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>

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:

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. 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

You may 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. 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

You may 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>
    </schemaupdate>
</target>