Chapter 4. Persistent Classes

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.

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 Map instances, for example.

4.1. A simple POJO example

Most Java applications require a persistent class representing felines.

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) {;
    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) {;
    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.setLitterId( kittens.size() ); 

There are four main rules to follow here:

4.1.1. Implement a no-argument constructor

Cat has a no-argument constructor. All persistent classes must have a default constructor (which may be non-public) so that Hibernate can instantiate them using Constructor.newInstance(). We strongly recommend having a default constructor with at least package visibility for runtime proxy generation in Hibernate.

4.1.2. Provide an identifier property (optional)

Cat has a property called id. 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, java.lang.String or java.util.Date. (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.)

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.

In fact, some functionality is available only to classes which declare an identifier property:

We recommend you declare consistently-named identifier properties on persistent classes. We further recommend that you use a nullable (ie. non-primitive) type.

4.1.3. Prefer non-final classes (optional)

A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the implementation of an interface that declares all public methods.

You can persist final 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.

You should also avoid declaring public final methods on the non-final classes. If you want to use a class with a public final method, you must explicitly disable proxying by setting lazy="false".

4.1.4. Declare accessors and mutators for persistent fields (optional)

Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance variables. We believe it is better to provide an indirection between the relational schema and internal data structures of the class. By default, Hibernate persists JavaBeans style properties, and recognizes method names of the form getFoo, isFoo and setFoo. You may switch to direct field access for particular properties, if needed.

Properties need not be declared public - Hibernate can persist a property with a default, protected or private get / set pair.

4.2. Implementing inheritance

A subclass must also observe the first and second rules. It inherits its identifier property from the superclass, Cat.

package eg;

public class DomesticCat extends Cat {
        private String name;

        public String getName() {
                return name;
        protected void setName(String name) {

4.3. Implementing equals() and hashCode()

You have to override the equals() and hashCode() methods if you

  • intend to put instances of persistent classes in a Set (the recommended way to represent many-valued associations) and

  • intend to use reattachment of detached instances

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 equals() and hashCode() if we wish to have meaningful semantics for Sets.

The most obvious way is to implement equals()/hashCode() 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 Set, we will only have one element in the Set). 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 Set, saving it will assign an identifier value to the object. If equals() and hashCode() are based on the identifier value, the hash code would change, breaking the contract of the Set. 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.

We recommend implementing equals() and hashCode() using Business key equality. Business key equality means that the equals() method compares only the properties that form the business key, a key that would identify our instance in the real world (a natural candidate key):

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;


Note that a business key does not have to be as solid as a database primary key candidate (see Section 11.1.3, “Considering object identity”). Immutable or unique properties are usually good candidates for a business key.

4.4. Dynamic models

Note that the following features are currently considered experimental and may change in the near future.

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

By default, Hibernate works in normal POJO mode. You may set a default entity representation mode for a particular SessionFactory using the default_entity_mode configuration option (see Table 3.3, “Hibernate Configuration Properties”.

The following examples demonstrates the representation using Maps. First, in the mapping file, an entity-name has to be declared instead of (or in addition to) a class name:


    <class entity-name="Customer">

        <id name="id"
            <generator class="sequence"/>

        <property name="name"

        <property name="address"

        <many-to-one name="organization"

        <bag name="orders"
            <key column="CUSTOMER_ID"/>
            <one-to-many class="Order"/>


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.

After setting the default entity mode to dynamic-map for the SessionFactory, we can at runtime work with Maps of Maps:

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"Customer", david);"Organization", foobar);


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.

Entity representation modes can also be set on a per Session basis:

Session dynamicSession = pojoSession.getSession(EntityMode.MAP);

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

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

More information about the XML representation capabilities can be found in Chapter 18, XML Mapping.

4.5. Tuplizers

org.hibernate.tuple.Tuplizer, and its sub-interfaces, are responsible for managing a particular representation of a piece of data, given that representation's org.hibernate.EntityMode. If a given piece of data is thought of as a data structure, then a tuplizer is the thing which knows how to create such a data structure and how to extract values from and inject values into such a data structure. For example, for the POJO entity mode, the correpsonding tuplizer knows how create the POJO through its constructor and how to access the POJO properties using the defined property accessors. There are two high-level types of Tuplizers, represented by the org.hibernate.tuple.EntityTuplizer and org.hibernate.tuple.ComponentTuplizer interfaces. EntityTuplizers are responsible for managing the above mentioned contracts in regards to entities, while ComponentTuplizers do the same for components.

