06 association of value types

Professional Open Source™

Association Mappings

© JBoss, Inc. 2003, 2004. 07/17/04 1

Fine Grained Domain models Professional Open Source™

A major objective of Hibernate is support for fine-grained domain models, which is the
most important requirement for a rich domain model. It’s one reason why we work with
POJOs. In crude terms, fine-grained means more classes than tables.

For example, a user may have both a billing address and a home address. In the
database, you may have a single USERS table with the columns
BILLING_STREET,BILLING_CITY, and BILLING_ZIPCODE, along with HOME_STREET,
HOME_CITY, and HOME_ZIPCODE. But there will be two classes User and Address

This fine grained domain model improves code reuse

© JBoss, Inc. 2003, 2004. 07/17/04 2

Collections of Value types

 An object of value type has no database identity; it belongs to an entity instance,
 and its persistent state is embedded in the table row of the owning entity—at least,
 if an entity has a reference to a single instance of a value type.

 If an entity class has a collection of value types (or a collection of references to value-
typed instances), you need an additional table, the so-called collection table.

 Before you map collections of value types to collection tables, remember that value-
typed classes don’t have identifiers or identifier properties.

 The lifespan of a value-type instance is bounded by the lifespan of the owning
entity instance. A value type doesn’t support shared references.

© JBoss, Inc. 2003, 2004. 3

ValueType Vs Entity

 When shared references to an object is needed, then model that
class as an Entity else make the class as a ValueType

© JBoss, Inc. 2003, 2004. 4

Understanding Java identity and equality

 There are three methods for identifying objects:

3. Objects are identical if they occupy the same memory location in the
JVM. This can be checked by using the == operator. This concept is
known as object identity.
4. Objects are equal if they have the same value, as defined by the
 equals(Object o) method. Classes that don’t explicitly override this
 method inherit the implementation defined by java.lang.Object, which
 compares object identity. This concept is known as equality.
3 Objects stored in a relational database are identical if they represent
the same row or, equivalently, if they share the same table and
primary key value. This concept is known as database identity.

© JBoss, Inc. 2003, 2004. 5

Handling database identity

 Hibernate exposes database identity to the application in two ways:
 ■ The value of the identifier property of a persistent instance
 ■ The value returned by Session.getIdentifier(Object entity)

 In addition to operations for testing Java object identity, (a == b), and
object equality, ( a.equals(b) ), you may now use
a.getId().equals( b.getId() ) to test database identity.

© JBoss, Inc. 2003, 2004. 6

Selecting a primary key

 The candidate key is a column or set of columns that could be used to
identify a particular row in a table.

 To become a primary key, a candidate key must satisfy the following
properties:
– Its value (for any column of the candidate key) is never null.
– Each row has a unique value.
– The value of a particular row never changes.

 If a table has only one identifying attribute, it’s, by definition, the
primary key.

 However, several columns or combinations of columns may satisfy
these properties for a particular table; you choose between candidate
keys to decide the best primary key for the table.

© JBoss, Inc. 2003, 2004. 7

Natural Key

 A natural key is a key with business meaning: an attribute or
combination of attributes that is unique by virtue of its business
semantics.

 Examples  the U.S. Social Security Number and Australian Tax File
Number.

 Distinguishing natural keys is simple : If a candidate key attribute has
meaning outside the database context, it’s a natural key, whether or
not it’s automatically generated.

© JBoss, Inc. 2003, 2004. 8

Surrogate Key

 Surrogate keys have no business meaning—they’re unique values
generated by the database or application. Application users ideally
 don’t see or refer to these key values; they’re part of the system
internals.

© JBoss, Inc. 2003, 2004. 9

Selecting a key Generator

 Hibernate has several built-in identifier-generation strategies.

