DataNucleus Object Store

1. DataNucleus Object Store

The DataNucleus Object Store enables domain objects to be persisted to relational as well as NoSQL databases. The object store is implemented using DataNucleus.

This user guide discuss end-user features, configuration and customization of the DataNucleus object store.

DataNucleus as a product also supports the JPA API; Apache Isis is likely to also support JPA in the future.

Apache Isis documentation is broken out into a number of user and reference guides.

The user guides available are:

The reference guides are:

The remaining guides are:

  • Developers' Guide (how to set up a development environment for Apache Isis and contribute back to the project)

  • Committers' Guide (release procedures and related practices)

2. Configuring DataNucleus

Apache Isis programmatically configures DataNucleus; any Apache Isis properties with the prefix isis.persistor.datanucleus.impl are passed through directly to the JDO/DataNucleus objectstore (with the prefix stripped off, of course).

DataNucleus will for itself also and read the META-INF/persistence.xml; at a minimum this defines the name of the "persistence unit". In theory it could also hold mappings, though in Apache Isis we tend to use annotations instead.

Furthermore, DataNucleus will search for various other XML mapping files, eg mappings.jdo. A full list can be found here. The metadata in these XML can be used to override the annotations of annotated entities; see Overriding JDO Annotatons for further discussion.

2.1. Configuration Properties

These configuration properties are typically stored in WEB-INF/persistor_datanucleus.properties. However, you can place all configuration properties into WEB-INF/isis.properties if you wish (the configuration properties from all config files are merged together).

2.1.1. Configuration Properties for Apache Isis itself

Table 1. JDO/DataNucleus Objectstore Configuration Properties
Property Value
(default value)
Description

isis.persistor.
datanucleus.
classMetadataLoadedListener

FQCN

The default (o.a.i.os.jdo.dn.CreateSchemaObjectFromClassMetadata) creates a DB schema object

isis.persistor.datanucleus.
RegisterEntities.packagePrefix

fully qualified package names (CSV)

that specifies the entities early rather than allow DataNucleus to find the entities lazily. Further discussion below.

This property is IGNORED if the isis.appManifest configuration property is specified, or if an AppManifest is provided programmatically.

isis.persistor.datanucleus.
PublishingService.serializedForm

zipped

2.1.2. Configuration Properties passed through directly to DataNucleus.

Table 2. JDO/DataNucleus Objectstore Configuration Properties
Property Value
(default value)
Description

isis.persistor.datanucleus.impl.*

Passed through directly to Datanucleus (with isis.persistor.datanucleus.impl prefix stripped)

isis.persistor.datanucleus.impl.
datanucleus.persistenceByReachabilityAtCommit

false

We recommend this setting is disabled.
Further discussion below.

2.2. persistence.xml

DataNucleus will for itself also and read the META-INF/persistence.xml. In theory it can hold mappings and even connection strings. However, with Apache Isis we tend to use annotations instead and externalize connection strings. so its definition is extremely simply, specifying just the name of the "persistence unit".

Here’s the one provided by the SimpleApp archetype:

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

    <persistence-unit name="simple">
    </persistence-unit>
</persistence>

Normally all one needs to do is to change the persistence-unit name.

If you use Eclipse IDE on Windows then note the importance of the persistence.xml file to make DataNucleus enhancer work correctly.

See DataNucleus' documentation on persistence.xml to learn more.

2.3. Eagerly Registering Entities

Both Apache Isis and DataNucleus have their own metamodels of the domain entities. Apache Isis builds its metamodel by walking the graph of types of the domain services. The JDO/DataNucleus objectstore then takes these types and registers them with DataNucleus.

In some cases, though, not every entity type is discoverable from the API of the service actions. This is especially the case if you have lots of subtypes (where the action method specifies only the supertype). In such cases the Isis and JDO metamodels is built lazily, when an instance of that (sub)type is first encountered.

Apache Isis is quite happy for the metamodel to be lazily created, and - to be fair - DataNucleus also works well in most cases. In some cases, though, we have found that the JDBC driver (eg HSQLDB) will deadlock if DataNucleus tries to submit some DDL (for a lazily discovered type) intermingled with DML (for updating). In any case, it’s probably not good practice to have DataNucleus work this way.

