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20.2. Automatic Schema Generation
20.2.1. Customizing the Schema
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 20.1. 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.
20.2.2. Running the Tool
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_classpathsorg.hibernate.tool.hbm2ddl.SchemaExport
options mapping_files
Table 20.2. 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);
20.2.3. Database Properties
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 20.3. 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 |
20.2.4. Using Ant
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>
20.2.5. Incremental Schema Updates
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_classpathsorg.hibernate.tool.hbm2ddl.SchemaUpdate
options mapping_files
Table 20.4. 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);
20.2.6. 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="class.path"/> <schemaupdate properties="hibernate.properties" quiet="no"> <fileset dir="src"> <include name="**/*.hbm.xml"/> </fileset> </schemaupdate> </target>
20.2.7. Schema Validation
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_classpathsorg.hibernate.tool.hbm2ddl.SchemaValidator
options mapping_files
The following table displays the
SchemaValidator
command line options:
Table 20.5. 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();
20.2.8. 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="class.path"/> <schemavalidator properties="hibernate.properties"> <fileset dir="src"> <include name="**/*.hbm.xml"/> </fileset> </schemavalidator> </target>