Kodo JDO includes a reverse mapping tool for generating persistent class definitions, complete with JDO metadata and object-relational mapping data, from an existing database schema. You do not have to use the reverse mapping tool to access an existing schema; you are free to write your classes and mapping information by hand. The reverse mapping tool, however, can give you an excellent starting point from which to grow your persistent classes.
To use the reverse mapping tool, follow the steps below:
Use the schema generator to export your current schema to an XML schema file. You can skip this step and the next step if you want to run the reverse mapping tool directly against the database schema.
Examine the generated schema file. JDBC drivers often provide incomplete or faulty metadata, in which case the file will not exactly match the actual schema. Alter the XML file to match the true schema. The XML format for the schema file is described in Section 8.3, “XML Schema Format”.
After fixing any errors in the schema file, modify the XML to include foreign keys between all related tables. The schema generator will have automatically detected existing foreign key constraints; many schemas, however, do not employ database foreign keys for every table relation. By manually adding any missing foreign keys, you will give the reverse mapping tool the information it needs to reflect the proper relations between the persistent classes it creates.
Run the reverse mapping tool on the finished schema file (if you do not supply the schema file to reverse map, the tool will run directly against the schema in the database). The tool can be run via the included reversemappingtool script, or through its Java class, kodo.jdbc.meta.ReverseMappingTool .
Example 5.14. Using the Reverse Mapping Tool
reversemappingtool -pkg com.xyz -d ~/src -cp customizer.properties schema.xml
-schemas/-s <schema and table names> : A comma-separated list of schema and table names to reverse map, if no XML schema file is supplied. Each element of the list must follow the naming conventions for the kodo.jdbc.Schemas property. In fact, if this flag is omitted, it defaults to the value of the Schemas property. If the Schemas property is not defined, all schemas will be reverse-mapped.
-package/-pkg <package name>: The package name of the generated classes. If no package name is given, the generated code will not contain package declarations.
-directory/-d <output directory> : The path to the directory to output all generated code and metadata to. If the directory does not match the package of a class, the package structure will be created beneath this directory. Defaults to the current directory.
-useSchemaName/-sn <true/t | false/f> : Set this flag to true to include the schema as well as table name in the name of each generated class. This can be useful when dealing with multiple schemas with same-named tables.
-useForeignKeyName/-fkn <true/t | false/f>: Set this flag to true if you would like field names for relations to be based on the database foreign key name. By default, relation field names are derived from the name of the related class.
-nullableAsObject/-no <true/t | false/f> : By default, all non-foreign key columns are mapped to primitives. Set this flag to true to generate primitive wrapper fields instead for columns that allow null values.
-blobAsObject/-bo <true/t | false/f> : By default, all binary columns are mapped to byte fields. Set this flag to true to map them to Object fields instead. Note that whem mapped this way, the column is presumed to contain a serialized Java object.
-primaryKeyOnJoin/-pkj <true/t | false/f> : The standard reverse mapping tool behavior is to map all tables with primary keys to persistent classes. If your schema has primary keys on many-many join tables as well, set this flag to true to avoid creating classes for those tables.
-oneToManyRelations/-omr <true/t | false/f>: Set to false to prevent the creation of inverse 1-many relations for every 1-1 relation detected.
-useDatastoreIdentity/-ds <true/t | false/f>: Set to true to use datastore JDO identity for tables that have single numeric primary key columns. The tool typically uses application identity for all generated classes.
-innerAppId/-inn <true/t | false/f>: Set to true to have any generated application identity classes be created as static inner classes within the persistent classes. Defaults to false.
-metadata/-md <package | class> : Whether to write a single package-level JDO metadata file, or to write a JDO metadata file per generated class. Defaults to package.
-customizerClass/-cc <class name> : The full class name of a kodo.jdbc.meta.ReverseCustomizer customization plugin. If you do not specify a reverse customizer of your own, the system defaults to a PropertiesReverseCustomizer . This customizer allows you to specify simple customization properties in the properties file given with the -customizerProperties flag below. We present the available property keys below.