The framework thus provide mechanisms to search for all @PersistenceCapable entities under specified package(s), and registers them all eagerly. In fact there are two:

  • as of 1.9.0 the recommended (and simpler) approach is to specify an AppManifest, either as a isis.appManifest configuration property or programmatically.

  • for earlier versions the isis.persistor.datanucleus.RegisterEntities.packagePrefix configuration property can be specified. To bootstrap as a webapp this is usually specified in persistor_datanucleus.properties. (This is also supported in 1.9.0 if no AppManifest is specified. For integration testing this can be specified programatically.

Further discussion on specifying the package(s) in integration testing (for either approach) can be found in the user guide.

2.4. Persistence by Reachability

By default, JDO/DataNucleus supports the concept of persistence-by-reachability. That is, if a non-persistent entity is associated with an already-persistent entity, then DataNucleus will detect this and will automatically persist the associated object. Put another way: there is no need to call Apache Isis' DomainObjectContainer#persist(.) or DomainObjectContainer#persistIfNotAlready(.) methods.

However, convenient though this feature is, you may find that it causes performance issues.

DataNucleus' persistence-by-reachability may cause performance issues. We strongly recommend that you disable it.

One scenario in particular where this performance issues can arise is if your entities implement the java.lang.Comparable interface, and you have used Apache Isis' ObjectContracts utility class. The issue here is that ObjectContracts implementation can cause DataNucleus to recursively rehydrate a larger number of associated entities. (More detail below).

We therefore recommend that you disable persistence-by-reachability by adding the following to persistor_datanucleus.properties:

isis.persistor.datanucleus.impl.datanucleus.persistenceByReachabilityAtCommit=false

This change has been made to the SimpleApp archetype

If you do disable this feature, then you will (of course) need to ensure that you explicitly persist all entities using the DomainObjectContainer#persist(.) or DomainObjectContainer#persistIfNotAlready(.) methods.

2.4.1. The issue in more detail

Consider these entities (yuml.me/b8681268):

party agreementrole agreement

In the course of a transaction, the Agreement entity is loaded into memory (not necessarily modified), and then new AgreementRoles are associated to it.

All these entities implement Comparable using ObjectContracts, and the implementation of AgreementRole's (simplified) is:

public class AgreementRole {
    ...
    public int compareTo(AgreementRole other) {
        return ObjectContracts.compareTo(this, other, "agreement","startDate","party");
    }
}

while Agreement's is implemented as:

public class Agreement {
    ...
    public int compareTo(Agreement other) {
        return ObjectContracts.compareTo(this, other, "reference");
    }
}

and Party's is similarly implemented as:

public class Party {
    ...
    public int compareTo(Party other) {
        return ObjectContracts.compareTo(this, other, "reference");
    }
}

DataNucleus’s persistence-by-reachability algorithm adds the AgreementRole instances into a SortedSet, which causes AgreementRole#compareTo() to fire:

  • the evaluation of the "agreement" property delegates back to the Agreement, whose own Agreement#compareTo() uses the scalar reference property. As the Agreement is already in-memory, this does not trigger any further database queries

  • the evaluation of the "startDate" property is just a scalar property of the AgreementRole, so will already in-memory

  • the evaluation of the "party" property delegates back to the Party, whose own Party#compareTo() requires the uses the scalar reference property. However, since the Party is not yet in-memory, using the reference property triggers a database query to "rehydrate" the Party instance.

In other words, in figuring out whether AgreementRole requires the persistence-by-reachability algorithm to run, it causes the adjacent associated entity Party to also be retrieved.

2.5. Using JNDI DataSource

Isis' JDO objectstore can be configured either to connect to the database using its own connection pool, or by using a container-managed datasource.

2.5.1. Application managed

Using a connection pool managed directly by the application (that is, by Apache Isis' JDO objectstore and ultimately by DataNucleus) requires a single set of configuration properties to be specified.

In either WEB-INF\isis.properties file (or WEB-INF\persistor.properties, or WEB-INF\persistor_datanucleus.properties), specify the connection driver, url, username and password.

For example:

isis.persistor.datanucleus.impl.javax.jdo.option.ConnectionDriverName=net.sf.log4jdbc.DriverSpy
isis.persistor.datanucleus.impl.javax.jdo.option.ConnectionURL=jdbc:log4jdbc:hsqldb:mem:test
isis.persistor.datanucleus.impl.javax.jdo.option.ConnectionUserName=sa
isis.persistor.datanucleus.impl.javax.jdo.option.ConnectionPassword=

Those configuration properties that start with the prefix isis.persistor.datanucleus.impl. are passed through directly to DataNucleus (with the prefix removed).

