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 the mapping of collections, the 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.
| Collection type |
Mapping and Description |
| java.util.Set |
This is mapped with a <set> element and initialized with java.util.HashSet |
| 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. |
| java.util.List |
This is mapped with a <list> element and initialized with java.util.ArrayList |
| java.util.Collection |
This is mapped with a <bag> or <ibag> element and initialized with java.util.ArrayList |
| java.util.Map |
This is mapped with a <map> element and initialized with java.util.HashMap |
| 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'm 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.
| Mapping type |
Description |
| Many-to-One |
Mapping many-to-one relationship using Hibernate |
| One-to-One |
Mapping one-to-one relationship using Hibernate |
| One-to-Many |
Mapping one-to-many relationship using Hibernate |
| 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
it's 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.
| Mapping type |
Description |
| Component Mappings |
Mapping for a class having a reference to another class as a member variable. |