Persistent classes are classes in an application that implement the entities of the business problem (e.g. Customer and Order in an E-commerce application). Not all instances of a persistent class are considered to be in the persistent state - an instance may instead be transient or detached. Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object (POJO) programming model. However none of these rules are hard requirements. Indeed, Hibernate3 assumes very little about the nature of your persistent objects. You may express a domain model in other ways: using trees of Map instances, for example. Most Java applications require a persistent class representing felines. There are four main rules to follow here: Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance variables. We believe it is far better to decouple this implementation detail from the persistence mechanism. Hibernate persists JavaBeans style properties, and recognizes method names of the form getFoo, isFoo and setFoo. You may however switch to direct field access for particular properties, if needed. Properties need not be declared public - Hibernate can persist a property with a default, protected or private get / set pair. Cat has a no-argument constructor. All persistent classes must have a default constructor (which may be non-public) so Hibernate can instantiate them using Constructor.newInstance(). We recommend having a constructor with at least package visibility for runtime proxy generation in Hibernate. Cat has a property called id. This property maps to the primary key column of a database table. The property might have been called anything, and its type might have been any primitive type, any primitive "wrapper" type, java.lang.String or java.util.Date. (If your legacy database table has composite keys, you can even use a user-defined class with properties of these types - see the section on composite identifiers later.) The identifier property is strictly optional. You can leave them off and let Hibernate keep track of object identifiers internally. We do not recommend this, however. In fact, some functionality is available only to classes which declare an identifier property: Transitive reattachment for detached objects (cascade update or cascade merge) - see Session.saveOrUpdate() Session.merge() We recommend you declare consistently-named identifier properties on persistent classes. We further recommend that you use a nullable (ie. non-primitive) type. A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the implementation of an interface that declares all public methods. You can persist final classes that do not implement an interface with Hibernate, but you won't be able to use proxies for lazy association fetching - which will limit your options for performance tuning. A subclass must also observe the first and second rules. It inherits its identifier property from the superclass, Cat. You have to override the equals() and hashCode() methods if you intend to put instances of persistent classes in a Set (the recommended way to represent many-valued associations) and intend to use reattachment of detached instances Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a particular session scope. So as soon as we mix instances retrieved in different sessions, we must implement equals() and hashCode() if we wish to have meaningful semantics for Sets. The most obvious way is to implement equals()/hashCode() by comparing the identifier value of both objects. If the value is the same, both must be the same database row, they are therefore equal (if both are added to a Set, we will only have one element in the Set). Unfortunately, we can't use that approach with generated identifiers! Hibernate will only assign identifier values to objects that are persistent, a newly created instance will not have any identifier value! Furthermore, if an instance is unsaved and currently in a Set, saving it will assign an identifier value to the object. If equals() and hashCode() are based on the identifier value, the hash code would change, breaking the contract of the Set. See the Hibernate website for a full discussion of this problem. Note that this is not a Hibernate issue, but normal Java semantics of object identity and equality. We recommend implementing equals() and hashCode() using Business key equality. Business key equality means that the equals() method compares only the properties that form the business key, a key that would identify our instance in the real world (a natural candidate key): Note that a business key does not have to be as solid as a database primary key candidate (see ). Immutable or unique properties are usually good candidates for a business key. Note that the following features are currently considered experimental and may change in the near future. Persistent entities don't necessarily have to be represented as POJO classes or as JavaBean objects at runtime. Hibernate also supports dynamic models (using Maps of Maps at runtime) and the representation of entities as DOM4J trees. With this approach, you don't write persistent classes, only mapping files. By default, Hibernate works in normal POJO mode. You may set a default entity representation mode for a particular SessionFactory using the default_entity_mode configuration option (see . The following examples demonstrates the representation using Maps. First, in the mapping file, an entity-name has to be declared instead of (or in addition to) a class name: ]]> Note that even though associations are declared using target class names, the target type of an associations may also be a dynamic entity instead of a POJO. After setting the default entity mode to dynamic-map for the SessionFactory, we can at runtime work with Maps of Maps: The advantages of a dynamic mapping are quick turnaround time for prototyping without the need for entity class implementation. However, you lose compile-time type checking and will very likely deal with many exceptions at runtime. Thanks to the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a proper domain model implementation on top later on. Entity representation modes can also be set on a per Session basis: Please note that the call to getSession() using an EntityMode is on the Session API, not the SessionFactory. That way, the new Session shares the underlying JDBC connection, transaction, and other context information. This means you don't have tocall flush() and close() on the secondary Session, and also leave the transaction and connection handling to the primary unit of work. More information about the XML representation capabilities can be found in . TODO: Document user-extension framework in the property and proxy packages