390 lines
14 KiB
XML
390 lines
14 KiB
XML
<chapter id="persistent-classes" revision="2">
|
|
<title>Persistent Classes</title>
|
|
|
|
<para>
|
|
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.
|
|
</para>
|
|
|
|
<para>
|
|
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 <literal>Map</literal> instances, for example.
|
|
</para>
|
|
|
|
<sect1 id="persistent-classes-pojo">
|
|
<title>A simple POJO example</title>
|
|
|
|
<para>
|
|
Most Java applications require a persistent class representing felines.
|
|
</para>
|
|
|
|
<programlisting><![CDATA[package eg;
|
|
import java.util.Set;
|
|
import java.util.Date;
|
|
|
|
public class Cat {
|
|
private Long id; // identifier
|
|
|
|
private Date birthdate;
|
|
private Color color;
|
|
private char sex;
|
|
private float weight;
|
|
private int litterId;
|
|
|
|
private Cat mother;
|
|
private Set kittens = new HashSet();
|
|
|
|
private void setId(Long id) {
|
|
this.id=id;
|
|
}
|
|
public Long getId() {
|
|
return id;
|
|
}
|
|
|
|
void setBirthdate(Date date) {
|
|
birthdate = date;
|
|
}
|
|
public Date getBirthdate() {
|
|
return birthdate;
|
|
}
|
|
|
|
void setWeight(float weight) {
|
|
this.weight = weight;
|
|
}
|
|
public float getWeight() {
|
|
return weight;
|
|
}
|
|
|
|
public Color getColor() {
|
|
return color;
|
|
}
|
|
void setColor(Color color) {
|
|
this.color = color;
|
|
}
|
|
|
|
void setSex(char sex) {
|
|
this.sex=sex;
|
|
}
|
|
public char getSex() {
|
|
return sex;
|
|
}
|
|
|
|
void setLitterId(int id) {
|
|
this.litterId = id;
|
|
}
|
|
public int getLitterId() {
|
|
return litterId;
|
|
}
|
|
|
|
void setMother(Cat mother) {
|
|
this.mother = mother;
|
|
}
|
|
public Cat getMother() {
|
|
return mother;
|
|
}
|
|
void setKittens(Set kittens) {
|
|
this.kittens = kittens;
|
|
}
|
|
public Set getKittens() {
|
|
return kittens;
|
|
}
|
|
|
|
// addKitten not needed by Hibernate
|
|
public void addKitten(Cat kitten) {
|
|
kitten.setMother(this);
|
|
kitten.setLitterId( kittens.size() );
|
|
kittens.add(kitten);
|
|
}
|
|
}]]></programlisting>
|
|
|
|
<para>
|
|
There are four main rules to follow here:
|
|
</para>
|
|
|
|
|
|
<sect2 id="persistent-classes-pojo-accessors" revision="1">
|
|
<title>Declare accessors and mutators for persistent fields</title>
|
|
|
|
<para>
|
|
<literal>Cat</literal> 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 <literal>getFoo</literal>, <literal>isFoo</literal> and
|
|
<literal>setFoo</literal>. You may however switch to direct field access for
|
|
particular properties, if needed.
|
|
</para>
|
|
|
|
<para>
|
|
Properties need <emphasis>not</emphasis> be declared public - Hibernate can
|
|
persist a property with a default, <literal>protected</literal> or <literal>
|
|
private</literal> get / set pair.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="persistent-classes-pojo-constructor" revision="1">
|
|
<title>Implement a no-argument constructor</title>
|
|
|
|
<para>
|
|
<literal>Cat</literal> has a no-argument constructor. All
|
|
persistent classes must have a default constructor (which may be non-public) so
|
|
Hibernate can instantiate them using <literal>Constructor.newInstance()</literal>.
|
|
We recommend having a constructor with at least <emphasis>package</emphasis>
|
|
visibility for runtime proxy generation in Hibernate.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="persistent-classes-pojo-identifier" revision="2">
|
|
<title>Provide an identifier property (optional)</title>
|
|
|
|
<para>
|
|
<literal>Cat</literal> has a property called <literal>id</literal>. 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, <literal>java.lang.String</literal> or <literal>java.util.Date</literal>. (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.)
|
|
</para>
|
|
|
|
<para>
|
|
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.
|
|
</para>
|
|
|
|
<para>
|
|
In fact, some functionality is available only to classes which declare an
|
|
identifier property:
|
|
</para>
|
|
|
|
<itemizedlist spacing="compact">
|
|
<listitem>
|
|
<para>
|
|
Transitive reattachment for detached objects (cascade update or cascade
|
|
merge) - see <xref linkend="objectstate-transitive"/>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<literal>Session.saveOrUpdate()</literal>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<literal>Session.merge()</literal>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>
|
|
We recommend you declare consistently-named identifier properties on persistent
|
|
classes. We further recommend that you use a nullable (ie. non-primitive) type.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="persistent-classes-pojo-final">
|
|
<title>Prefer non-final classes (optional)</title>
|
|
<para>
|
|
A central feature of Hibernate, <emphasis>proxies</emphasis>, depends upon the
|
|
persistent class being either non-final, or the implementation of an interface
|
|
that declares all public methods.
|
|
</para>
|
|
<para>
|
|
You can persist <literal>final</literal> 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.
