Many NHibernate mapping elements define an optional attribute named length. You may set the length of a column with this attribute. (Or, for numeric/decimal data types, the precision.)

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).

Some tags accept an index attribute for specifying the name of an index for that column. A unique-key attribute can be used to group columns in a single unit key constraint. Currently, the specified value of the unique-key attribute is not used to name the constraint, only to group the columns in the mapping file.

Examples:

<property name="Foo" type="String" length="64" not-null="true"/>

<many-to-one name="Bar" foreign-key="fk_foo_bar" not-null="true"/>

<element column="serial_number" type="Int64" not-null="true" unique="true"/>

Alternatively, these elements also accept a child <column> element. This is particularly useful for multi-column types:

<property name="Foo" type="String">
    <column name="foo" length="64" not-null="true" sql-type="text"/>
</property>

<property name="Bar" type="My.CustomTypes.MultiColumnType, My.CustomTypes"/>
    <column name="fee" not-null="true" index="bar_idx"/>
    <column name="fi" not-null="true" index="bar_idx"/>
    <column name="fo" not-null="true" index="bar_idx"/>
</property>

The sql-type attribute allows the user to override the default mapping of NHibernate type to SQL datatype.

The check attribute allows you to specify a check constraint.

<property name="Foo" type="Int32">
    <column name="foo" check="foo > 10"/>
</property>

<class name="Foo" table="foos" check="bar < 100.0">
    ...
    <property name="Bar" type="Single"/>
</class>

The SchemaExport tool writes a DDL script to standard out and/or executes the DDL statements.

java -cp hibernate_classpaths net.sf.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="net.sf.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>

If you don't specify properties or a config file, the SchemaExportTask will try to use normal Ant project properties instead. In other words, if you don't want or need an external configuration or properties file, you may put hibernate.* configuration properties in your build.xml or build.properties.

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 net.sf.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="net.sf.hibernate.tool.hbm2ddl.SchemaUpdateTask"
        classpathref="class.path"/>
    
    <schemaupdate
        properties="hibernate.properties"
        quiet="no">
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>
        </fileset>
    </schemaupdate>
</target>

The config file provides for a way to specify multiple "renderers" for the source code and to declare <meta> attributes that is "global" in scope. See more about this in the <meta> attribute section.

<codegen>
    <meta attribute="implements">codegen.test.IAuditable</meta>
    <generate renderer="net.sf.hibernate.tool.hbm2java.BasicRenderer"/>
    <generate
        package="autofinders.only"
        suffix="Finder"
        renderer="net.sf.hibernate.tool.hbm2java.FinderRenderer"/>
</codegen>

This config file declares a global meta attribute "implements" and specify two renderers, the default one (BasicRenderer) and a renderer that generates Finder's (See more in "Basic Finder generation" below).

The second renderer is provided with a package and suffix attribute.

The package attribute specifies that the generated source files from this renderer should be placed here instead of the package scope specified in the .hbm files.

The suffix attribute specifies the suffix for generated files. E.g. here a file named Foo.java would be FooFinder.java instead.

It is also possible to send down arbitrary parameters to the renders by adding <param> attributes to the <generate> elements.

hbm2java currently has support for one such parameter, namely generate-concrete-empty-classes which informs the BasicRenderer to only generate empty concrete classes that extends a base class for all your classes. The following config.xml example illustrate this feature

            <codegen>
              <generate prefix="Base" renderer="net.sf.hibernate.tool.hbm2java.BasicRenderer"/> 
              <generate renderer="net.sf.hibernate.tool.hbm2java.BasicRenderer">
                <param name="generate-concrete-empty-classes">true</param>
                <param name="baseclass-prefix">Base</param>
              </generate>
            </codegen>

Notice that this config.xml configure 2 (two) renderers. One that generates the Base classes, and a second one that just generates empty concrete classes.

The <meta> tag is a simple way of annotating the hbm.xml with information, so tools have a natural place to store/read information that is not directly related to the Hibernate core.

You can use the <meta> tag to tell hbm2java to only generate "protected" setters, have classes always implement a certain set of interfaces or even have them extend a certain base class and even more.

