7.  Additional JPA Mappings

7.1. Datastore Identity Mapping
7.2. Surrogate Version Mapping
7.3. Multi-Column Mappings
7.4. Join Column Attribute Targets
7.5. Embedded Mapping
7.6. Collections
7.6.1. Container Table
7.6.2. Element Join Columns
7.6.3. Order Column
7.7. One-Sided One-Many Mapping
7.8. Maps
7.9. Indexes and Constraints
7.9.1. Indexes
7.9.2. Foreign Keys
7.9.3. Unique Constraints
7.10. XML Column Mapping

OpenJPA supports many persistence strategies beyond those of the JPA specification. Section 2, “ Additional JPA Metadata ” covered the logical metadata for OpenJPA's additional persistence strategies. We now demonstrate how to map entities using these strategies to the database.

7.1.  Datastore Identity Mapping

Section 3, “ Object Identity ” describes how to use datastore identity in JPA. OpenJPA requires a single numeric primary key column to hold datastore identity values. The org.apache.openjpa.persistence.jdbc.DataStoreIdColumn annotation customizes the datastore identity column. This annotation has the following properties:

  • String name: Defaults to ID.

  • int precision

  • String columnDefinition

  • boolean insertable

  • boolean updatable

All properties correspond exactly to the same-named properties on the standard Column annotation, described in Section 3, “ Column ”.

Example 7.14.  Datastore Identity Mapping

import org.apache.openjpa.persistence.*;
import org.apache.openjpa.persistence.jdbc.*;

public class LogEntry {

    private String content;


7.2.  Surrogate Version Mapping

OpenJPA supports version fields as defined by the JPA specification, but allows you to use a surrogate version column in place of a version field if you like. You map the surrogate version column with the org.apache.openjpa.persistence.jdbc.VersionColumn annotation. You can also use the org.apache.openjpa.persistence.jdbc.VersionColumns annotation to declare an array of VersionColumn values. Each VersionColumn has the following properties:

  • String name: Defaults to VERSN.

  • int length

  • int precision

  • int scale

  • String columnDefinition

  • boolean nullable

  • boolean insertable

  • boolean updatable

All properties correspond exactly to the same-named properties on the standard Column annotation, described in Section 3, “ Column ”.

By default, OpenJPA assumes that surrogate versioning uses a version number strategy. You can choose a different strategy with the VersionStrategy annotation described in Section 9.1.4, “ Version Strategy ”.

7.3.  Multi-Column Mappings

OpenJPA makes it easy to create multi-column custom mappings. The JPA specification includes a Column annotation, but is missing a way to declare multiple columns for a single field. OpenJPA remedies this with the org.apache.openjpa.persistence.jdbc.Columns annotation, which contains an array of Column values.

Remember to annotate custom field types with Persistent, as described in Section 2.3, “ Persistent Field Values ”.

7.4.  Join Column Attribute Targets

Section 8.4, “ Direct Relations ” in the JPA Overview introduced you to the JoinColumn annotation. A JoinColumn's referencedColumnName property declares which column in the table of the related type this join column links to. Suppose, however, that the related type is unmapped, or that it is part of a table-per-class inheritance hierarchy. Each subclass that might be assigned to the field could reside in a different table, and could use entirely different names for its primary key columns. It becomes impossible to supply a single referencedColumnName that works for all subclasses.

OpenJPA rectifies this by allowing you to declare which attribute in the related type each join column links to, rather than which column. If the attribute is mapped differently in various subclass tables, OpenJPA automatically forms the proper join for the subclass record at hand. The org.apache.openjpa.persistence.jdbc.XJoinColumn annotation has all the same properties as the standard JoinColumn annotation, but adds an additional referencedAttributeName property for this purpose. Simply use a XJoinColumn in place of a JoinColumn whenever you need to access this added functionality.

