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<title>6.&nbsp; Inheritance</title><link rel="stylesheet" href="css/docbook.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.72.0"><link rel="start" href="manual.html" title="Apache OpenJPA User's Guide"><link rel="up" href="jpa_overview_mapping.html" title="Chapter&nbsp;12.&nbsp; Mapping Metadata"><link rel="prev" href="jpa_overview_mapping_sequence.html" title="5.&nbsp; Generators"><link rel="next" href="jpa_overview_mapping_discrim.html" title="7.&nbsp; Discriminator"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">6.&nbsp;
Inheritance
</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="jpa_overview_mapping_sequence.html">Prev</a>&nbsp;</td><th width="60%" align="center">Chapter&nbsp;12.&nbsp;
Mapping Metadata
</th><td width="20%" align="right">&nbsp;<a accesskey="n" href="jpa_overview_mapping_discrim.html">Next</a></td></tr></table><hr></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="jpa_overview_mapping_inher"></a>6.&nbsp;
Inheritance
</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_single">6.1.
Single Table
</a></span></dt><dd><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_single_adv">6.1.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_single_disadv">6.1.2.
Disadvantages
</a></span></dt></dl></dd><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_joined">6.2.
Joined
</a></span></dt><dd><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_joined_adv">6.2.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_joined_disadv">6.2.2.
Disadvantages
</a></span></dt></dl></dd><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_tpc">6.3.
Table Per Class
</a></span></dt><dd><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_tpc_adv">6.3.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_tpc_disadv">6.3.2.
Disadvantages
</a></span></dt></dl></dd><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_together">6.4.
Putting it All Together
</a></span></dt></dl></div><a class="indexterm" name="d0e10307"></a><a class="indexterm" name="d0e10314"></a><a class="indexterm" name="d0e10319"></a><a class="indexterm" name="d0e10326"></a><p>
In the 1990's programmers coined the term <span class="emphasis"><em>impedance mismatch
</em></span> to describe the difficulties in bridging the object and relational
worlds. Perhaps no feature of object modeling highlights the impedance mismatch
better than inheritance. There is no natural, efficient way to represent an
inheritance relationship in a relational database.
</p><p>
<a class="indexterm" name="d0e10336"></a>
Luckily, JPA gives you a choice of inheritance strategies, making
the best of a bad situation. The base entity class defines the inheritance
strategy for the hierarchy with the <code class="classname">Inheritance</code>
annotation. <code class="classname">Inheritance</code> has the following properties:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">InheritanceType strategy</code>: Enum value declaring the
inheritance strategy for the hierarchy. Defaults to <code class="literal">
InheritanceType.SINGLE_TABLE</code>. We detail each of the available
strategies below.
</p></li></ul></div><p>
The corresponding XML element is <code class="literal">inheritance</code>, which has a
single attribute:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">strategy</code>: One of <code class="literal">SINGLE_TABLE</code>, <code class="literal">
JOINED</code>, or <code class="literal">TABLE_PER_CLASS</code>.
</p></li></ul></div><p>
The following sections describe JPA's standard inheritance strategies.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
OpenJPA allows you to vary your inheritance strategy for each class, rather than
forcing a single strategy per inheritance hierarchy. See
<a href="ref_guide_mapping_jpa.html" title="7.&nbsp; Additional JPA Mappings">Section&nbsp;7, &#8220;
Additional JPA Mappings
&#8221;</a> in the Reference Guide for
details.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="jpa_overview_mapping_inher_single"></a>6.1.&nbsp;
Single Table
</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_single_adv">6.1.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_single_disadv">6.1.2.
Disadvantages
</a></span></dt></dl></div><a class="indexterm" name="d0e10391"></a><a class="indexterm" name="d0e10398"></a><p>
The <code class="literal">InheritanceType.SINGLE_TABLE</code> strategy maps all classes in
the hierarchy to the base class' table.