It is also possible to specify the ` datanucleus.ConnectionPasswordDecrypter ` property; see the DataNucleus documentation for further details.

2.5.2. Container managed (JNDI)

Using a datasource managed by the servlet container requires three separate bits of configuration.

Firstly, specify the name of the datasource in the WEB-INF\persistor_datanucleus.properties file. For example:

If connection pool settings are also present in this file, they will simply be ignored. Any other configuration properties that start with the prefix isis.persistor.datanucleus.impl. are passed through directly to DataNucleus (with the prefix removed).

Secondly, in the WEB-INF/web.xml, declare the resource reference:

<resource-ref>
    <description>db</description>
    <res-ref-name>jdbc/simpleapp</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>

Finally, declare the datasource as required by the servlet container. For example, if using Tomcat 7, the datasource can be specified by adding the following to $TOMCAT_HOME/conf/context.xml:

<Resource name="jdbc/simpleapp"
  auth="Container"
  type="javax.sql.DataSource"
  maxActive="100"
  maxIdle="30"
  maxWait="10000"
  username="sa"
  password="p4ssword"
  driverClassName="com.microsoft.sqlserver.jdbc.SQLServerDriver"
  url="jdbc:sqlserver://127.0.0.1:1433;instance=.;databaseName=simpleapp"/>

You will also need to make sure that the JDBC driver is on the servlet container’s classpath. For Tomcat, this means copying the driver to $TOMCAT_HOME/lib.

According to Tomcat’s documentation, it is supposedly possible to copy the conf/context.xml to the name of the webapp, eg conf/mywebapp.xml, and scope the connection to that webapp only. I was unable to get this working, however.

3. JDO Mappings

3.1. 1-m Bidirectional relationships

Consider a bidirectional one-to-many association between two entities; a collection member in the "parent" and a property member on the "child".

We can tell DataNucleus about the bidirectionality using @Persistent(mappedBy=…​), or we can take responsibility for this aspect ourselves.

In addition, the two entities can be associated either without or with a join table (indicated by the @Join annotation):

  • without a join table is more common; a regular foreign key in the child table for FermentationVessel points back up to the associated parent Batch

  • with a join table; a link table holds the tuple representing the linkage.

Testing (as of 1.13.0, against dn-core 4.1.7/dn-rdbms 4.1.9) has determined there are two main rules:

  • If not using @Join, then the association must be maintained by setting the child association on the parent.

    It is not sufficient to simply add the child object to the parent’s collection.

  • @Persistent(mappedBy=…​) and @Join cannot be used together.

    Put another way, if using @Join then you must maintain both sides of the relationship in the application code.

In the examples that follow, we use two entities, Batch and FermentationVessel (from a brewery domain). In the original example domain the relationship between these two entities was optional (a FermentationVessel may have either none or one Batch associated with it); for the purpose of this article we’ll explore both mandatory and optional associations.

3.1.1. Mandatory, no @Join

In the first scenario we have use @Persistent(mappedBy=…​) to indicate a bidirectional association, without any @Join:

public class Batch {

    // getters and setters omitted

    @Persistent(mappedBy = "batch", dependentElement = "false")     (1)
    private SortedSet<FermentationVessel> vessels = new TreeSet<FermentationVessel>();
}
1 "mappedBy" means this is bidirectional

and

public class FermentationVessel implements Comparable<FermentationVessel> {

    // getters and setters omitted

    @Column(allowsNull = "false")       (1)
    private Batch batch;

    @Column(allowsNull = "false")
    private State state;                (2)
}
1 mandatory association up to parent
2 State is an enum (omitted)

Which creates this schema:

CREATE TABLE "batch"."Batch"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    ...
    "version" BIGINT NOT NULL,
    CONSTRAINT "Batch_PK" PRIMARY KEY ("id")
)
CREATE TABLE "fvessel"."FermentationVessel"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    "batch_id_OID" BIGINT NOT NULL,
    "state" NVARCHAR(255) NOT NULL,
    ...
    "version" TIMESTAMP NOT NULL,
    CONSTRAINT "FermentationVessel_PK" PRIMARY KEY ("id")
)

That is, there is an mandatory foreign key from FermentationVessel to Batch.

