I have 2 objects, User and Message, User has a one-to-many relationship with Message. I scanned through <http://www.xylax.net/hibernate/onetomany.html> and made the code just lik described in the article:

User.hbm.xml
=======
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >

<hibernate-mapping>

<class
name="users.User"
table="users"
>

<id
name="id"
type="java.lang.Integer"
column="id"
unsaved-value="-1"
>
<generator class="identity" />
</id>

<property
name="username"
type="java.lang.String"
column="username"
not-null="true"
length="32"
/>
<property
name="password"
type="java.lang.String"
column="password"
not-null="true"
length="32"
/>

<set role="messages" table="messages">
<key column="user_id"/>
<one-to-many class="Message"/>
</set>



</class>
</hibernate-mapping>
=======

Message.hbm.xml
=======
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd" >

<hibernate-mapping>

<class
name="users.Message"
table="messages"
>

<id
name="id"
type="java.lang.Integer"
column="id"
unsaved-value="-1"
>
<generator class="identity" />
</id>

<property
name="message"
type="java.lang.String"
column="message"
not-null="true"
length="32"
/>
<property
name="receivedUtc"
type="java.sql.Timestamp"
column="received_utc"
not-null="true"
length="19"
/>

<property
name="user_id"
type="java.lang.Integer"
column="user_id"
not-null="true"
/>


</class>
</hibernate-mapping>
=======

Afterwards i try to generate java code from the .hbm.xml file with "net.sf.hibernate.tool.hbm2java.CodeGenerator", but this gives me a:

=======
java.lang.NullPointerException
at net.sf.hibernate.tool.hbm2java.BasicRenderer.makeSupportField(BasicRenderer.java:509)
at net.sf.hibernate.tool.hbm2java.BasicRenderer.render(BasicRenderer.java:111)
at net.sf.hibernate.tool.hbm2java.Generator.write(Generator.java:113)
at net.sf.hibernate.tool.hbm2java.Generator.writeRecur(Generator.java:92)
at net.sf.hibernate.tool.hbm2java.Generator.generate(Generator.java:81)
at net.sf.hibernate.tool.hbm2java.CodeGenerator.main(CodeGenerator.java:116)
=======

I suspect the problem lies in User.hbm.xml, cause the java code of Message.hbm.xml gets generated well.

When i remove:
=======
<set role="messages" table="messages">
<key column="user_id"/>
<one-to-many class="Message"/>
</set>
=======
User.hbm.xml compiles fine.

I looked throught the code for a long time, but can't see what i'm doing wrong. Anybody got a idea?

You need to give the full class name including package in your one-to-many Element I think.

Thanks for the reply. I tryed to replace the "Message" with "users.Message", but i still got the same error:

<set role="messages" table="messages">
<key column="user_id"/>
<one-to-many class="users.Message"/>
</set>

I'm feeling a little stupid here, cause i can't seem to find where the error comes from ;)

There is no role attribute in set, it is called name

That did indeed fix it, thanks! It's a bit of a shame the article which mentioned "role" is not correctly written (or maybe just out-dated).