If we have two entities with a bi-directional association between them and define at class scope level the meta attributes: use-in-string, use-in-equals:

<hibernate-mapping>
  <class name="Person">
    <meta attribute="use-in-tostring">true</meta>
    <meta attribute="use-in-equals">true</meta>
    ...
  </class>
</hibernate-mapping>

and for Event.hbm file:

<hibernate-mapping>              
  <class name="events.Event" table="EVENTS">
    <meta attribute="use-in-tostring">true</meta>
    <meta attribute="use-in-equals">true</meta>                  
    <id name="id" column="EVENT_ID">
        <generator class="native"/>
    </id>
    <property name="date" type="timestamp" column="EVENT_DATE"/>
    <property name="title"/>
    <set name="participants" table="PERSON_EVENT" inverse="true">
        <key column="EVENT_ID"/>
        <many-to-many column="PERSON_ID" class="events.Person"/>
    </set>                    
  </class>
</hibernate-mapping>

Then <hbm2java> will assume you want to include all properties and collections in the toString()/equals() methods and this can result in infinite recursive calls.

To remedy this you have to decide which side of the association will include the other part (if at all) in the toString()/equals() methods. Therefore it is not a good practice to put at class scope such meta attributes, unless you are defining a class without bi-directional associations

We recomend instead to add the meta attributes at the property level:

<hibernate-mapping>              
  <class name="events.Event" table="EVENTS">                  
    <id name="id" column="EVENT_ID">
        <meta attribute="use-in-tostring">true</meta>
        <generator class="native"/>
    </id>
    <property name="date" type="timestamp" column="EVENT_DATE"/>
    <property name="title">
      <meta attribute="use-in-tostring">true</meta>
      <meta attribute="use-in-equals">true</meta>      
    </property>
    <set name="participants" table="PERSON_EVENT" inverse="true">
        <key column="EVENT_ID"/>
        <many-to-many column="PERSON_ID" class="events.Person"/>
    </set>                    
  </class>
</hibernate-mapping>

and now for Person:

<hibernate-mapping>
    <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>
        <meta attribute="use-in-tostring">true</meta>        
        <generator class="increment"/>
    </id>
    <property name="name" type="string">
        <meta attribute="field-description">The name of the person</meta>
        <meta attribute="use-in-tostring">true</meta>
    </property>
  </class>
</hibernate-mapping>

For equal()/hashCode() method generation, you have to take into account that the attributes that participate on such method definition, should take into account only attributes with business meaning (the name, social security number, etc, but no generated id's, for example).

This is important because Java's hashbased collections, such as java.util.Set relies on equals() and hashcode() to be correct and not change for objects in the set; this can be a problem if the id gets assigned for an object after you inserted it into a set.

Therefore automatically configuration the generation of equals()/hashCode() methods specifying at class scope level the <meta> attribute use-in-equals could be a dangerous decision that could produce non expected side-effect.

Here you can get more in-depth explanation on the subject of equals() and hashcode().

With an <meta attribute="class-code">, you can add addional methods on a given class, nevertheless such <meta> attribute can not be used at property scope level and Hibernate Tools does not provide such <meta> attributes.

A possibly solution for this is to modify the freemarker templates responsable for generating the POJO's. If you look inside hibernate-tools.jar, you can find the template: pojo/PojoPropertyAccessor.ftl

This file is as the named indicates used to generate property accessors for pojo's.

Extract the PojoPropertyAccessor.ftl into a local folder i.e. ${hbm.template.path}, respecting the whole path, for example: ${hbm.template.path}/pojo/PojoPropertyAccessor.ftl

The contents of the file is something like this:

<#foreach property in pojo.getAllPropertiesIterator()>
    ${pojo.getPropertyGetModifiers(property)} ${pojo.getJavaTypeName(property, jdk5)} ${pojo.getGetterSignature(property)}() {
        return this.${property.name};
    }
    
    ${pojo.getPropertySetModifiers(property)} void set${pojo.getPropertyName(property)}(${pojo.getJavaTypeName(property, jdk5)} ${property.name}) {
        this.${property.name} = ${property.name};
    }
</#foreach>

We can add conditionally pre/post-conditions on our set method generation just adding a little Freemarker syntax to the above source code:

<#foreach property in pojo.getAllPropertiesIterator()>
    ${pojo.getPropertyGetModifiers(property)} ${pojo.getJavaTypeName(property, jdk5)} ${pojo.getGetterSignature(property)}() {
        return this.${property.name};
    }
    
    ${pojo.getPropertySetModifiers(property)} void set${pojo.getPropertyName(property)}(${pojo.getJavaTypeName(property, jdk5)} ${property.name}) {
      <#if pojo.hasMetaAttribute(property, "pre-cond")> 
       ${c2j.getMetaAsString(property, "pre-cond","\n")} 
      </#if>      
      this.${property.name} = ${property.name};
      <#if pojo.hasMetaAttribute(property, "post-cond")> 
       ${c2j.getMetaAsString(property, "post-cond","\n")} 
      </#if>        
}
</#foreach>

Now if in any *hbm.xml file we define the <meta> attributes: pre-cond or post-cond, their contents will be generated into the body of the relevant set method.

As an examlpe let us add a pre-condition for property name preventing no Person can have an empty name. So we have to modify the Person.hbm.xml file like this:

<hibernate-mapping>
  <class name="Person">
  <id name="id" type="long">        
      <generator class="increment"/>
  </id>
  <property name="firstName" type="string">
      <meta attribute="pre-cond"><![CDATA[
      if ((firstName != null) && (firstName.length() == 0) ) {
        throw new IllegalArgumentException("firstName can not be an empty String");
      }]]>
      </meta>
  </property>
</class>
</hibernate-mapping>

Finally we have to generate the Person.java class, for this we can use both Eclipse and Ant as long as you remember to set or fill in the templatepath setting. For Ant we configure <hibernatetool> task via the templatepath attribute as in:

    <target name="hbm2java">
        <taskdef name="hibernatetool"
          classname="org.hibernate.tool.ant.HibernateToolTask"
          classpathref="lib.classpath"/>
        <hibernatetool destdir="${hbm2java.dest.dir}"
          templatepath="${hbm.template.path}">
          <classpath>
            <path refid="pojo.classpath"/>
          </classpath>        
          <configuration>
            <fileset dir="${hbm2java.src.dir}">
              <include name="**/*.hbm.xml"/>
            </fileset>
          </configuration>
          <hbm2java/>
        </hibernatetool>
    </target>

Invoking the target <hbm2java> will generate on the ${hbm2java.dest.dir} the file: Person.java:

// default package
import java.io.Serializable;
public class Person implements Serializable {

    public Long id;

    public String name;

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

    public Person() {
    }

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

    public void setId(java.lang.Long id) {
        this.id = id;
    }
    
    public java.lang.String getName() {
        return this.name;
    }

    public void setName(java.lang.String name) {
        if ((name != null) && (name.length() == 0)) {
            throw new IllegalArgumentException("name can not be an empty String");
        }
        this.name = name;
    }
}