-customizerProperties/-cp <properties file or resource>: The path or resource name of a properties file to pass to the reverse customizer on initialization.
-customizer./-c.<property name> <property value>: The given property name will be matched with the corresponding Java bean property in the specified reverse customizer, and set to the given value.
Running the tool will generate .java files for each generated class and its application identity class, package-level or per-class .jdo files, depending on the -metadata flag, and package-level or per-class .mapping files, depending on the same flag.
Examine the generated class, metadata, and mapping information, and modify it as necessary. Information on object-relational mapping in Kodo JDO is available in Chapter 7, Object-Relational Mapping. Remember that the reverse mapping tool only provides a starting point, and you are free to make whatever modifications you want to the code it generates.
After you are satisfied with the generated classes and their mappings, you should first compile them with javac, jikes, or your favorite Java compiler. Make sure the classes and their JDO metadata are located in the directory corresponding to the -package flag you gave the reverse mapping tool.
Finally, use the mapping tool to import the object-relational mapping data in the mapping files into the mapping factory you have chosen to use. Note that if you are using the default FileMappingFactory, you can simply leave the mapping file(s) in place, without importing them.
Your persistent classes are now ready to access your existing schema. Remember to enhance them before use.
The reverse mapping process can be customized with the kodo.jdbc.meta.ReverseCustomizer plugin interface. See the class Javadoc for details on the hooks that this interface provides. Specify the concrete plugin implementation to use with the -customizerClass/-cc command-line flag, described in the preceding section.
By default, the reverse mapping tool uses a kodo.jdbc.meta.PropertiesReverseCustomizer . This customizer allows you to perform relatively simple customizations through the properties file named with the -customizerProperties tool flag. The customizer recognizes the following properties:
<class name>.rename <new class name> : Override the given tool-generated class name with a new value. Use full class names, including package. You are free to rename a class to a new package. Specify a value of none to reject the class and leave the corresponding table unmapped.
<class name>.identity <datastore | identity class name>: Set this property to datastore to use datastore identity for the named class, rather than the default application identity. Any value other than datastore will be considered a new class name for the generated application identity class. Give full class names, including package. You are free to change the package of the identity class this way. If the persistent class has been renamed, use the new class name for this property key. Remember that datastore identity requires a table with a single numeric primary key column.
<class name>.<field name>.rename <new field name>: Override the tool-generated field name with the given one. Use the field owner's full class name in the property key. If the field owner's class was renamed, use the new class name. The property value should be the new field name, without the preceding class name. Use a value of none to reject the generated mapping and remove the field from the class.
<class name>.<field name>.type <field type>: The type to give the named field. Use full class names. If the field or the field's owner class has been renamed, use the new name.
<class name>.<field name>.value : The initial value for the named field. The given string will be placed as-is in the generated Java code, so be sure to add quotes to strings, etc. If the field or the field's owner class has been renamed, use the new name.
All property keys are optional; if not specified, the customizer keeps the default value generated by the reverse mapping tool.
Example 5.16. Customizing Reverse Mapping with Properties
reversemappingtool -pkg com.xyz -cp custom.properties schema.xml
com.xyz.TblMagazine.rename: com.xyz.Magazine com.xyz.TblArticle.rename: com.xyz.Article com.xyz.TblPubCompany.rename: com.xyz.pub.Company com.xyz.TblSysInfo.rename: none com.xyz.Magazine.allArticles.rename: articles com.xyz.Magazine.articles.type: java.util.Collection com.xyz.Magazine.articles.value: new TreeSet() com.xyz.Magazine.identity: datastore com.xyz.pub.Company.identity: com.xyz.pub.CompanyId
Your Kodo download includes the PropertiesReverseCustomizer source code. You can use this code as an example when writing your own customization class.