Hibernate features an intuitive, extensible criteria query API. The interface org.hibernate.Criteria represents a query against a particular persistent class. The Session is a factory for Criteria instances. An individual query criterion is an instance of the interface org.hibernate.criterion.Criterion. The class org.hibernate.criterion.Restrictions defines factory methods for obtaining certain built-in Criterion types. Restrictions may be grouped logically. There are quite a range of built-in criterion types (Restrictions subclasses), but one that is especially useful lets you specify SQL directly. The {alias} placeholder with be replaced by the row alias of the queried entity. An alternative approach to obtaining a criterion is to get it from a Property instance. You can create a Property by calling Property.forName(). You may order the results using org.hibernate.criterion.Order. You may easily specify constraints upon related entities by navigating associations using createCriteria(). note that the second createCriteria() returns a new instance of Criteria, which refers to the elements of the kittens collection. The following, alternate form is useful in certain circumstances. (createAlias() does not create a new instance of Criteria.) Note that the kittens collections held by the Cat instances returned by the previous two queries are not pre-filtered by the criteria! If you wish to retrieve just the kittens that match the criteria, you must use returnMaps(). You may specify association fetching semantics at runtime using setFetchMode(). This query will fetch both mate and kittens by outer join. See for more information. The class org.hibernate.criterion.Example allows you to construct a query criterion from a given instance. Version properties, identifiers and associations are ignored. By default, null valued properties are excluded. You can adjust how the Example is applied. You can even use examples to place criteria upon associated objects. The class org.hibernate.criterion.Projections is a factory for Projection instances. We apply a projection to a query by calling setProjection(). There is no explicit "group by" necessary in a criteria query. Certain projection types are defined to be grouping projections, which also appear in the SQL group by clause. An alias may optionally be assigned to a projection, so that the projected value may be referred to in restrictions or orderings. Here are two different ways to do this: The alias() and as() methods simply wrap a projection instance in another, aliased, instance of Projection. As a shortcut, you can assign an alias when you add the projection to a projection list: You can also use Property.forName() to express projections: The DetachedCriteria class lets you create a query outside the scope of a session, and then later execute it using some arbitrary Session. A DetachedCriteria may also be used to express a subquery. Criterion instances involving subqueries may be obtained via Subqueries or Property. Even correlated subqueries are possible: For most queries, including criteria queries, the query cache is not very efficient, because query cache invalidation occurs too frequently. However, there is one special kind of query where we can optimize the cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides special provision for this use case. First, you should map the natural key of your entity using <natural-id>, and enable use of the second-level cache. ]]> Note that this functionality is not intended for use with entities with mutable natural keys. Next, enable the Hibernate query cache. Now, Restrictions.naturalId() allows us to make use of the more efficient cache algorithm.