Table of Contents
The JPA Overview's Chapter 12, Mapping Metadata explains object-relational mapping under JPA. This chapter reviews the mapping utilities OpenJPA provides and examines OpenJPA features that go beyond the JPA specification.
Forward mapping is the process of creating mappings and
their corresponding database schema from your object model. OpenJPA supports
forward mapping through the mapping tool. The next section
presents several common mapping tool use cases. You can invoke the tool through
its Java class,
org.apache.openjpa.jdbc.meta.MappingTool.
Section 1.4, “ Mapping Tool Ant Task ” describes the mapping tool Ant task.
In addition to the universal flags of the configuration framework, the mapping tool accepts the following command line arguments:
-schemaAction/-sa <add | refresh | drop | build | retain | reflect | createDB | dropDB | import | export | none>
: The action to take on the schema. These options correspond to the
same-named actions on the schema tool described in
Section 12, “
Schema Tool
”. Actions can be composed in a
comma-separated list. Unless you are running the mapping tool on all of
your persistent types at once or dropping a mapping, we strongly
recommend you use the default add action or the
build action. Otherwise you may end up inadvertently
dropping schema components that are used by classes you are not
currently running the tool over.
-schemaFile/-sf <stdout | output file>: Use this
option to write the planned schema to an XML document rather than modify the
database. The document can then be manipulated and committed to the database
with the schema tool.
-sqlFile/-sql <stdout | output file>: Use this option
to write the planned schema modifications to a SQL script rather than modify the
database. Combine this with a schemaAction of build
to generate a script that recreates the schema for the current
mappings, even if the schema already exists.
-dropTables/-dt <true/t | false/f>: Corresponds to the
same-named option on the schema tool.
-dropSequences/-dsq <true/t | false/f>: Corresponds to
the same-named option on the schema tool.
-openjpaTables/-ot <true/t | false/f>: Corresponds to
the same-named option on the schema tool.
-ignoreErrors/-i <true/t | false/f>: Corresponds to
the same-named option on the schema tool.
-schemas/-s <schema and table names>: Corresponds to
the same-named option on the schema tool. This option is ignored if
readSchema is not set to true.
-readSchema/-rs <true/t | false/f>: Set this option to
true to read the entire existing schema when the tool runs.
Reading the existing schema ensures that OpenJPA does not generate any mappings
that use table, index, primary key, or foreign key names that conflict with
existing names. Depending on the JDBC driver, though, it can be a slow process
for large schemas.
-primaryKeys/-pk <true/t | false/f>: Whether to read
and manipulate primary key information of existing tables. Defaults to false.
-foreignKeys/-fk <true/t | false/f>: Whether to read
and manipulate foreign key information of existing tables. Defaults to false.
This means that to add any new foreign keys to a class that has already been
mapped, you must explicitly set this flag to true.
-indexes/-ix <true/t | false/f>: Whether to read and
manipulate index information of existing tables. Defaults to false. This means
that to add any new indexes to a class that has already been mapped once, you
must explicitly set this flag to true.
-sequences/-sq <true/t | false/f>: Whether to
manipulate sequences. Defaults to true.
-meta/-m <true/t | false/f>: Whether the given action
applies to metadata rather than or in addition to mappings.
The mapping tool also uses an -action/-a argument to specify
the action to take on each class. The available actions are:
buildSchema: This is the default action. It
makes the database schema match your existing mappings. If your provided
mappings conflict with your class definitions, OpenJPA will fail with an
informative exception.
validate: Ensure that the mappings for the given classes are
valid and that they match the schema. No mappings or tables will be changed. An
exception is thrown if any mappings are invalid.
Each additional argument to the tool should be one of:
The full name of a persistent class.
The .java file for a persistent class.
The .class file of a persistent class.
If you do not supply any arguments to the mapping tool, it will run on the classes in your persistent classes list (see Section 1, “ Persistent Class List ”).
The mappings generated by the mapping tool are stored by the system mapping factory. Section 5, “ Mapping Factory ” discusses your mapping factory options.
The JPA specification defines a comprehensive set of defaults for missing
mapping information. Thus, forward mapping in JPA is virtually automatic. After
using the mapping annotations covered in Chapter 12,
Mapping Metadata
of the JPA Overview to override any unsatisfactory defaults, run the
mapping tool on your persistent classes. The default buildSchema
mapping tool action manipulates the database schema to
match your mappings. It fails if any of your mappings don't match your object
model.
Example 7.2. Creating the Relational Schema from Mappings
java org.apache.openjpa.jdbc.meta.MappingTool Magazine.java
To drop the schema for a persistent class, set the mapping tool's
schemaAction to drop.
Example 7.4. Dropping Mappings and Association Schema
java org.apache.openjpa.jdbc.meta.MappingTool -schemaAction drop Magazine.java
The examples below show how to use the mapping tool to generate DDL SQL scripts, rather than modifying the database directly.
Example 7.5. Create DDL for Current Mappings
This example uses your existing mappings to determine the needed schema, then
writes the SQL to create that schema to create.sql.
java org.apache.openjpa.jdbc.meta.MappingTool -schemaAction build -sql create.sql Magazine.java
Example 7.6. Create DDL to Update Database for Current Mappings
This example uses your existing mappings to determine the needed schema. It then
writes the SQL to add any missing tables and columns to the current schema to
update.sql.
java org.apache.openjpa.jdbc.meta.MappingTool -sql update.sql Magazine.java
You can configure OpenJPA to automatically run the mapping tool at runtime
through the
openjpa.jdbc.SynchronizeMappings configuration property. Using
this property saves you the trouble of running the mapping tool manually, and is
meant for use during rapid test/debug cycles.
In order to enable automatic runtime mapping, you must first list all your persistent classes as described in Section 1, “ Persistent Class List ”.
OpenJPA will run the mapping tool on these classes when your application obtains
its first EntityManager.
The openjpa.jdbc.SynchronizeMappings property is a plugin
string (see Section 4, “
Plugin Configuration
”) where the class
name is the mapping tool action to invoke, and the properties are the
MappingTool class' JavaBean properties. These properties
correspond go the long versions of the tool's command line flags.
Example 7.7. Configuring Runtime Forward Mapping
<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/>
The setting above corresponds to running the following command:
java org.apache.openjpa.jdbc.meta.MappingTool -action buildSchema -foreignKeys true