Users may also plug in their own tuplizers. Perhaps you require that a java.util.Map implementation other than java.util.HashMap be used while in the dynamic-map entity-mode; or perhaps you need to define a different proxy generation strategy than the one used by default. Both would be achieved by defining a custom tuplizer implementation. Tuplizers definitions are attached to the entity or component mapping they are meant to manage. Going back to the example of our customer entity:

    <class entity-name="Customer">
            Override the dynamic-map entity-mode
            tuplizer for the customer entity
        <tuplizer entity-mode="dynamic-map"

        <id name="id" type="long" column="ID">
            <generator class="sequence"/>

        <!-- other properties -->

public class CustomMapTuplizerImpl
        extends org.hibernate.tuple.DynamicMapEntityTuplizer {
    // override the buildInstantiator() method to plug in our custom map...
    protected final Instantiator buildInstantiator(
            org.hibernate.mapping.PersistentClass mappingInfo) {
        return new CustomMapInstantiator( mappingInfo );

    private static final class CustomMapInstantiator
            extends org.hibernate.tuple.DynamicMapInstantitor {
        // override the generateMap() method to return our custom map...
	    protected final Map generateMap() {
		    return new CustomMap();

4.6. EntityNameResolvers

The org.hibernate.EntityNameResolver interface is a contract for resolving the entity name of a given entity instance. The interface defines a single method resolveEntityName which is passed the entity instance and is expected to return the appropriate entity name (null is allowed and would indicate that the resolver does not know how to resolve the entity name of the given entity instance). Generally speaking, an org.hibernate.EntityNameResolver is going to be most useful in the case of dynamic models. One example might be using proxied interfaces as your domain model. The hibernate test suite has an example of this exact style of usage under the <package>org.hibernate.test.dynamicentity.tuplizer2</package>. Here is some of the code from that package for illustration.

 * A very trivial JDK Proxy InvocationHandler implementation where we proxy an interface as
 * the domain model and simply store persistent state in an internal Map.  This is an extremely
 * trivial example meant only for illustration.
public final class DataProxyHandler implements InvocationHandler {
	private String entityName;
	private HashMap data = new HashMap();

	public DataProxyHandler(String entityName, Serializable id) {
		this.entityName = entityName;
		data.put( "Id", id );

	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
		String methodName = method.getName();
		if ( methodName.startsWith( "set" ) ) {
			String propertyName = methodName.substring( 3 );
			data.put( propertyName, args[0] );
		else if ( methodName.startsWith( "get" ) ) {
			String propertyName = methodName.substring( 3 );
			return data.get( propertyName );
		else if ( "toString".equals( methodName ) ) {
			return entityName + "#" + data.get( "Id" );
		else if ( "hashCode".equals( methodName ) ) {
			return new Integer( this.hashCode() );
		return null;

	public String getEntityName() {
		return entityName;

	public HashMap getData() {
		return data;

public class ProxyHelper {
    public static String extractEntityName(Object object) {
        // Our custom java.lang.reflect.Proxy instances actually bundle
        // their appropriate entity name, so we simply extract it from there
        // if this represents one of our proxies; otherwise, we return null
        if ( Proxy.isProxyClass( object.getClass() ) ) {
            InvocationHandler handler = Proxy.getInvocationHandler( object );
            if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) {
                DataProxyHandler myHandler = ( DataProxyHandler ) handler;
                return myHandler.getEntityName();
        return null;

    // various other utility methods ....


 * The EntityNameResolver implementation.
 * IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names should be
 * resolved.  Since this particular impl can handle resolution for all of our entities we want to
 * take advantage of the fact that SessionFactoryImpl keeps these in a Set so that we only ever
 * have one instance registered.  Why?  Well, when it comes time to resolve an entity name,
 * Hibernate must iterate over all the registered resolvers.  So keeping that number down
 * helps that process be as speedy as possible.  Hence the equals and hashCode impls
public class MyEntityNameResolver implements EntityNameResolver {
    public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();

    public String resolveEntityName(Object entity) {
        return ProxyHelper.extractEntityName( entity );

    public boolean equals(Object obj) {
        return getClass().equals( obj.getClass() );

    public int hashCode() {
        return getClass().hashCode();

public class MyEntityTuplizer extends PojoEntityTuplizer {
	public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
		super( entityMetamodel, mappedEntity );

	public EntityNameResolver[] getEntityNameResolvers() {
		return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };

    public String determineConcreteSubclassEntityName(Object entityInstance, SessionFactoryImplementor factory) {
        String entityName = ProxyHelper.extractEntityName( entityInstance );
        if ( entityName == null ) {
            entityName = super.determineConcreteSubclassEntityName( entityInstance, factory );
        return entityName;


In order to register an org.hibernate.EntityNameResolver users must either:

  1. Implement a custom Tuplizer, implementing the getEntityNameResolvers method.

  2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the implementation class for org.hibernate.SessionFactory) using the registerEntityNameResolver method.

TODO: Document user-extension framework in the property and proxy packages