© JBoss, Inc. 2003, 2004. 10

Configuring a generator

 Some of the built-in identifier generators can be configured with
options. In a native Hibernate XML mapping, you define options as
pairs of keys and values:

 <id name="id" type="long" column="person_id">
 <generator class="sequence">
 <param name="sequence">person_id_sequence</param>
 </generator>
 </id>

© JBoss, Inc. 2003, 2004. 15


CLASS MAPPING OPTIONS

© JBoss, Inc. 2003, 2004. 16

Dynamic SQL generation

 By default, Hibernate creates SQL statements for each persistent
class on startup. These statements are simple create, read, update,
and delete operations for reading a single row, deleting a row, and so
on.
 The generated SQL statement updates all columns, and if the value
of a particular column isn’t modified, the statement sets it to its old
value.
 In some situations, such as a legacy table with hundreds of columns
where the SQL statements will be large for even the simplest
operations (say, only one column needs updating), you have to turn
off this startup SQL generation and switch to dynamic statements
generated at runtime.
 Two attributes for disabling CRUD SQL generation on startup are
available on the <class> mapping element:

© JBoss, Inc. 2003, 2004. 17

Making an entity immutable

 Instances of a particular class may be immutable.
 A Bid made for an item is immutable. Hence, no UPDATE statement
ever needs to be executed on the BID table.

 Hibernate can also make a few other optimizations, such as avoiding
dirty checking, if you map an immutable class with the mutable
 attribute set to false:

A POJO is immutable if no public setter methods for any properties
of the class are exposed—all values are set in the constructor.
Instead of private setter methods, you often prefer direct field
access by Hibernate for immutable persistent classes, so you don’t
have to write useless accessor methods.

© JBoss, Inc. 2003, 2004. 18

Naming entities for querying

 By default, all class names are automatically “imported” into the
namespace of the Hibernate query language, HQL. In other words,
you can use the short class names without a package prefix in HQL,
which is convenient. However, this auto import can be turned off if two
classes with the same name exist for a given SessionFactory, maybe
in different packages of the domain model.
 You can turn off auto-import of names into the HQL namespace for
particular mapping files with the autoimport=“false" setting on the
<hibernate-mapping> root element.
 Entity names can also be imported explicitly into the HQL
namespace. You can even import classes and interfaces that aren’t
explicitly mapped, so a short name can be used in polymorphic HQL
queries:

© JBoss, Inc. 2003, 2004. 19

Naming entities for querying

 You can now use an HQL query such as from IAuditable to retrieve all
persistent instances of classes that implement the
auction.model.Auditable interface.

 Note that the <import> element , like all other immediate child
elements of <hibernate-mapping>, is an application-wide declaration,
so you don’t have to (and can’t) duplicate this in other mapping files.

© JBoss, Inc. 2003, 2004. 20

Declaring a package name

© JBoss, Inc. 2003, 2004. 21

Mapping components

 See the composition example.

© JBoss, Inc. 2003, 2004. 22

Selecting a collection interface

A java.util.Set is mapped with a <set> element. Initialize the collection
with a java.util.HashSet. The order of its elements isn’t preserved,
and duplicate elements aren’t allowed. This is the most common
persistent collection in a typical Hibernate application.

A java.util.SortedSet can be mapped with <set>, and the sort attribute
can be set to either a comparator or natural ordering for in-memory
sorting. Initialize the collection with a java.util.TreeSet instance.

A java.util.List can be mapped with <list>, preserving the position of
each element with an additional index column in the collection table.
Initialize with a java.util.ArrayList.

© JBoss, Inc. 2003, 2004. 23


 A java.util.Collection can be mapped with <bag> or <idbag>. Java
 doesn’t have a Bag interface or an implementation; however,
java.util.Collection allows bag semantics (possible duplicates,
no element order is preserved). Hibernate supports persistent bags
(it uses lists internally but ignores the index of the elements). Use a
java.util.ArrayList to initialize a bag collection.

 A java.util.Map can be mapped with <map>, preserving key and value
 pairs. Use a java.util.HashMap to initialize a property.

 A java.util.SortedMap can be mapped with <map> element, and the
sort attribute can be set to either a comparator or natural ordering for
in-memory sorting. Initialize the collection with a java.util.TreeMap
instance.

© JBoss, Inc. 2003, 2004. 24

Mapping a set

 The simplest implementation is a Set of String image filenames. First,
add a collection property to the Item class:

© JBoss, Inc. 2003, 2004. 25


 The image filenames are stored in a table named ITEM_IMAGE, the
collection table. From the point of view of the database, this table is a
separate entity, a separate table, but Hibernate hides this for you.

 The <key> element declares the foreign key column in the collection
table that references the primary key ITEM_ID of the owning entity.
The <element> tag declares this collection as a collection of value
type instances—in this case, of strings.

 A set can’t contain duplicate elements, so the primary key of the
ITEM_IMAGE collection table is a composite of both columns in
the <set> declaration: ITEM_ID and FILENAME.

