Chapter 20. Toolset Guide

Roundtrip engineering with Hibernate is possible using a set of Eclipse plug-ins, command line tools, and Ant tasks.
Hibernate Tools currently include plug-ins 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.

20.1. Automatic schema generation

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.

20.1.1. 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"/>
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"/>
<version name="version" type="integer" insert="false">
    <column name="version" default="0"/>
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)"/>
The check attribute allows you to specify a check constraint.
<property name="foo" type="integer">
    <column name="foo" check="foo > 10"/>
<class name="Foo" table="foos" check="bar < 100.0">
    <property name="bar" type="float"/>
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>
<property name="balance">
    <column name="bal">
        <comment>Balance in USD</comment>
This results in a comment on table or comment on column statement in the generated DDL where supported.

20.1.2. 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_classpaths org.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 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);

20.1.3. Properties

Database properties can be specified:
  • as system properties with -D<property>
  • in
  • 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

20.1.4. Using Ant

You can call SchemaExport from your Ant build script:
<target name="schemaexport">
    <taskdef name="schemaexport"
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>

20.1.5. 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_classpaths org.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 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);

20.1.6. Using Ant for incremental schema updates

You can call SchemaUpdate from the Ant script:
<target name="schemaupdate">
    <taskdef name="schemaupdate"
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>

20.1.7. 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_classpaths org.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 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();

20.1.8. Using Ant for schema validation

You can call SchemaValidator from the Ant script:
<target name="schemavalidate">
    <taskdef name="schemavalidator"
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>