For compound keys, use the org.apache.openjpa.persistence.jdbc.XJoinColumns annotation. The value of this annotation is an array of individual XJoinColumns.

7.5.  Embedded Mapping

JPA uses the AttributeOverride annotation to override the default mappings of an embeddable class. The JPA Overview details this process in Section 8.3, “ Embedded Mapping ”. AttributeOverrides suffice for simple mappings, but do not allow you to override complex mappings. Also, JPA has no way to differentitate between a null embedded object and one with default values for all of its fields.

OpenJPA overcomes these shortcomings with the org.apache.openjpa.persistence.jdbc.EmbeddedMapping annotation. This annotation has the following properties:

  • String nullIndicatorColumnName: If the named column's value is NULL, then the embedded object is assumed to be null. If the named column has a non- NULL value, then the embedded object will get loaded and populated with data from the other embedded fields. This property is entirely optional. By default, OpenJPA always assumes the embedded object is non-null, just as in standard JPA mapping.

    If the column you name does not belong to any fields of the embedded object, OpenJPA will create a synthetic null-indicator column with this name. In fact, you can specify a value of true to simply indicate that you want a synthetic null-indicator column, without having to come up with a name for it. A value of false signals that you explicitly do not want a null-indicator column created for this mapping (in case you have configured your mapping defaults to create one by default).

  • String nullIndicatorFieldName: Rather than name a null indicator column, you can name a field of the embedded type. OpenJPA will use the column of this field as the null-indicator column.

  • MappingOverride[] overrides: This array allows you to override any mapping of the embedded object.

The EmbeddedMapping's overrides array serves the same purpose as standard JPA's AttributeOverride s and AssociationOverride s. In fact, you can also use the MappingOverride annotation on an entity class to override a complex mapping of its mapped superclass, just as you can with AttributeOverride and AssociationOverride s. The MappingOverrides annotation, whose value is an array of MappingOverride s, allows you to overide multiple mapped superclass mappings.

Each org.apache.openjpa.persistence.jdbc.MappingOverride annotation has the following properties:

  • String name: The name of the field that is being overridden.

  • Column[] columns: Columns for the new field mapping.

  • XJoinColumn[] joinColumns: Join columns for the new field mapping, if it is a relation field.

  • ContainerTable containerTable: Table for the new collection or map field mapping. We cover collection mappings in Section 7.6, “ Collections ”, and map mappings in Section 7.8, “ Maps ”.

  • ElementJoinColumn[] elementJoinColumns: Element join columns for the new collection or map field mapping. You will see how to use element join columns in Section 7.6.2, “ Element Join Columns ”.

The following example defines an embeddable PathCoordinate class with a custom mapping of a java.awt.Point field to two columns. It then defines an entity which embeds a PointCoordinate and overrides the default mapping for the point field. The entity also declares that if the PathCoordinate 's siteName field column is null, it means that no PathCoordinate is stored in the embedded record; the owning field will load as null.

Example 7.15.  Overriding Complex Mappings

import org.apache.openjpa.persistence.jdbc.*;

public class PathCoordinate {

    private String siteName;

    private Point point;


public class Path {

    @EmbeddedMapping(nullIndicatorFieldName="siteName", overrides={
        @MappingOverride(name="siteName", columns=@Column(name="START_SITE")),
        @MappingOverride(name="point", columns={
    private PathCoordinate start;


7.6.  Collections

In Section 2.4, “Persistent Collection Fields”, we explored the PersistentCollection annotation for persistent collection fields that aren't a standard OneToMany or ManyToMany relation. To map these non-standard collections, combine OpenJPA's ContainerTable annotation with ElementJoinColumns. We explore the annotations below.