</p><div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" cellspacing="0" cellpadding="0" width="177"><tr><td><img src="img/inher-superclass-table.png"></td></tr></table></div><p>
In our model, <code class="classname">Subscription</code> is mapped to the <code class="literal">
CNTRCT.SUB</code> table. <code class="classname"> LifetimeSubscription</code>, which
extends <code class="classname"> Subscription</code>, adds its field data to this table
as well.
</p><div class="example"><a name="jpa_overview_mapping_inher_singleex"></a><p class="title"><b>Example&nbsp;12.5.&nbsp;
Single Table Mapping
</b></p><div class="example-contents"><pre class="programlisting">
@Entity
@Table(name="SUB", schema="CNTRCT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Subscription {
...
}
@Entity(name="Lifetime")
public class LifetimeSubscription
extends Subscription {
...
}
</pre><p>
The same metadata expressed in XML form:
</p><pre class="programlisting">
&lt;entity class="org.mag.subcribe.Subscription"&gt;
&lt;table name="SUB" schema="CNTRCT"/&gt;
&lt;inheritance strategy="SINGLE_TABLE"/&gt;
...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.LifetimeSubscription"&gt;
...
&lt;/entity&gt;
</pre></div></div><br class="example-break"><p>
Single table inheritance is the default strategy. Thus, we could omit the
<code class="literal">@Inheritance</code> annotation in the example above and get the same
result.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
<a class="indexterm" name="d0e10443"></a>
<a class="indexterm" name="d0e10449"></a>
Mapping subclass state to the superclass table is often called <span class="emphasis"><em>flat
</em></span> inheritance mapping.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_single_adv"></a>6.1.1.&nbsp;
Advantages
</h4></div></div></div><a class="indexterm" name="d0e10461"></a><p>
Single table inheritance mapping is the fastest of all inheritance models, since
it never requires a join to retrieve a persistent instance from the database.
Similarly, persisting or updating a persistent instance requires only a single
<code class="literal">INSERT</code> or <code class="literal">UPDATE</code> statement. Finally,
relations to any class within a single table inheritance hierarchy are just as
efficient as relations to a base class.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_single_disadv"></a>6.1.2.&nbsp;
Disadvantages
</h4></div></div></div><a class="indexterm" name="d0e10479"></a><p>
The larger the inheritance model gets, the "wider" the mapped table gets, in
that for every field in the entire inheritance hierarchy, a column must exist in
the mapped table. This may have undesirable consequence on the database size,
since a wide or deep inheritance hierarchy will result in tables with many
mostly-empty columns.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="jpa_overview_mapping_inher_joined"></a>6.2.&nbsp;
Joined
</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_joined_adv">6.2.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_joined_disadv">6.2.2.
Disadvantages
</a></span></dt></dl></div><a class="indexterm" name="d0e10491"></a><a class="indexterm" name="d0e10498"></a><p>
The <code class="literal">InheritanceType.JOINED</code> strategy uses a different table
for each class in the hierarchy. Each table only includes state declared in its
class. Thus to load a subclass instance, the JPA implementation must
read from the subclass table as well as the table of each ancestor class, up to
the base entity class.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
<a class="indexterm" name="d0e10511"></a>
<a class="indexterm" name="d0e10517"></a>
Using joined subclass tables is also called <span class="emphasis"><em>vertical</em></span>
inheritance mapping.
</p></div><div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" cellspacing="0" cellpadding="0" width="171"><tr><td><img src="img/jpa-inher-joined.png"></td></tr></table></div><p>
<code class="classname">PrimaryKeyJoinColumn</code> annotations tell the JPA
implementation how to join each subclass table record to the corresponding
record in its direct superclass table. In our model, the <code class="literal">LINE_ITEM.ID
</code> column joins to the <code class="literal">CONTRACT.ID</code> column. The
<code class="classname">PrimaryKeyJoinColumn</code> annotation has the following
properties:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">String name</code>: The name of the subclass table column. When
there is a single identity field, defaults to that field's column name.