The following example:

<class name="Person">
    <meta attribute="class-description">
        Javadoc for the Person class
        @author Frodo
    </meta>
    <meta attribute="implements">IAuditable</meta>
    <id name="id" type="long">
        <meta attribute="scope-set">protected</meta>
        <generator class="increment"/>
    </id>
    <property name="name" type="string">
        <meta attribute="field-description">The name of the person</meta>
    </property>
</class>

will produce something like the following (code shortened for better understanding). Notice the Javadoc comment and the protected set methods:

// default package

import java.io.Serializable;
import org.apache.commons.lang.builder.EqualsBuilder;
import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.ToStringBuilder;

/** 
 *         Javadoc for the Person class
 *         @author Frodo
 *     
 */
public class Person implements Serializable, IAuditable {

    /** identifier field */
    public Long id;

    /** nullable persistent field */
    public String name;

    /** full constructor */
    public Person(java.lang.String name) {
        this.name = name;
    }

    /** default constructor */
    public Person() {
    }

    public java.lang.Long getId() {
        return this.id;
    }

    protected void setId(java.lang.Long id) {
        this.id = id;
    }

    /** 
     * The name of the person
     */
    public java.lang.String getName() {
        return this.name;
    }

    public void setName(java.lang.String name) {
        this.name = name;
    }

}

Attributes declared via the <meta> tag are per default "inherited" inside an hbm.xml file.

What does that mean? It means that if you e.g want to have all your classes implement IAuditable then you just add an <meta attribute="implements">IAuditable</meta> in the top of the hbm.xml file, just after <hibernate-mapping>. Now all classes defined in that hbm.xml file will implement IAuditable! (Except if a class also has an "implements" meta attribute, because local specified meta tags always overrules/replaces any inherited meta tags).

Note: This applies to all <meta>-tags. Thus it can also e.g. be used to specify that all fields should be declare protected, instead of the default private. This is done by adding <meta attribute="scope-field">protected</meta> at e.g. just under the <class> tag and all fields of that class will be protected.

To avoid having a <meta>-tag inherited then you can simply specify inherit="false" for the attribute, e.g. <meta attribute="scope-class" inherit="false">public abstract</meta> will restrict the "class-scope" to the current class, not the subclasses.

It is now possible to have hbm2java generate basic finders for Hibernate properties. This requires two things in the hbm.xml files.

The first is an indication of which fields you want to generate finders for. You indicate that with a meta block inside a property tag such as:

<property name="name" column="name" type="string">
     <meta attribute="finder-method">findByName</meta>
</property>

The finder method name will be the text enclosed in the meta tags.

The second is to create a config file for hbm2java of the format:

<codegen>
    <generate renderer="net.sf.hibernate.tool.hbm2java.BasicRenderer"/>
    <generate suffix="Finder" renderer="net.sf.hibernate.tool.hbm2java.FinderRenderer"/>
</codegen>

And then use the param to hbm2java --config=xxx.xml where xxx.xml is the config file you just created.

An optional parameter is meta tag at the class level of the format:

<meta attribute="session-method">
    com.whatever.SessionTable.getSessionTable().getSession();
</meta>

Which would be the way in which you get sessions if you use the Thread Local Session pattern (documented in the Design Patterns area of the Hibernate website).

It is now possible to use velocity as an alternative rendering mechanism. The follwing config.xml shows how to configure hbm2java to use its velocity renderer.

    <codegen>
     <generate renderer="net.sf.hibernate.tool.hbm2java.VelocityRenderer">
      <param name="template">pojo.vm</param>
     </generate>
    </codegen>

The parameter named template is a resource path to the velocity macro file you want to use. This file must be available via the classpath for hbm2java. Thus remember to add the directory where pojo.vm is located to your ant task or shell script. (The default location is ./tools/src/velocity)

Be aware that the current pojo.vm generates only the most basic parts of the java beans. It is not as complete and feature rich as the default renderer - primarily a lot of the meta tags are not supported.

The tool writes XML mappings to standard out and/or to a file.

When invoking the tool you must place your compiled classes on the classpath.

java -cp hibernate_and_your_class_classpaths net.sf.hibernate.tool.class2hbm.MapGenerator options and classnames

There are two modes of operation: command line or interactive.

The interactive mode is selected by providing the single command line argument --interact. This mode provides a prompt response console. Using it you can set the UID property name for each class using the uid=XXX command where XXX is the UID property name. Other command alternatives are simply a fully qualified class name, or the command done which emits the XML and terminates.

In command line mode the arguments are the options below interspersed with fully qualified class names of the classes to be processed. Most of the options are meant to be used multiple times; each use affects subsequently added classes.

The abstract switch directs the map generator tool to ignore specific super classes so that classes with common inheritance are not mapped to one large table. For instance, consider these class hierarchies:

Animal-->Mammal-->Human

Animal-->Mammal-->Marsupial-->Kangaroo

If the --abstractswitch is not used, all classes will be mapped as subclasses of Animal, resulting in one large table containing all the properties of all the classes plus a discriminator column to indicate which subclass is actually stored. If Mammal is marked as abstract, Human and Marsupial will be mapped to separate <class> declarations and stored in separate tables. Kangaroo will still be a subclass of Marsupial unless Marsupial is also marked as abstract.