When persisting a class to an RDBMS datastore there is a mapping process from class/field to table/column. The most common thing to configure is the JavaTypeMapping so that it maps between the java type and the column(s) in the desired way. We can also configure the mapping to the datastore type. So, for example, we define what JDBC types we can map a particular Java type to. By datastore type we mean the JDBC type, such as BLOB, INT, VARCHAR etc. DataNucleus provides datastore mappings for the vast majority of JDBC types and the handlings will almost always be adequate. What you could do though is make a particular Java type persistable using a different JDBC type if you wished.
So you can define how to convert the datastore column value to/from common Java types. Look at datanucleus-rdbms for examples. Note that your XXXDatastoreMapping should have a single constructor taking in JavaTypeMapping mapping, StoreManager storeMgr, Column col.
To give an example of what the plugin specification looks like
<?xml version="1.0"?> <plugin id="mydomain.myplugins" name="DataNucleus plug-ins" provider-name="My Company"> <extension point="org.datanucleus.store.rdbms.datastore_mapping"> <mapping java-type="java.lang.Character" rdbms-mapping-class="org.datanucleus.store.rdbms.mapping.datastore.CharRDBMSMapping" jdbc-type="CHAR" sql-type="CHAR" default="true"/> <mapping java-type="java.lang.Character" rdbms-mapping-class="org.datanucleus.store.rdbms.mapping.datastore.IntegerRDBMSMapping" jdbc-type="INTEGER" sql-type="INT" default="false"/> </extension> </plugin>
Note that you also require a MANIFEST.MF file as per the Extensions Guide. So in this definition we have defined that a field of type Character can be mapped to JDBC type of CHAR or INTEGER (with CHAR the default)
While DataNucleus attempts to provide all useful JDBC type mappings out of the box, occasionally you may come across one that is not defined, even though we provide an RDBMSMapping class for that JDBC type. For example, if you got this error message
JDBC type XXX declared for field "org.datanucleus.test.MyClass.yyyField" of java type java.lang.YYY cant be mapped for this datastore.
This means that while the JDBC driver may support this JDBC type, we havent defined it for the java type YYY in the plugin.xml file. All that you need to do is add a plugin.xml
<?xml version="1.0"?> <plugin id="mydomain.myplugins" name="DataNucleus plug-ins" provider-name="My Company"> <extension point="org.datanucleus.store.rdbms.datastore_mapping"> <mapping java-type="java.lang.YYY" rdbms-mapping-class="org.datanucleus.store.rdbms.mapping.XXXRDBMSMapping" jdbc-type="XXX" sql-type="XXX" default="false"/> </extension> </plugin>
together with MANIFEST.MF to your application, and it will be supported. Obviously if it is of a type that you think we ought to support out-of-the-box then raise an issue against the datanucleus-rdbms plugin with your plugin.xml definition and mention the datastore it was needed for.