Question

I have three classes:

xyz/url/core/datastore/ObjectBase.java
xyz/url/core/test/hibernate/BaseClass.java
xyz/url/core/test/hibernate/ChildClass.java

Their code:

ObjectBase

package xyz.url.core.datastore;

import java.util.Date;

public abstract class ObjectBase {
    private final long  m_id;
    private final long  m_version;

    private final Date  m_creation_time;

    public ObjectBase() {
        this.m_id = 0;
        this.m_version = 0;

        this.m_creation_time = new Date();
    }

    public long get_id() {
        return this.m_id;
    }

    public long get_version() {
        return this.m_version;
    }

    public Date get_creation_time() {
        return this.m_creation_time;
    }
}

BaseClass

package xyz.url.core.test.hibernate;

import java.util.Date;

import xyz.url.core.datastore.ObjectBase;

public abstract class BaseClass extends ObjectBase {

    private final Date  m_another_time;

    public BaseClass() {
        this.m_another_time = new Date();
    }

    public void say_something() {
        final Class<?> my_class = this.getClass();
        final String output = String.format(
                "Hello from the `%s` class! My id is %d!", my_class.getName(),
                this.get_id());
        System.out.println(output);
    }
}

ChildClass

package xyz.url.core.test.hibernate;

public class ChildClass extends BaseClass {
    private String  m_text;

    public ChildClass(final String text) {
        this.m_text = text;
    }

    public void set_text(final String text) {
        this.m_text = text;
    }

    public String get_text() {
        return this.m_text;
    }
}

Currently I use implicit polymorphism; I have one HBM.XML file for the only “concrete” class (ChildClass), named “ChildClass.hbm.xml”:

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="xyz.url.core.test.hibernate"
    default-access="field">

    <class name="ChildClass" table="child_class">
        <!-- Attributes of ObjectBase -->
        <id name="m_id" column="id">
            <generator class="increment" />
        </id>

        <version name="m_version" column="version" type="long" />

        <property name="m_creation_time" column="creation_time" type="date" />

        <!-- Attributes of BaseClass -->
        <property name="m_another_time" column="another_time" type="date" />

        <!-- Attributes of ChildClass -->
        <property name="m_text" column="text" type="string" />
    </class>

</hibernate-mapping>

See the above? I aggregate all the attributes of the three classes into one table.

I want to do the same as above, get one table as above (“child_class”), but break it into three HBM.XML files.

I was hoping Hibernate (v4.1) supports some sort of an “import” keyword, so I can create three HBM.XML files, one for each class, and link them all into one.
Unfortunately they got “too smart” and made things a lot more complicated. If I’m wrong, enlighten me please!

Take note that ObjectBase and BaseClass are abstract classes.

Another thing worth mentioning is that when I get an object from the database I know exactly what type it should be, so maybe I should not use “discriminators”…?..

Here is some console output when I test what I currently have, and I wish it stays this way, I mean, one table is created, one table is being fetched when reading (instead of the “join strategy” I’ve read in Hibernate’s docs):

Hibernate: 
    drop table child_class if exists
Hibernate: 
    create table child_class (
        id bigint not null,
        version bigint not null,
        creation_time date,
        another_time date,
        text varchar(255),
        primary key (id)
    )
APR 14, 2012 8:49:47 AM org.hibernate.tool.hbm2ddl.SchemaExport execute
INFO: HHH000230: Schema export complete
Hibernate: 
    select
        max(id) 
    from
        child_class
Hibernate: 
    /* insert xyz.url.core.test.hibernate.ChildClass
        */ insert 
        into
            child_class
            (version, creation_time, another_time, text, id) 
        values
            (?, ?, ?, ?, ?)

Again, all I want is to break my HBM.XML to three different files so I don’t have to write the same properties in every concrete class’s descriptor. That’s all.

Thank you!

Answer 1

union-subclass is all I needed. Simple, OOP-like inheritance.