hibernate-orm/reference/en/modules/persistent_classes.xml

<chapter id="persistent-classes" revision="1">
    <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).
        Persistent classes have, as the name implies, transient and also persistent
        instance stored in the database.
    </para>

    <para>
        Hibernate works best if these classes follow some simple rules, also known
        as the Plain Old Java Object (POJO) programming model. However, Hibernate3
        allows you to 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">
            <title>Provide an identifier property (optional)</title>

            <para>
                <literal>Cat</literal> has a property called <literal>id</literal>. This property 
                holds 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 optional. You can leave it off and let Hibernate keep track 
                of object identifiers internally. However, for many applications it is still
                a good (and very popular) design decision.
            </para>

            <para>
                What's more, 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) - 
			see "Lifecycle Objects"
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <literal>Session.saveOrUpdate()</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 associationfetching - 
		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">
        <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! We recommend 
	    implementing <literal>equals()</literal> and <literal>hashCode()</literal> using
            <emphasis>Business key equality</emphasis>.
        </para>

        <para>
            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>

    </sect1>

    <sect1 id="persistent-classes-dynamic" revision="1">
        <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>
	
    <dynamic-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>
	
    </dynamic-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 package
    </para>

</chapter>