Hibernate - O/R Mappings

So far, we have seen very basic O/R mapping using hibernate, but there are three most important mapping topics, which we have to learn in detail.

These are −

• Mapping of collections,
• Mapping of associations between entity classes, and
• Component Mappings.

Collections Mappings

If an entity or class has collection of values for a particular variable, then we can map those values using any one of the collection interfaces available in java. Hibernate can persist instances of java.util.Map, java.util.Set, java.util.SortedMap, java.util.SortedSet, java.util.List, and any array of persistent entities or values.

Sr.No.	Collection type & Mapping Description
1	java.util.Set This is mapped with a <set> element and initialized with java.util.HashSet
2	java.util.SortedSet This is mapped with a <set> element and initialized with java.util.TreeSet. The sort attribute can be set to either a comparator or natural ordering.
3	java.util.List This is mapped with a <list> element and initialized with java.util.ArrayList
4	java.util.Collection This is mapped with a <bag> or <ibag> element and initialized with java.util.ArrayList
5	java.util.Map This is mapped with a <map> element and initialized with java.util.HashMap
6	java.util.SortedMap This is mapped with a <map> element and initialized with java.util.TreeMap. The sort attribute can be set to either a comparator or natural ordering.

Arrays are supported by Hibernate with <primitive-array> for Java primitive value types and <array> for everything else. However, they are rarely used, so I am not going to discuss them in this tutorial.

If you want to map a user defined collection interfaces, which is not directly supported by Hibernate, you need to tell Hibernate about the semantics of your custom collections, which is not very easy and not recommend to be used.

Association Mappings

The mapping of associations between entity classes and the relationships between tables is the soul of ORM. Following are the four ways in which the cardinality of the relationship between the objects can be expressed. An association mapping can be unidirectional as well as bidirectional.

Sr.No.	Mapping type & Description
1	Many-to-One Mapping many-to-one relationship using Hibernate
2	One-to-One Mapping one-to-one relationship using Hibernate
3	One-to-Many Mapping one-to-many relationship using Hibernate
4	Many-to-Many Mapping many-to-many relationship using Hibernate

Component Mappings

It is very much possible that an Entity class can have a reference to another class as a member variable. If the referred class does not have its own life cycle and completely depends on the life cycle of the owning entity class, then the referred class hence therefore is called as the Component class.

The mapping of Collection of Components is also possible in a similar way just as the mapping of regular Collections with minor configuration differences. We will see these two mappings in detail with examples.

Sr.No.	Mapping type & Description
1	Component Mappings Mapping for a class having a reference to another class as a member variable.