type from a library that needs to be persisted but does not
provide the accessors needed to map it as a component. You should consider implementing
org.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 might benefit from
direct JDBC. Do not assume, however, that JDBC is necessarily faster. Please wait until you know something is a bottleneck.
If you need to use direct JDBC,
you can open a Hibernate Session, wrap your JDBC operation as a org.hibernate.jdbc.Work object and using that JDBC connection. This
way you can still use the same transaction strategy and underlying connection provider.
Session flushing:Sometimes the Session synchronizes its persistent state with the database. Performance will be affected if this process occurs too often. You can 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 can 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.merge() or Session.saveOrUpdate() to
synchronize objects with the database.
Database Transactions have to be as short as possible for best scalability. However, it is often necessary to implement long running application transactions, a single unit-of-work from the point of view of a user. An application transaction might span several client request/response cycles. It is common to use detached objects to implement application transactions. An appropriate alternative in a two tiered architecture, is to maintain a single open persistence contact session for the whole life cycle of the application transaction. Then simply disconnect from the JDBC connection at the end of each request and reconnect at the beginning of the subsequent request. Never share a single session across more than one application transaction or you will be working with 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 do not do this, Hibernate
cannot guarantee that in-memory state accurately represents the persistent state. For example,
do not use Session.load() to determine if an instance with the given identifier
exists on the database; use Session.get() or a query instead.
Use eager fetching sparingly. Use proxies and lazy collections for most associations to classes that
are not likely to be completely held in the second-level cache. For associations to cached classes,
where there is an a extremely high probability of a cache hit, explicitly disable eager fetching using
lazy="false". When join fetching is appropriate to a particular use
case, use a query with a left join fetch.
Hibernate frees the developer from writing tedious Data Transfer Objects (DTO). In a traditional EJB architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, they implicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOs before returning control to the presentation tier. Hibernate eliminates the first purpose. Unless you are prepared to hold the persistence context (the session) open across the view rendering process, you will still need an assembly phase. Think of your business methods as having a strict contract with the presentation tier about what data is available in the detached objects. This is not a limitation of Hibernate. It is a fundamental requirement of safe transactional data access.
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, however,
intended for "sufficiently large" applications. It is not appropriate for an application with
five tables.
Practical test cases for 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, most associations are one-to-many and many-to-one. For this reason, you should proceed cautiously when using any other association style.
Unidirectional associations are more difficult to query. In a large application, almost all associations must be navigable in both directions in queries.
Copyright © 2004 Red Hat, Inc.