Hibernate.orgCommunity Documentation
Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and Ant tasks.
Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering of existing databases:
Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and syntax highlighting. It also supports semantic auto-completion for class names and property/field names, making it more versatile than a normal XML editor.
Console: the console is a new view in Eclipse. In addition to a tree overview of your console configurations, you are also provided with an interactive view of your persistent classes and their relationships. The console allows you to execute HQL queries against your database and browse the result directly in Eclipse.
Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can use a wizard to quickly generate Hibernate configuration (cfg.xml) files, or to reverse engineer an existing database schema into POJO source files and Hibernate mapping files. The reverse engineering wizard supports customizable templates.
Please refer to the Hibernate Tools package documentation for more information.
However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka
hbm2ddl.It can even
be used from "inside" Hibernate.
DDL can 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, you must customize your mapping files to improve the generated schema. The next section covers schema customization.
Many Hibernate mapping elements define optional attributes named length,
precision and scale. You can 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 can 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. It is only used 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 can be
grouped into the same index by simply specifying the same index name.
<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>
A foreign-key attribute can 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 allows you to 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>
The following table summarizes these optional attributes.
Table 21.1. Summary
| Attribute | Values | Interpretation |
|---|---|---|
length | number | column length |
precision | number | column decimal precision |
scale | number | column decimal scale |
not-null | true|false | specifies 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 |
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.
The following table displays the SchemaExport command line options
java -cp hibernate_classpaths
org.hibernate.tool.hbm2ddl.SchemaExport options mapping_files
Table 21.2. SchemaExport Command Line Options
| Option | Description |
|---|---|
--quiet | do not output the script to stdout |
--drop | only drop the tables |
--create | only create the tables |
--text | do not export to the database |
--output=my_schema.ddl | output the ddl script to a file |
--naming=eg.MyNamingStrategy | select a NamingStrategy |
--config=hibernate.cfg.xml | read Hibernate configuration from an XML file |
--properties=hibernate.properties | read database properties from a file |
--format | format the generated SQL nicely in the script |
--delimiter=; | set an end of line delimiter for the script |
You can even embed SchemaExport in your application:
Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);
Database properties can be specified:
as system properties with -D<property>
in hibernate.properties
in a named properties file with --properties
The needed properties are:
Table 21.3. SchemaExport Connection Properties
| 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 |
You can call SchemaExport from your Ant build script:
<target name="schemaexport">
<taskdef name="schemaexport"
classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
classpathref="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>
The SchemaUpdate tool will update an existing schema with "incremental" changes.
The SchemaUpdate depends upon the JDBC metadata API and, as such, will
not work with all JDBC drivers.
java -cp hibernate_classpaths
org.hibernate.tool.hbm2ddl.SchemaUpdate options mapping_files
Table 21.4. SchemaUpdate Command Line Options
| Option | Description |
|---|---|
--quiet | do not output the script to stdout |
--text | do not 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 |
You can embed SchemaUpdate in your application:
Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);
You can call SchemaUpdate from the Ant script:
<target name="schemaupdate">
<taskdef name="schemaupdate"
classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
classpathref="class.path"/>
<schemaupdate
properties="hibernate.properties"
quiet="no">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemaupdate>
</target>
The SchemaValidator tool will validate that the existing database schema "matches"
your mapping documents. The SchemaValidator depends heavily upon the JDBC
metadata API and, as such, 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
The following table displays the SchemaValidator command line options:
Table 21.5. SchemaValidator Command Line Options
| 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 |
You can embed SchemaValidator in your application:
Configuration cfg = ....;
new SchemaValidator(cfg).validate();
You can call SchemaValidator from the Ant script:
<target name="schemavalidate">
<taskdef name="schemavalidator"
classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
classpathref="class.path"/>
<schemavalidator
properties="hibernate.properties">
<fileset dir="src">
<include name="**/*.hbm.xml"/>
</fileset>
</schemavalidator>
</target>
Copyright © 2004 Red Hat, Inc.