In this case we can use this code:

public Batch transfer(final FermentationVessel vessel) {
    vessel.setBatch(this);                                  (1)
    vessel.setState(FermentationVessel.State.FERMENTING);
    return this;
}
1 set the parent on the child

This sets up the association correctly, using this SQL:

UPDATE "fvessel"."FermentationVessel"
   SET "batch_id_OID"=<0>
       ,"state"=<'FERMENTING'>
       ,"version"=<2016-07-07 12:37:14.968>
 WHERE "id"=<0>

The following code will also work:

public Batch transfer(final FermentationVessel vessel) {
    vessel.setBatch(this);                                  (1)
    getVessels().add(vessel);                               (2)
    vessel.setState(FermentationVessel.State.FERMENTING);
    return this;
}
1 set the parent on the child
2 add the child to the parent’s collection.

However, obviously the second statement is redundant.

3.1.2. Optional, no @Join

If the association to the parent is made optional:

public class FermentationVessel implements Comparable<FermentationVessel> {

    // getters and setters omitted

    @Column(allowsNull = "true")       (1)
    private Batch batch;

    @Column(allowsNull = "false")
    private State state;
}
1 optional association up to parent

Which creates this schema:

CREATE TABLE "batch"."Batch"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    ...
    "version" BIGINT NOT NULL,
    CONSTRAINT "Batch_PK" PRIMARY KEY ("id")
)
CREATE TABLE "fvessel"."FermentationVessel"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    "batch_id_OID" BIGINT NULL,
    "state" NVARCHAR(255) NOT NULL,
    ...
    "version" TIMESTAMP NOT NULL,
    CONSTRAINT "FermentationVessel_PK" PRIMARY KEY ("id")
)

This is almost exactly the same, except the foreign key from FermentationVessel to Batch is now nullable.

In this case then setting the parent on the child still works:

public Batch transfer(final FermentationVessel vessel) {
    vessel.setBatch(this);                                  (1)
    vessel.setState(FermentationVessel.State.FERMENTING);
    return this;
}
1 set the parent on the child

HOWEVER, if we (redundantly) update both sides, then - paradoxically - the association is NOT set up

public Batch transfer(final FermentationVessel vessel) {
    vessel.setBatch(this);                                  (1)
    getVessels().add(vessel);                               (2)
    vessel.setState(FermentationVessel.State.FERMENTING);
    return this;
}
1 set the parent on the child
2 add the child to the parent’s collection.

It’s not clear if this is a bug in dn-core 4.1.7/dn-rdbms 4.19; an earlier thread on the mailing list from 2014 actually gave the opposite advice, see this thread and in particular this message.

In fact we also have a different case which argues that the parent should only be set on the child, and the child not added to the parent’s collection. This concurs with the most recent testing.

Therefore, the simple advice is that, for bidirectional associations, simply set the parent on the child, and this will work reliably irrespective of whether the association is mandatory or optional.

3.1.3. With @Join

Although DataNucleus does not complain if @Persistence(mappedBy=…​) and @Join are combined, testing (against dn-core 4.1.7/dn-rdbms 4.19) has shown that the bidirectional association is not properly maintained.

Therefore, we recommend that if @Join is used, then manually maintain both sides of the relationship and do not indicate that the association is bidirectional.

For example:

public class Batch {

    // getters and setters omitted

    @Join(table = "Batch_vessels")
    @Persistent(dependentElement = "false")
    private SortedSet<FermentationVessel> vessels = new TreeSet<FermentationVessel>();
}

and

public class FermentationVessel implements Comparable<FermentationVessel> {

    // getters and setters omitted

    @Column(allowsNull = "true")       (1)
    private Batch batch;

    @Column(allowsNull = "false")
    private State state;
}
1 optional association up to parent

creates this schema:

CREATE TABLE "batch"."Batch"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    ...
    "version" BIGINT NOT NULL,
    CONSTRAINT "Batch_PK" PRIMARY KEY ("id")
)
CREATE TABLE "fvessel"."FermentationVessel"
(
    "id" BIGINT GENERATED BY DEFAULT AS IDENTITY,
    "state" NVARCHAR(255) NOT NULL,
    ...
    "version" TIMESTAMP NOT NULL,
    CONSTRAINT "FermentationVessel_PK" PRIMARY KEY ("id")
)
CREATE TABLE "batch"."Batch_vessels"
(
    "id_OID" BIGINT NOT NULL,
    "id_EID" BIGINT NOT NULL,
    CONSTRAINT "Batch_vessels_PK" PRIMARY KEY ("id_OID","id_EID")
)

That is, there is NO foreign key from FermentationVessel to Batch, instead the Batch_vessels table links the two together.