7.6.1.  Container Table

The org.apache.openjpa.persistence.jdbc.ContainerTable annotation describes a database table that holds collection (or map) elements. This annotation has the following properties:

  • String name

  • String catalog

  • String schema

  • XJoinColumn[] joinColumns

  • ForeignKey joinForeignKey

  • Index joinIndex

The name, catalog, schema , and joinColumns properties describe the container table and how it joins to the owning entity's table. These properties correspond to the same-named properties on the standard JoinTable annotation, described in Section 8.5, “ Join Table ” . If left unspecified, the name of the table defaults to the first five characters of the entity table name, plus an underscore, plus the field name. The joinForeignKey and joinIndex properties override default foreign key and index generation for the join columns. We explore foreign keys and indexes later in this chapter.

You may notice that the container table does not define how to store the collection elements. That is left to separate annotations, which are the subject of the next sections.

7.6.2.  Element Join Columns

Element join columns are equivalent to standard JPA join columns, except that they represent a join to a collection or map element entity rather than a direct relation. You represent an element join column with OpenJPA's org.apache.openjpa.persistence.jdbc.ElementJoinColumn annotation. To declare a compound join, enclose an array of ElementJoinColumns in the org.apache.openjpa.persistence.jdbc.ElementJoinColumns annotation.

An ElementJoinColumn always resides in a container table, so it does not have the table property of a standard JoinColumn. Like XJoinColumns above, ElementJoinColumns can reference a linked attribute rather than a static linked column. Otherwise, the ElementJoinColumn and standard JoinColumn annotations are equivalent. See Section 8.4, “ Direct Relations ” in the JPA Overview for a review of the JoinColumn annotation.

7.6.3.  Order Column

Relational databases do not guarantee that records are returned in insertion order. If you want to make sure that your collection elements are loaded in the same order they were in when last stored, you must declare an order column. OpenJPA's org.apache.openjpa.persistence.jdbc.OrderColumn annotation has the following properties:

  • String name: Defaults to ORDR.

  • boolean enabled

  • int precision

  • String columnDefinition

  • boolean insertable

  • boolean updatable

Order columns are always in the container table. You can explicitly turn off ordering (if you have enabled it by default via your mapping defaults) by setting the enabled property to false. All other properties correspond exactly to the same-named properties on the standard Column annotation, described in Section 3, “ Column ”.

7.7.  One-Sided One-Many Mapping

The previous section covered the use of ElementJoinColumn annotations in conjunction with a ContainerTable for mapping collections to dedicate tables. ElementJoinColumn s, however, have one additional use: to create a one-sided one-many mapping. Standard JPA supports OneToMany fields without a mappedBy inverse, but only by mapping these fields to a JoinTable (see Section 8.5, “ Join Table ” in the JPA Overview for details). Often, you'd like to create a one-many association based on an inverse foreign key (logical or actual) in the table of the related type.

Consider the model above. Subscription has a collection of LineItem s, but LineItem has no inverse relation to Subscription. To retrieve all of the LineItem records for a Subscription , we join the SUB_ID inverse foreign key column in the LINE_ITEM table to the primary key column of the SUB table. The example below shows how to represent this model in mapping annotations. Note that OpenJPA automatically assumes an inverse foreign key mapping when element join columns are given, but no container or join table is given.

Example 7.16.  One-Sided One-Many Mapping

package org.mag.subscribe;

import org.apache.openjpa.persistence.jdbc.*;

@Table(name="LINE_ITEM", schema="CNTRCT")
public class LineItem {

@Table(name="SUB", schema="CNTRCT")
public class Subscription {

    @Id private long id;

    @ElementJoinColumn(name="SUB_ID", target="ID")
    private Collection<LineItem> items;


7.8.  Maps

We detailed the ContainerTable annotation in Section 7.6.1, “ Container Table ”. Custom map mappings may also use this annotation to represent a map table.

7.9.  Indexes and Constraints

OpenJPA uses index information during schema generation to index the proper columns. OpenJPA uses foreign key and unique constraint information during schema creation to generate the proper database constraints, and also at runtime to order SQL statements to avoid constraint violations while maximizing SQL batch size.

