Hello :
this is the build.xml in Ant:
<?xml version="1.0"?>
<project name="Harnessing Hibernate: The Developer's Notebook"

default ="db" basedir=".">
<!-- Set up properties containing important project directories -->
<property name = "source.root" value="src"/>
<property name = "class.root" value ="classes"/>
<property name ="lib.dir" value="lib"/>
<property name="data.dir" value="data"/>
<!--Set up the class path for compilation and execution-->
<path id="project.class.path">
<!-- Include our own classes ,of course -->
<pathelement location="${class.root}"/>
<!-- include jars in the project library directory -->
<fileset dir="${lib.dir}">
<include name="**/*.jar"/>
</fileset>
</path>
<target name="db" description = "RUNS HSQLDB database management UI against

the database file -- use when application is not running">
<java classname="org.hsqldb.util.DatabaseManager"

fork="yes">
<classpath refid="project.class.path"/>
<arg value="-driver"/>
<arg value="org.hsqldb.jdbcDriver"/>
<arg value="-url"/>
<arg value="jdbc:hsqldb:${data.dir}/music"/>
<arg value="-user"/>
<arg value="sa"/>
</java>
</target>
<!-- Teach Ant how to use Hibernate's code generation tool -->
<taskdef name="hbm2java"
classname="net.sf.hibernate.tool.hbm2java.Hbm2JavaTask"

classpathref="project.class.path"/>
<!-- Generate the java code for all mapping files in our source tree -->
<target name="codegen"

description="Generate Java source from the O/R mapping files">
<hbm2java output="${source.root}">
<fileset dir="${source.root}">
<include name="**/*.hbm.xml"/>
</fileset>

</hbm2java>
</target>
</project>


and this is track.hbm.xml:
<?xml version ="1.0"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="com.oreilly.hh.Track" table="TRACK">

<meta attribute = "class-description">
Represents a single playable track in the music database.
@author Jim Elliott (with help from hibernate)
</meta>
<id name="id" type ="int" column="TRACK_ID">

<meta attribute="scope-set">protected</meta>
<generator class="native"/>
</id>

<property name="title" type="string" not-null="true"/>
<property name="filepath" type="string" not-null="true"/>
<property name="playTime" type="time">
<meta attribute="field-description">Plying time</meta>
</property>

<property name="added" type="date">
<meta attribute="field-description">When the track was created</meta>
</property>
<property name="volume" type="short">
<meta attribute"field-description">How loud to play the track</meta>
</property>

</class>
</hibernate-mapping>

ok when I type the following command on the prompt:
ant codegen

it send me back this error:Build failed E:\HibernateProjects\ch01\build.xml:60: taskdef class net.sf.hibernate.tool.hbm2 java.HBM2JavaTask cannot be found.
I should mention I use hibernate-3.1.2 version.
Thank you very much for your attention please help me

you said you're using 3.1.2, but that looks like the old toolset specs. The v3 tools use
<taskdef ... classname="org.hibernate.tool.ant.HibernateToolTask" ...>
See
http://www.hibernate.org/hib_docs/tools ... tml#d0e651
for an example.