These should then be maintained using:

public Batch transfer(final FermentationVessel vessel) {
    vessel.setBatch(this);                                  (1)
    getVessels().add(vessel);                               (2)
    vessel.setState(FermentationVessel.State.FERMENTING);
    return this;
}
1 set the parent on the child
2 add the child to the parent’s collection.

that is, explicitly update both sides of the relationship.

This generates this SQL:

INSERT INTO "batch"."Batch_vessels" ("id_OID","id_EID") VALUES (<0>,<0>)
UPDATE "batch"."Batch"
   SET "version"=<3>
 WHERE "id"=<0>
UPDATE "fvessel"."FermentationVessel"
   SET "state"=<'FERMENTING'>
      ,"version"=<2016-07-07 12:49:21.49>
 WHERE "id"=<0>

It doesn’t matter in these cases whether the association is mandatory or optional; it will be the same SQL generated.

3.2. Mandatory Properties in Subtypes

If you have a hierarchy of classes then you need to decide which inheritance strategy to use.

  • "table per hierarchy", or "rollup" (InheritanceStrategy.SUPERCLASS_TABLE)

    whereby a single table corresponds to the superclass, and also holds the properties of the subtype (or subtypes) being rolled up

  • "table per class" (InheritanceStrategy.NEW_TABLE)

    whereby there is a table for both superclass and subclass, in 1:1 correspondence

  • "rolldown" (InheritanceStrategy.SUBCLASS_TABLE)

    whereby a single table holds the properties of the subtype, and also holds the properties of its supertype

In the first "rollup" case, we can have a situation where - logically speaking - the property is mandatory in the subtype - but it must be mapped as nullable in the database because it is n/a for any other subtypes that are rolled up.

In this situation we must tell JDO that the column is optional, but to Apache Isis we want to enforce it being mandatory. This can be done using the @Property(optionality=Optionality.MANDATORY) annotation.

For example:

@javax.jdo.annotations.Inheritance(strategy = InheritanceStrategy.SUPER_TABLE)
public class SomeSubtype extends SomeSuperType {
    @javax.jdo.annotations.Column(allowsNull="true")
    @Property(optionality=Optionality.MANDATORY)
    @lombok.Getter @lombok.Setter
    private LocalDate date;
}

The @Property(optionality=…​) annotation is equivalent to the older but still supported @Optional annotation and @Mandatory annotations.

3.3. Mapping to a View

JDO/DataNucleus supports the ability to map the entity that is mapped to a view rather than a database table. Moreover, DataNucleus itself can create/maintain this view.

One use case for this is to support use cases which act upon aggregate information. An example is in the (non-ASF) Estatio application, which uses a view to define an "invoice run": a representatoin of all pending invoices to be sent out for a particular shopping centre. (Note that example also shows the entity as being "non-durable", but if the view is read/write then — I think — that this isn’t necessary required).

For more on this topic, see the DataNucleus documentation.

4. Database Schemas

In the same way that Java packages act as a namespace for domain objects, it’s good practice to map domain entities to their own (database) schemas.

As of 1.9.0, all the Isis Addons (non-ASF) modules do this. For example:

@javax.jdo.annotations.PersistenceCapable( ...
        schema = "isissecurity",
        table = "ApplicationUser")
public class ApplicationUser ... { ... }

results in a CREATE TABLE statement of:

CREATE TABLE isissecurity."ApplicationUser" (
    ...
)

while:

@javax.jdo.annotations.PersistenceCapable( ...
        schema = "isisaudit",
        table="AuditEntry")
public class AuditEntry ... { ... }

similarly results in:

CREATE TABLE isisaudit."AuditEntry" (
    ...
)

If for some reason you don’t want to use schemas (though we strongly recommend that you do), then note that you can override the @PersistenceCapable annotation by providing XML metadata (the mappings.jdo file). See the section on configuring DataNucleus Overriding Annotations for more details.