OpenJPA assumes certain columns have indexes or constraints based on your mapping defaults, as detailed in Section 4, “ Mapping Defaults ”. You can override the configured defaults on individual joins, field values, collection elements, map keys, or map values using the annotations presented in the following sections.

7.9.1.  Indexes

The org.apache.openjpa.persistence.jdbc.Index annotation represents an index on the columns of a field. It is also used within the ContainerTable annotation to index join columns. To index the columns of a collection element, use the org.apache.openjpa.persistence.jdbc.ElementIndex annotation. These annotations have the following properties:

  • boolean enabled: Set this property to false to explicitly tell OpenJPA not to index these columns, when OpenJPA would otherwise do so.

  • String name: The name of the index. OpenJPA will choose a name if you do not provide one.

  • boolean unique: Whether to create a unique index. Defaults to false.

7.9.2.  Foreign Keys

The org.apache.openjpa.persistence.jdbc.ForeignKey annotation represents a foreign key on the columns of a field. It is also used within the ContainerTable annotation to set a database foreign key on join columns. To set a constraint to the columns of a collection element, use the org.apache.openjpa.persistence.jdbc.ElementForeignKey annotation. These annotations have the following properties:

  • boolean enabled: Set this property to false to explicitly tell OpenJPA not to set a foreign key on these columns, when OpenJPA would otherwise do so.

  • String name: The name of the foreign key. OpenJPA will choose a name if you do not provide one, or will create an anonymous key.

  • boolean deferred: Whether to create a deferred key if supported by the database.

  • ForeignKeyAction deleteAction: Value from the org.apache.openjpa.persistence.jdbc.ForeignKeyAction enum identifying the desired delete action. Defaults to RESTRICT.

  • ForeignKeyAction updateAction: Value from the org.apache.openjpa.persistence.jdbc.ForeignKeyAction enum identifying the desired update action. Defaults to RESTRICT.

Keep in mind that OpenJPA uses foreign key information at runtime to avoid constraint violations; it is important, therefore, that your mapping defaults and foreign key annotations combine to accurately reflect your existing database constraints, or that you configure OpenJPA to reflect on your database schema to discover existing foreign keys (see Section 11.2, “ Schema Factory ”).

7.9.3.  Unique Constraints

The org.apache.openjpa.persistence.jdbc.Unique annotation represents a unqiue constraint on the columns of a field. It is more convenient than using the uniqueConstraints property of standard JPA Table and SecondaryTable annotations, because you can apply it directly to the constrained field. The Unique annotation has the following properties:

  • boolean enabled: Set this property to false to explicitly tell OpenJPA not to constrain these columns, when OpenJPA would otherwise do so.

  • String name: The name of the constraint. OpenJPA will choose a name if you do not provide one, or will create an anonymous constraint.

  • boolean deferred: Whether to create a deferred constraint if supported by the database.

7.10.  XML Column Mapping

DB2, Oracle and SQLServer support XML column types and XPath queries and indexes over these columns.OpenJPA supports mapping of an entity property mapped to an XML column.

Annotate the entity property using the XMLValueHandler strategy:


The default fetch type is EAGER but can be changed to LAZY by using:


The entity property class is required to have jaxb binding annotations. This is produced when the classes are generated from an xml schema using the jaxb generator XJC.Ensure that @XmlRootElement appears in the root class. In some case this annotation needs to be added manually if it is missing.

The jaxb jar files must be on the application classpath (jaxb-api.jar, jaxb-impl.jar, jsr173_1.0_api.jar or equivalent).

EJB Query path expressions can navigate into the mapped class and its subfields to any level.

The path expression is rewritten into an equivalent XPATH expression using SQL XML functions.

The path expression must be single valued.Path expressions over xml mapped classes can only be used in WHERE as an operand to a simple predicate (= <> < > >= <=).