© JBoss, Inc. 2003, 2004. 26

Disadvantages of <set>

 A set is very inefficient if/when adding to a large collection,
because Hibernate has to load the entire collection into memory
in order to ensure that uniqueness is preserved.

© JBoss, Inc. 2003, 2004. 27

Mapping an identifier bag

 An unordered collection that permits duplicate elements is
called a bag.
 Curiously, the Java Collections framework doesn’t include a bag
implementation.However, the java.util.Collection interface has bag
semantics, so you only need a matching implementation.

 Write the collection property with the java.util.Collection interface,
 and, on declaration, initialize it with an ArrayList of the JDK. Map the
 collection in Hibernate with a standard <bag> or <idbag> element.

 Hibernate has a built-in PersistentBag that can deal with lists;
however, consistent with the contract of a bag, it ignores the
position of elements in the ArrayList. In other words, you get a
persistent Collection.

© JBoss, Inc. 2003, 2004. 28


You also have to modify the collection table to permit duplicate FILENAMEs;
the table needs a different primary key. An <idbag> mapping adds a surrogate
key column to the collection table, much like the synthetic identifiers you use for
entity classes.
Note that the native generator for primary keys isn’t supported for <idbag>
mappings
© JBoss, Inc. 2003, 2004. 29

Mapping a list

 If you wish to preserve the order in which images are attached to the
Item, u can use list

© JBoss, Inc. 2003, 2004. 30


 A <list> mapping requires the addition of an index column to the
collection table.
 The index column defines the position of the element in the collection.
Thus,Hibernate is able to preserve the ordering of the collection
elements.

 The primary key of the collection table is a composite of ITEM_ID and
POSITION.
 Notice that duplicate elements (FILENAME) are now allowed,

© JBoss, Inc. 2003, 2004. 31

Mapping a map

The primary key of the collection table is a composite of ITEM_ID and
IMAGENAME. The IMAGENAME column holds the keys of the map. Again,
duplicate elements are allowed;

© JBoss, Inc. 2003, 2004. 32

Sorted and ordered collections

A sorted collection is sorted in memory using a Java comparator.
An ordered collection is ordered at the database level using an SQL
query with an order by clause.

© JBoss, Inc. 2003, 2004. 33


 By specifying sort="natural", you tell Hibernate to use a SortedMap
and to sort the image names according to the compareTo() method of
java.lang.String. If you need some other sort algorithm (for example,
reverse alphabetical order),you may specify the name of a class that
implements java.util.Comparator in the sort attribute. For example:

© JBoss, Inc. 2003, 2004. 34

Ordering Collections

 Instead of switching to the Sorted* interfaces and the (Tree*
 implementations), you may want to work with a linked map and to sort
elements on the database side, not in memory.

© JBoss, Inc. 2003, 2004. 36

Ordering Map

 The expression in the order-by attribute is a fragment of an SQL order
by clause. In this case, Hibernate orders the collection elements by
the IMAGENAME column in ascending order during loading of the
collection. You can even include an SQL function call in the order-by
attribute:

© JBoss, Inc. 2003, 2004. 37

Collections of components

As you can see from the composition association style (the black diamond),
Image is a component of Item, and Item is the entity that is responsible for
the lifecycle of Image instances.

The multiplicity of the association further declares this association as many-
valued—that is, many (or zero) Image instances for the same Item instance.

© JBoss, Inc. 2003, 2004. 40

Mapping Collection of Components

 Collections of components are mapped similarly to collections of JDK
value type. The only difference is the use of <composite-element>
instead of an <element> tag. An ordered set of images (internally, a
LinkedHashSet) can be mapped like this:

See next page for example data

© JBoss, Inc. 2003, 2004. 41


 The tables with example data are shown in figure

This is a set, so the primary key of the collection table is a composite of the
key column and all element columns: ITEM_ID, IMAGENAME,
FILENAME, SIZEX, andSIZEY.

Because these columns all appear in the primary key, you needed to
declare them with not-null="true" (or make sure they’re NOT NULL in any
existing schema). No column in a composite primary key can be nullable—
you can’t identify what you don’t know. This is probably a disadvantage of
this particular mapping.

© JBoss, Inc. 2003, 2004. 42

Avoiding not-null columns

 Analogous to the additional surrogate identifier property an <idbag>
offers, a surrogate key column would come in handy now.

© JBoss, Inc. 2003, 2004. 43

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