4.1. Listener to create schema

JDO/DataNucleus does not automatically create these schema objects, but it does provide a listener callback API on the initialization of each class into the JDO metamodel.

Actually, the above statement isn’t quite true. In DN 3.2.x (as used by Apache Isis up to v1.8.0) there was no support for schemas. As of Apache Isis 1.9.0 and DN 4.0 there is now support. But we implemented this feature initially against DN 3.2.x, and it still works, so for now we’ve decided to leave it in.

Therefore Apache Isis attaches a listener, CreateSchemaObjectFromClassMetadata, that checks for the schema’s existence, and creates the schema if required.

The guts of its implementation is:

public class CreateSchemaObjectFromClassMetadata
        implements MetaDataListener,
                   DataNucleusPropertiesAware {
    @Override
    public void loaded(final AbstractClassMetaData cmd) { ... }

    protected String buildSqlToCheck(final AbstractClassMetaData cmd) {
        final String schemaName = schemaNameFor(cmd);
        return String.format(
            "SELECT count(*) FROM INFORMATION_SCHEMA.SCHEMATA where SCHEMA_NAME = '%s'", schemaName);
    }
    protected String buildSqlToExec(final AbstractClassMetaData cmd) {
        final String schemaName = schemaNameFor(cmd);
        return String.format("CREATE SCHEMA \"%s\"", schemaName);
    }
}

where MetaDataListener is the DataNucleus listener API:

public interface MetaDataListener {
    void loaded(AbstractClassMetaData cmd);
}

Although not formal API, the default CreateSchemaObjectFromClassMetadata has been designed to be easily overrideable if you need to tweak it to support other RDBMS'. Any implementation must implement org.datanucleus.metadata.MetaDataListener:

The implementation provided has has been tested for HSQLDB, PostgreSQL and MS SQL Server, and is used automatically unless an alternative implementation is specified (as described in the section below).

4.2. Alternative implementation

An alternative implementation can be registered and used through the following configuration property:

isis.persistor.datanucleus.classMetadataLoadedListener=\
        org.apache.isis.objectstore.jdo.datanucleus.CreateSchemaObjectFromClassMetadata

Because this pertains to the JDO Objectstore we suggest you put this configuration property in WEB-INF/persistor_datanucleus.properties; but putting it in isis.properties will also work.

Any implementation must implement org.datanucleus.metadata.MetaDataListener. In many cases simply subclassing from CreateSchemaObjectFromClassMetadata and overriding buildSqlToCheck(…​) and buildSqlToExec(…​) should suffice.

If you do need more control, your implementation can also optionally implement org.apache.isis.objectstore.jdo.datanucleus.DataNucleusPropertiesAware:

public interface DataNucleusPropertiesAware {
    public void setDataNucleusProperties(final Map<String, String> properties);
}

This provides access to the properties passed through to JDO/DataNucleus.

If you do extend Apache Isis' CreateSchemaObjectFromClassMetadata class for some other database, please contribute back your improvements.

5. Hints and Tips

This chapter provides some solutions for problems we’ve encountered ourselves or have been raised on the Apache Isis mailing lists.

See also hints-n-tips chapters in the:

5.1. Overriding JDO Annotations

The JDO Objectstore (or rather, the underlying DataNucleus implementation) builds its own persistence metamodel by reading both annotations on the class and also by searching for metadata in XML files. The metadata in the XML files takes precedence over the annotations, and so can be used to override metadata that is "hard-coded" in annotations.

In fact, JDO/DataNucleus provides two different XML files that have slightly different purposes and capabilities:

  • first, a .jdo file can be provided which - if found - completely replaces the annotations.

    The idea here is simply to use XML as the means by which metadata is specified.

  • second, an .orm file can be provided which - if found - provides individual overrides for a particular database vendor.

    The idea here is to accommodate for subtle differences in support for SQL between vendors. A good example is the default schema for a table: dbo for SQL Server, public for HSQLDB, sys for Oracle, and so on.

If you want to use the first approach (the .jdo file), you’ll find that you can download the effective XML representation of domain entities using the downloadJdoMetadata mixin action available in prototyping mode. This then needs to be renamed and placed in the appropriate location on the classpath; see the DataNucleus documentation for details.

However, using this first approach does create a maintenance effort; if the domain entity’s class structure changes over time, then the XML metadata file will need to be updated.