Path expressions over XML mapped fields can not be:

  • an input to a EJB query scalar function

  • an operand of BETWEEN, IS NULL, LIKE or IN predicate

  • used to project out subfields in the SELECT clause

  • used in the FROM , GROUP BY, HAVING, ORDER BY clauses

XML schema must not contain namespace declarations. The EJB query path expressions can not refer to java fields generated from XML ANY type or XML mixed element types.

The datatype generated by JAXB must be a valid EJB query type to use the property in an EJB query predicate.

Shown below is a sample XML schema myaddress.xsd, in which the JPA entity Order has <shipAddress> persistent field that maps to an XML column.

Example 7.17.  myaddress.xsd

<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" >

<xs:complexType name="Address">
  <xs:element name="Name" type="xs:string" />
  <xs:element name="Street" type="xs:string"
  	minOccurs="1" maxOccurs="3" />
  <xs:element name="City" type="xs:string" />

<xs:complexType name="CAN_Address">
  <xs:extension base="Address">
    <xs:element name="Province" type="xs:string" />
    <xs:element name="PostalCode" type="xs:string" />

<xs:simpleType name="USPS_ZIP">
 <xs:restriction base="xs:integer">
  <xs:minInclusive value="01000" />
  <xs:maxInclusive value="99999" />

<xs:complexType name="USA_Address">
  <xs:extension base="Address">
    <xs:element name="State" type="xs:string" />
    <xs:element name="ZIP" type="USPS_ZIP" />

<xs:element name="MailAddress" type="Address" />
<xs:element name="AddrCAN" type="CAN_Address"
   substitutionGroup="MailAddress" />
<xs:element name="AddrUSA" type="USA_Address"
   substitutionGroup="MailAddress" />

Java classes Address, USAAddress and CANAddress are produced using jaxb XJC generator from myaddress schema.

Example 7.18.  Address.Java

@XmlType(name = "Address", propOrder = {
public class Address {
    @XmlElement(name = "Name", required = true)
    protected String name;
    @XmlElement(name = "Street", required = true)
    protected List<String> street;
    @XmlElement(name = "City", required = true)
    protected String city;

     * Getter and Setter methods.

Example 7.19.  USAAddress.java

@XmlType(name = "USA_Address", propOrder = {
public class USAAddress
    extends Address

    @XmlElement(name = "State")
    protected String state;
    @XmlElement(name = "ZIP")
    protected int zip;

     * Getter and Setter methods.

Example 7.20.  CANAddress.java

@XmlType(name = "CAN_Address", propOrder = {
public class CANAddress
    extends Address

    @XmlElement(name = "Province")
    protected String province;
    @XmlElement(name = "PostalCode")
    protected String postalCode;

     * Getter and Setter methods.

Example 7.21.  Showing annotated Order entity with XML mapping strategy

public class Order {
    @Id  private into id;
    @Strategy ("org.apache.openjpa.jdbc.meta.strats.XMLValueHandler")
    private Address shipAddress;

Example 7.22.  Showing creation of Order Entity having shipAddress mapped to XML column

myaddress.ObjectFactory addressFactory = new myaddress.ObjectFactory();
Customer c1 = new Customer();
c1.setCid( new Customer.CustomerKey("USA", 1) );
c1.setName("Harry's Auto");
Order o1 = new Order( 850, false, c1);
USAAddress addr1 = addressFactory.createUSAAddress();
addr1.setCity("San Jose");
addr1.setZIP(new Integer("95141"));
addr1.getStreet().add("12500 Monterey");
addr1.setName( c1.getName());

Example 7.23.  Sample EJB Queries for XML Column mapping

. select o from Order o where o.shipAddress.city = "San Jose" or
			      o.shipAddress.city = "San Francisco"  (OK)

. select o.shipaAddress from Order o  (OK)

. select o.shipAddress.city from Order o  (INVALID)

. select o from Order o where o.shipAddress.street = "San Jose"  (INVALID multi valued)