hibernate-orm/reference/en/modules/persistent_classes.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 (see <xref linkend="transactions-basics-identity"/>).
            Immutable or unique properties are usually good
            candidates for a business key.
        </para>

    </sect1>

    <sect1 id="persistent-classes-dynamicmodels">
        <title>Dynamic modelss</title>

        <para>
            <emphasis>Note that the following features are currently considered
            experimental and may change in the near future.</emphasis>
        </para>

        <para>
            Persistent entities don't necessarily have to be represented as POJO classes
            or as JavaBean objects at runtime. Hibernate also supports dynamic models
            (using <literal>Map</literal>s of <literal>Map</literal>s at runtime) and the
            representation of entities as DOM4J trees. With this approach, you don't
            write persistent classes, only mapping files.
        </para>

        <para>
            By default, Hibernate works in normal POJO mode. You may set a default entity
            representation mode for a particular <literal>SessionFactory</literal> using the
            <literal>default_entity_mode</literal> configuration option (see
            <xref linkend="configuration-optional-properties"/>.
        </para>

        <para>
            The following examples demonstrates the representation using <literal>Map</literal>s.
            First, in the mapping file, an <literal>entity-name</literal> has to be declared
            instead of (or in addition to) a class name:
        </para>

        <programlisting><![CDATA[<hibernate-mapping>

    <class entity-name="Customer">

        <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="organization"
            column="ORGANIZATION_ID"
            class="Organization"/>

        <bag name="orders"
            inverse="true"
            lazy="false"
            cascade="all">
            <key column="CUSTOMER_ID"/>
            <one-to-many class="Order"/>
        </bag>

    </class>
    
</hibernate-mapping>]]></programlisting>

        <para>

            Note that even though associations are declared using target class names,
            the target type of an associations may also be a dynamic entity instead
            of a POJO.
        </para>

        <para>
            After setting the default entity mode to <literal>dynamic-map</literal>
            for the <literal>SessionFactory</literal>, we can at runtime work with
            <literal>Map</literal>s of <literal>Map</literal>s:
        </para>

        <programlisting><![CDATA[Session s = openSession();
Transaction tx = s.beginTransaction();
Session s = openSession();

// Create a customer
Map david = new HashMap();
david.put("name", "David");

// Create an organization
Map foobar = new HashMap();
foobar.put("name", "Foobar Inc.");

// Link both
david.put("organization", foobar);

// Save both
s.save("Customer", david);
s.save("Organization", foobar);

tx.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>

        <para>
            Entity representation modes can also be set on a per <literal>Session</literal>
            basis:
        </para>

        <programlisting><![CDATA[Session dynamicSession = pojoSession.getSession(EntityMode.MAP);

// Create a customer
Map david = new HashMap();
david.put("name", "David");
dynamicSession.save("Customer", david);
...
dynamicSession.flush();
dynamicSession.close()
...
// Continue on pojoSession
]]></programlisting>


        <para>
            Please note that the call to <literal>getSession()</literal> using an
            <literal>EntityMode</literal> is on the <literal>Session</literal> API, not the
            <literal>SessionFactory</literal>. That way, the new <literal>Session</literal>
            shares the underlying JDBC connection, transaction, and other context
            information. This means you don't have tocall <literal>flush()</literal>
            and <literal>close()</literal> on the secondary <literal>Session</literal>, and
            also leave the transaction and connection handling to the primary unit of work.
        </para>

        <para>
            More information about the XML representation capabilities can be found
            in <xref linkend="xml"/>.
        </para>

    </sect1>

    <para>
        TODO: Document user-extension framework in the property and proxy packages
    </para>

</chapter>