The second approach (using an .orm file) is therefore often more useful than the first, because the metadata provided overrides rather than replaces the annotations (and annotations not overridden continue to be honoured).

A typical use case is to change the database schema for an entity. For example, as of 1.9.0 the various Isis addons modules (not ASF) use schemas for each entity. For example, the AuditEntry entity in the audit module is annotated as:

@javax.jdo.annotations.PersistenceCapable(
        identityType=IdentityType.DATASTORE,
        schema = "IsisAddonsAudit",
        table="AuditEntry")
public class AuditEntry {
    ...
}

This will map the AuditEntry class to a table "IsisAddonsAudit"."AuditEntry"; that is using a custom schema to own the object.

Suppose though that for whatever reason we didn’t want to use a custom schema but would rather use the default. Also suppose we are using SQL Server as our target database.

We can override the above annotation using a AuditEntry-sqlserver.orm file, placed in the same package as the AuditEntry entity. For example:

AuditEntry-sqlserver.orm
<?xml version="1.0" encoding="UTF-8" ?>
<orm xmlns="http://xmlns.jcp.org/xml/ns/jdo/orm"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/orm http://xmlns.jcp.org/xml/ns/jdo/orm_3_0.xsd">

    <package name="org.isisaddons.module.audit.dom">
        <class name="AuditEntry"
               schema="isisaudit"
               table="AuditEntry">
        </class>
    </package>
</orm>

It’s also necessary to tell JDO/DataNucleus about which vendor is being used (sqlserver in the example above). This is done using a configuration property:

isis.properties
isis.persistor.datanucleus.impl.datanucleus.Mapping=sqlserver

5.2. Subtype not fully populated

Taken from this thread on the Apache Isis users mailing list…​

If it seems that Apache Isis (or rather DataNucleus) isn’t fully populating domain entities (ie leaving some properties as null), then check that your actions are not accessing the fields directly. Use getters instead.

Properties of domain entities should always be accessed using getters. The only code that should access to fields should be the getters themselves.

Why so? Because DataNucleus will potentially lazy load some properties, but to do this it needs to know that the field is being requested. This is the purpose of the enhancement phase: the bytecode of the original getter method is actually wrapped in code that does the lazy loading checking. But hitting the field directly means that the lazy loading code does not run.

This error can be subtle: sometimes "incorrect" code that accesses the fields will seem to work. But that will be because the field has been populated already, for whatever reason.

One case where you will find the issue highlighted is for subtype tables that have been mapped using an inheritance strategy of NEW_TABLE, eg:

@javax.jdo.annotations.PersistenceCapable
@javax.jdo.annotations.Inheritance(strategy = InheritanceStrategy.NEW_TABLE)
public class SupertypeEntity {
    ...
}

and then:

@javax.jdo.annotations.PersistenceCapable
@javax.jdo.annotations.Inheritance(strategy = InheritanceStrategy.NEW_TABLE)
public class SubtypeEntity extends SupertypeEntity {
    ...
}

This will generate two tables in the database, with the primary key of the supertype table propagated as a foreign key (also primary key) of the subtype table (sometimes called "table per type" strategy). This means that DataNucleus might retrieve data from only the supertype table, and the lazily load the subtype fields only as required. This is preferable to doing a left outer join from the super- to the subtype tables to retrieve data that might not be needed.

On the other hand, if the SUPERCLASS_TABLE strategy (aka "table per hierarchy" or roll-up) or the SUBCLASS_TABLE strategy (roll-down) was used, then the problem is less likely to occur because DataNucleus would obtain all the data for any given instance from a single table.

Final note: references to other objects (either scalar references or in collections) in particular require that getters rather than fields to be used to obtain them: it’s hopefully obvious that DataNucleus (like all ORMs) should not and will not resolve such references (otherwise, where to stop…​ and the whole database would be loaded into memory).

In summary, there’s just one rule: always use the getters, never the fields.

5.3. Java8

DataNucleus 4.x supports Java 7, but can also be used with Java 8, eg for streams support against collections managed by DataNucleus.

Just include within <dependencies> of your dom module’s pom.xml:

<dependency>
    <groupId>org.datanucleus</groupId>
    <artifactId>datanucleus-java8</artifactId>
    <version>4.2.0-release</version>t
</dependency>

The DataNucleus website includes a page listing version compatibility of these extensions vis-a-vis the core DataNucleus platform.