The JDO API provides a dynamic API for defining metadata for classes, as an alternative to using annotations or XML metadata.

Defining Metadata for classes

The basic idea behind the Metadata API is that the developer obtains a metadata object from the PersistenceManagerFactory, and adds the definition to that as required, before registering it for use in the persistence process.

PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(propsFile);
...
JDOMetadata md = pmf.newMetadata();

So we have a JDOMetadata object and want to define the persistence for our class mydomain.MyClass, so we do as follows

PackageMetadata pmd = md.newPackageMetadata("mydomain");
ClassMetadata cmd = pmd.newClassMetadata("MyClass");

So we follow the same structure of the JDO XML Metadata file adding packages to the top level, and classes to the respective package. Note that we could have achieved this by a simple typesafe invocation

ClassMetadata cmd = md.newClassMetadata(MyClass.class);

So now we have the class defined, we need to set its key information

cmd.setTable("CLIENT").setDetachable(true).setIdentityType(IdentityType.DATASTORE);
cmd.setPersistenceModifier(ClassPersistenceModifier.PERSISTENCE_CAPABLE);

InheritanceMetadata inhmd = cmd.newInheritanceMetadata();
inhmd.setStrategy(InheritanceStrategy.NEW_TABLE);
DiscriminatorMetadata dmd = inhmd.newDiscriminatorMetadata();
dmd.setColumn("disc").setValue("Client");
dmd.setStrategy(DiscriminatorStrategy.VALUE_MAP).setIndexed(Indexed.TRUE);

VersionMetadata vermd = cmd.newVersionMetadata();
vermd.setStrategy(VersionStrategy.VERSION_NUMBER);
vermd.setColumn("version").setIndexed(Indexed.TRUE);

And we define also define fields/properties via the API in a similar way

FieldMetadata fmd = cmd.newFieldMetadata("name");
fmd.setNullValue(NullValue.DEFAULT).setColumn("client_name");
fmd.setIndexed(true).setUnique(true);

Note that, just like with XML metadata, we don’t need to add information for all fields since they have their own default persistence settings based on the type of the field.

As you can see from the objects in this API, it follows the exact same structure as the JDO XML metadata, so you should be able to specify all by working your way through the respective javadocs for the API classes.

All that remains is to register the defined metadata with the persistence process

pmf.registerMetadata(md);

Accessing Metadata for classes

Maybe you have a class with its persistence defined in XML or annotations and you want to check its persistence information at runtime. With the JDO Metadata API you can do that

TypeMetadata compmd = pmf.getMetadata("mydomain.MyOtherClass");

and we can now inspect the information, casting the compmd to either javax.jdo.metadata.ClassMetadata or javax.jdo.metadata.InterfaceMetadata.

you cannot currently change metadata retrieved in this way, only view it

MetaData Reference

JDOMetadata

This represents a JDO context (the equivalent of a package.jdo file), and contains packages, named queries, fetch plans etc.

PackageMetadata

This represents a package (the package in package.jdo), and contains classes/interfaces, sequences etc.

ClassMetadata

This represents a persistable class (the class in package.jdo), and contains members (fields/properties), identity, version, indices, unique constraints, FKs, named queries, fetch groups for the class, and unmapped columns.

InterfaceMetadata

This represents a persistable interface (the interface in package.jdo), and contains members (fields/properties), identity, version, indices, unique constraints, FKs, named queries, fetch groups for the class, and unmapped columns.

FieldMetadata

This represents a persistable field (the field in package.jdo), and contains settings for the field, array/collection/map mapping, as well as any converters, columns etc.

PropertyMetadata

This represents a persistable property (the property in package.jdo), and contains settings for the property, array/collection/map mapping, as well as any converters, columns etc.

CollectionMetadata

This represents a collection member (the collection in package.jdo), and contains settings for the collection elements.

ArrayMetadata

This represents an array member (the array in package.jdo), and contains settings for the array elements.

MapMetadata

This represents a map member (the map in package.jdo), and contains settings for the map keys/values.

ElementMetadata

This represents a collection/array element (the element in package.jdo), and contains settings for the collection/array element.

KeyMetadata

This represents a map key (the key in package.jdo), and contains settings for the map key.

ValueMetadata

This represents a map value (the element in package.jdo), and contains settings for the map value.

JDO MetaData API Reference (v5.2)