5.3. Object Identity

5.3.1. Datastore Identity Objects
5.3.2. Application Identity Tool
5.3.3. Autoassign / Identity Strategy Caveats

Kodo supports both datastore and application JDO identity types, including single field identity. See Section 4.5, “JDO Identity” in the JDO Overview for coverage of the JDO identity types.

The JPA specification requires you to declare one or more identity fields in your persistent classes, as with JDO's application identity type. Kodo fully supports application identity for JPA entities; however, Kodo also allows you to use datastore identity in your entities. To use datastore identity, simply do not declare any primary key fields. Kodo will use a surrogate database key to track the identity of your objects.

You can control how your JPA datastore identity value is generated through Kodo's org.apache.openjpa.persistence.DataStoreId class annotation. This annotation has strategy and generator properties that mirror the same-named properties on the standard javax.persistence.GeneratedValue annotation described in Section 5.2.2, “Id” of the JPA Overview. The functionality of Kodo's DataStoreId annotation is also analogous to JDO's standard datastore-identity mapping element, described in Section 15.5, “Datastore Identity” of the JDO Overview.

To retrieve the identity value of a datastore identity entity, use the OpenJPAEntityManager.getObjectId (Object entity) method. See Section 9.2.2, “OpenJPAEntityManager” for more information on the OpenJPAEntityManager.

Example 5.4. JPA Datastore Identity Metadata

import org.apache.openjpa.persistence.*;

public class LineItem
    ... no @Id fields declared ...

5.3.1. Datastore Identity Objects

Internally, Kodo uses the public kodo.util.Id class for datastore identity objects. When writing Kodo plugins, you can manipulate datastore identity objects by casting them to this class. You can also create your own Id instances and pass them to any internal Kodo method that expects an identity object.

In JPA, you will never see Id instances directly. Instead, calling OpenJPAEntityManager.getObjectId on a datastore identity object will return the Long surrogate primary key value for that object. You can then use this value in calls to EntityManager.find for subsequent lookups of the same record.

In JDO, calling JDOHelper.getObjectId or PersistenceManager.getObjectId on a persistent object with datastore identity will return an Id instance. Remember, however, that datastore identity in JDO is meant to be opaque; if you find yourself having to use the Id class often, it may be a sign that you should be using application identity.

You can also use any Number in Kodo JDO to represent a datastore identity primary key value. The PersistenceManager.newObjectIdInstance(Class, Object) method will return an Id instance when passed a datastore identity persistent class and a Number representing a primary key value. And the PersistenceManager.getObjectById(Class, Object) method allows you to look up a datastore identity instance given its class and Number primary key. The JDO specification intended these methods for single field identity key values, but in Kodo they work for datastore identity values as well.

5.3.2. Application Identity Tool

If you choose to use application identity, you may want to take advantage of Kodo JPA/JDO's application identity tool. The application identity tool generates Java code implementing the identity class for any persistent type using application identity. The code satisfies all the requirements the specification places on identity classes. You can use it as-is, or simply use it as a starting point, editing it to meet your needs.

Before you can run the application identity tool on a persistent class, the class must be compiled and must have complete metadata. All primary key fields must be marked as such in the metadata.

In JPA metadata, do not attempt to specify the @IdClass annotation unless you are using the application identity tool to overwrite an existing identity class. Attempting to set the value of the @IdClass to a non-existent class will prevent your persistent class from compiling. Instead, use the -name or -suffix options described below to tell Kodo what name to give your generated identity class. Once the application identity tool has generated the class code, you can set the @IdClass annotation.

In JDO metadata, set the class' objectid-class attribute to the desired name of the generated object identity class.

The application identity tool can be invoked via the included appidtool shell/bat script or via its Java class, kodo.enhance.ApplicationIdTool.


Section 14.1.3, “Application Identity Tool Ant Task” describes the application identity tool's Ant task.

Example 5.5. Using the Application Identity Tool

appidtool -s Id Magazine.java
appidtool package.jdo

The application identity tool accepts the standard set of command-line arguments defined by the configuration framework (see Section 2.3, “Command Line Configuration”), including code formatting flags described in Section 2.3.1, “Code Formatting”. It also accepts the following arguments:

  • -directory/-d <output directory>: Path to the output directory. If the directory does not match the generated oid class' package, the package structure will be created beneath the directory. If not specified, the tool will first try to find the directory of the .java file for the persistence-capable class, and failing that will use the current directory.

  • -ignoreErrors/-i <true/t | false/f> : If false, an exception will be thrown if the tool is run on any class that does not use application identity, or is not the base class in the inheritance hierarchy (recall that subclasses never define the application identity class; they inherit it from their persistent superclass).

  • -token/-t <token>: The token to use to separate stringified primary key values in the string form of the object id. This option is only used if you have multiple primary key fields. It defaults to "::".

  • -name/-n <id class name>: The name of the identity class to generate. If this option is specified, you must run the tool on exactly one class. If the class metadata already names an object id class, this option is ignored. If the name is not fully qualified, the persistent class' package is prepended to form the qualified name.

  • -suffix/-s <id class suffix>: A string to suffix each persistent class name with to form the identity class name. This option is overridden by -name or by any object id class specified in metadata.

Each additional argument to the tool must be one of the following:

  • The full name of a persistent class.

  • The .java file for a persistent class.

  • The .class file of a persistent class.

  • A .jdo metadata file. The tool will run on each class listed in the metadata.

If you do not supply any arguments to the tool, it will act on the classes in your persistent classes list (see Section 5.1, “Persistent Class List”).

If you do not have a persistent classes list, the tool will scan your classpath for directories containing .jdo files, and run on all classes listed in those files.

5.3.3. Autoassign / Identity Strategy Caveats

Section 5.2.3, “Generated Value” explains how to use JPA's IDENTITY generation type. Section 15.5, “Datastore Identity” and Section 15.11.3, “Automatic Values” of the JDO Overview explain how to use JDO's autoassign and identity strategies for datastore identity and field values. However, here are some additional caveats you should be aware of when using these strategies:

  1. Your database must support auto-increment / identity columns, or some equivalent (see Section 4.4.3, “OracleDictionary Properties” for how to configure a combination of triggers and sequences to fake auto-increment support in Oracle).

  2. Auto-increment / identity columns must be an integer or long integer type.

  3. Databases support auto-increment / identity columns to varying degrees. Some do not support them at all. Others only allow a single such column per table, and require that it be the primary key column. More lenient databases may allow non-primary key auto-increment columns, and may allow more than one per table. See your database documentation for details.

  4. Statements inserting into tables with auto-increment / identity columns cannot be batched. After each insert, Kodo must go back to the database to retrieve the last inserted auto-increment value to set back in the persistent object. This can have a negative impact on performance.


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