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="persistent-classes-inheritance">
|
|
<title>Implementing inheritance</title>
|
|
|
|
<para>
|
|
A subclass must also observe the first and second rules. It inherits its
|
|
identifier property from the superclass, <literal>Cat</literal>.
|
|
</para>
|
|
|
|
<programlisting><![CDATA[package eg;
|
|
|
|
public class DomesticCat extends Cat {
|
|
private String name;
|
|
|
|
public String getName() {
|
|
return name;
|
|
}
|
|
protected void setName(String name) {
|
|
this.name=name;
|
|
}
|
|
}]]></programlisting>
|
|
</sect1>
|
|
|
|
<sect1 id="persistent-classes-equalshashcode" revision="1">
|
|
<title>Implementing <literal>equals()</literal> and <literal>hashCode()</literal></title>
|
|
|
|
<para>
|
|
You have to override the <literal>equals()</literal> and <literal>hashCode()</literal>
|
|
methods if you
|
|
</para>
|
|
<itemizedlist spacing="compact">
|
|
<listitem>
|
|
<para>
|
|
intend to put instances of persistent classes in a <literal>Set</literal>
|
|
(the recommended way to represent many-valued associations)
|
|
<emphasis>and</emphasis>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
intend to use reattachment of detached instances
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>
|
|
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 <literal>equals()</literal> and
|
|
<literal>hashCode()</literal> if we wish to have meaningful semantics for
|
|
<literal>Set</literal>s.
|
|
</para>
|
|
|
|
<para>
|
|
The most obvious way is to implement <literal>equals()</literal>/<literal>hashCode()</literal>
|
|
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 <literal>Set</literal>,
|
|
we will only have one element in the <literal>Set</literal>). 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 <literal>Set</literal>, saving it will assign
|
|
an identifier value to the object. If <literal>equals()</literal> and <literal>hashCode()</literal>
|
|
are based on the identifier value, the hash code would change, breaking the contract of the
|
|
<literal>Set</literal>. 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.
|
|
</para>
|
|
|
|
<para>
|
|
We recommend implementing <literal>equals()</literal> and <literal>hashCode()</literal>
|
|
using <emphasis>Business key equality</emphasis>. Business key equality means that the
|
|
<literal>equals()</literal> method compares only the properties that form the business
|
|
key, a key that would identify our instance in the real world (a
|
|
<emphasis>natural</emphasis> candidate key):
|
|
</para>
|
|
|
|
<programlisting><![CDATA[public class Cat {
|
|
|
|
...
|
|
public boolean equals(Object other) {
|
|
if (this == other) return true;
|
|
if ( !(other instanceof Cat) ) return false;
|
|
|
|
final Cat cat = (Cat) other;
|
|
|
|
if ( !cat.getLitterId().equals( getLitterId() ) ) return false;
|
|
if ( !cat.getMother().equals( getMother() ) ) return false;
|
|
|
|
return true;
|
|
}
|
|
|
|
public int hashCode() {
|
|
int result;
|
|
result = getMother().hashCode();
|
|
result = 29 * result + getLitterId();
|
|
return result;
|
|
}
|
|
|
|
}]]></programlisting>
|
|
|
|
<para>
|
|
Note that a business key does not have to be as solid as a database
|
|
primary key candidate. Immutable or unique properties are usually good
|
|
candidates for a business key.
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="persistent-classes-dynamic" revision="2">
|
|
<title>Dynamic models</title>
|
|
|
|
<para>
|
|
Hibernate also supports dynamic domain models, using <literal>Map</literal>s of
|
|
<literal>Map</literal>s. With this approach, you don't write persistent classes,
|
|
a Hibernate mapping file for each "entity" is sufficient:
|
|
</para>
|
|
|
|
<programlisting><![CDATA[<hibernate-mapping>
|
|
|
|
<class entity-name="TestMap">
|
|
|
|
<id name="id"
|
|
type="long"
|
|
column="ID">
|
|
<generator class="sequence"/>
|
|
</id>
|
|
|
|
<property name="name"
|
|
column="NAME"
|
|
type="string"/>
|
|
|
|
<property name="address"
|
|
column="ADDRESS"
|
|
type="string"/>
|
|
|
|
<many-to-one name="parent"
|
|
column="PARENT_ID"
|
|
class="TestMap"/>
|
|
|
|
<bag name="children"
|
|
inverse="true"
|
|
lazy="false"
|
|
cascade="all">
|
|
<key column="PARENT_ID"/>
|
|
<one-to-many class="TestMap"/>
|
|
</bag>
|
|
|
|
</class>
|
|
|
|
</hibernate-mapping>]]></programlisting>
|
|
|
|
<para>
|
|
At runtime, you just instantiate <literal>HashMap</literal>s and use
|
|
the Hibernate entity name to refer to a particular type:
|
|
</para>
|
|
|
|
<programlisting><![CDATA[Session s = openSession();
|
|
Transaction t = s.beginTransaction();
|
|
Map parent = new HashMap();
|
|
parent.put("name", "foo");
|
|
parent.put("address", "bar");
|
|
|
|
Map child = new HashMap();
|
|
child.put("name", "fooTwo");
|
|
child.put("address", "barTwo");
|
|
child.put("parent", parent);
|
|
|
|
s.save("TestMap", parent);
|
|
t.commit();
|
|
s.close();]]></programlisting>
|
|
|
|
<para>
|
|
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.
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
<para>
|
|
TODO: Document user-extension framework in the property and proxy packages
|
|
</para>
|
|
|
|
</chapter>
|
|
|