Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, as well as Ant tasks.
However, the Hibernate main package comes bundled with an integrated tool (it can even be used from "inside" Hibernate on-the-fly):
SchemaExport
aka hbm2ddl.
DDL may be generated from your mapping files by a Hibernate utility. The generated schema includes referential integrity constraints (primary and foreign keys) for entity and collection tables. Tables and sequences are also created for mapped identifier generators.
You
must
specify a SQL Dialect via the hibernate.dialect property when using this tool, as DDL is highly vendor specific.
First, customize your mapping files to improve the generated schema.
20.1.1. Customizing the schema
Many Hibernate mapping elements define optional attributes named length, precision and scale. You may set the length, precision and scale of a column with this attribute.
<property name="zip" length="5"/>
<property name="balance" precision="12" scale="2"/>
Some tags also accept a not-null attribute (for generating a NOT NULL constraint on table columns) and a unique attribute (for generating UNIQUE constraint on table columns).
<many-to-one name="bar" column="barId" not-null="true"/>
<element column="serialNumber" type="long" not-null="true" unique="true"/>
A unique-key attribute may be used to group columns in a single unique key constraint. Currently, the specified value of the unique-key attribute is
not
used to name the constraint in the generated DDL, only to group the columns in the mapping file.
<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
<property name="employeeId" unique-key="OrgEmployee"/>
An index attribute specifies the name of an index that will be created using the mapped column or columns. Multiple columns may be grouped into the same index, simply by specifying the same index name.
<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>
A foreign-key attribute may be used to override the name of any generated foreign key constraint.
<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>
Many mapping elements also accept a child <column> element. This is particularly useful for mapping multi-column types:
<property name="name" type="my.customtypes.Name"/>
<column name="last" not-null="true" index="bar_idx" length="30"/>
<column name="first" not-null="true" index="bar_idx" length="20"/>
<column name="initial"/>
</property>
The default attribute lets you specify a default value for a column (you should assign the same value to the mapped property before saving a new instance of the mapped class).
<property name="credits" type="integer" insert="false">
<column name="credits" default="10"/>
</property>
<version name="version" type="integer" insert="false">
<column name="version" default="0"/>
</property>
The sql-type attribute allows the user to override the default mapping of a Hibernate type to SQL datatype.
<property name="balance" type="float">
<column name="balance" sql-type="decimal(13,3)"/>
</property>
The check attribute allows you to specify a check constraint.
<property name="foo" type="integer">
<column name="foo" check="foo > 10"/>
</property>
<class name="Foo" table="foos" check="bar < 100.0">
...
<property name="bar" type="float"/>
</class>
|
Attribute
|
Values
|
Interpretation
|
length
|
number
|
column length
|
precision
|
number
|
column decimal precision
|
scale
|
number
|
column decimal scale
|
not-null
|
true|false
|
specfies that the column should be non-nullable
|
unique
|
true|false
|
specifies that the column should have a unique constraint
|
index
|
index_name
|
specifies the name of a (multi-column) index
|
unique-key
|
unique_key_name
|
specifies the name of a multi-column unique constraint
|
foreign-key
|
foreign_key_name
|
specifies the name of the foreign key constraint generated for an association, for a <one-to-one>, <many-to-one>, <key>, or <many-to-many> mapping element. Note that inverse="true" sides will not be considered by SchemaExport.
|
sql-type
|
SQL column type
|
overrides the default column type (attribute of <column> element only)
|
default
|
SQL expression
|
specify a default value for the column
|
check
|
SQL expression
|
create an SQL check constraint on either column or table
|
Table 20.1. Summary
The <comment> element allows you to specify comments for the generated schema.
<class name="Customer" table="CurCust">
<comment>Current customers only</comment>
...
</class>
<property name="balance">
<column name="bal">
<comment>Balance in USD</comment>
</column>
</property>
This results in a comment on table or comment on column statement in the generated DDL (where supported).
The SchemaExport tool writes a DDL script to standard out and/or executes the DDL statements.
java -cp
hibernate_classpaths
org.hibernate.tool.hbm2ddl.SchemaExport
options mapping_files
You may even embed SchemaExport in your application:
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
Database properties may be specified
The needed properties are:
|
Property Name
|
Description
|
hibernate.connection.driver_class
|
jdbc driver class
|
hibernate.connection.url
|
jdbc url
|
hibernate.connection.username
|
database user
|
hibernate.connection.password
|
user password
|
hibernate.dialect
|
dialect
|
Table 20.3. SchemaExport Connection Properties
You can call SchemaExport from your Ant build script:
<target name="schemaexport">
<taskdef name="schemaexport"
classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
classpathref="jboss_hibernate_class.path"/>
<schemaexport
properties="hibernate.properties"
quiet="no"
text="no"
drop="no"
delimiter=";"
output="schema-export.sql">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaexport>
</target>
20.1.5. Incremental schema updates
The SchemaUpdate tool will update an existing schema with "incremental" changes. Note that SchemaUpdate depends heavily upon the JDBC metadata API, so it will not work with all JDBC drivers.
java -cp
hibernate_classpaths
org.hibernate.tool.hbm2ddl.SchemaUpdate
options mapping_files
|
Option
|
Description
|
--quiet
|
don't output the script to stdout
|
--text
|
don't export the script to the database
|
--naming=eg.MyNamingStrategy
|
select a NamingStrategy
|
--properties=hibernate.properties
|
read database properties from a file
|
--config=hibernate.cfg.xml
|
specify a .cfg.xml file
|
Table 20.4. SchemaUpdate Command Line Options
You may embed SchemaUpdate in your application:
Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);
20.1.6. Using Ant for incremental schema updates
You can call SchemaUpdate from the Ant script:
<target name="schemaupdate">
<taskdef name="schemaupdate"
classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
classpathref="jboss_hibernate_class.path"/>
<schemaupdate
properties="hibernate.properties"
quiet="no">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaupdate>
</target>
20.1.7. Schema validation
The SchemaValidator tool will validate that the existing database schema "matches" your mapping documents. Note that SchemaValidator depends heavily upon the JDBC metadata API, so it will not work with all JDBC drivers. This tool is extremely useful for testing.
java -cp
hibernate_classpaths
org.hibernate.tool.hbm2ddl.SchemaValidator
options mapping_files
|
Option
|
Description
|
--naming=eg.MyNamingStrategy
|
select a NamingStrategy
|
--properties=hibernate.properties
|
read database properties from a file
|
--config=hibernate.cfg.xml
|
specify a .cfg.xml file
|
Table 20.5. SchemaValidator Command Line Options
You may embed SchemaValidator in your application:
Configuration cfg = ....;
new SchemaValidator(cfg).validate();
20.1.8. Using Ant for schema validation
You can call SchemaValidator from the Ant script:
<target name="schemavalidate">
<taskdef name="schemavalidator"
classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
classpathref="jboss_hibernate_class.path"/>
<schemavalidator
properties="hibernate.properties">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaupdate>
</target>