</p></li><li><p>
<code class="literal">String referencedColumnName</code>: The name of the superclass
table column this subclass table column joins to. When there is a single
identity field, defaults to that field's column name.
</p></li><li><p>
<code class="literal">String columnDefinition</code>: This property has the same meaning
as the <code class="literal">columnDefinition</code> property on the <code class="classname">Column
</code> annotation, described in
<a href="jpa_overview_mapping_column.html" title="3.&nbsp; Column">Section&nbsp;3, &#8220;
Column
&#8221;</a>.
</p></li></ul></div><p>
The XML equivalent is the <code class="literal">primary-key-join-column</code> element.
Its attributes mirror the annotation properties described above:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">name</code>
</p></li><li><p>
<code class="literal">referenced-column-name</code>
</p></li><li><p>
<code class="literal">column-definition</code>
</p></li></ul></div><p>
The example below shows how we use <code class="literal">InheritanceTable.JOINED</code>
and a primary key join column to map our sample model according to the diagram
above. Note that a primary key join column is not strictly needed, because there
is only one identity column, and the subclass table column has the same name as
the superclass table column. In this situation, the defaults suffice. However,
we include the primary key join column for illustrative purposes.
</p><div class="example"><a name="jpa_overview_mapping_inher_joinedex"></a><p class="title"><b>Example&nbsp;12.6.&nbsp;
Joined Subclass Tables
</b></p><div class="example-contents"><pre class="programlisting">
@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
public class Contract
extends Document {
...
}
public class Subscription {
...
@Entity
@Table(name="LINE_ITEM", schema="CNTRCT")
@PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
public static class LineItem
extends Contract {
...
}
}
</pre><p>
The same metadata expressed in XML form:
</p><pre class="programlisting">
&lt;entity class="org.mag.subcribe.Contract"&gt;
&lt;table schema="CNTRCT"/&gt;
&lt;inheritance strategy="JOINED"/&gt;
...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
&lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
&lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
...
&lt;/entity&gt;
</pre></div></div><br class="example-break"><p>
When there are multiple identity columns, you must define multiple <code class="classname">
PrimaryKeyJoinColumn</code>s using the aptly-named <code class="classname">
PrimaryKeyJoinColumns</code> annotation. This annotation's value is an
array of <code class="classname"> PrimaryKeyJoinColumn</code> s. We could rewrite
<code class="classname">LineItem</code>'s mapping as:
</p><pre class="programlisting">
@Entity
@Table(name="LINE_ITEM", schema="CNTRCT")
@PrimaryKeyJoinColumns({
@PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
})
public static class LineItem
extends Contract {
...
}
</pre><p>
In XML, simply list as many <code class="literal"> primary-key-join-column</code> elements
as necessary.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_joined_adv"></a>6.2.1.&nbsp;
Advantages
</h4></div></div></div><a class="indexterm" name="d0e10633"></a><p>
The joined strategy has the following advantages:
</p><div class="orderedlist"><ol type="1"><li><p>
<a class="indexterm" name="d0e10646"></a>
Using joined subclass tables results in the most <span class="emphasis"><em>normalized</em></span>
database schema, meaning the schema with the least spurious or redundant data.
</p></li><li><p>
As more subclasses are added to the data model over time, the only schema
modification that needs to be made is the addition of corresponding subclass
tables in the database (rather than having to change the structure of existing
tables).
</p></li><li><p>
Relations to a base class using this strategy can be loaded through standard
joins and can use standard foreign keys, as opposed to the machinations required
to load polymorphic relations to table-per-class base types, described below.
</p></li></ol></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_joined_disadv"></a>6.2.2.&nbsp;
Disadvantages
</h4></div></div></div><a class="indexterm" name="d0e10662"></a><p>
Aside from certain uses of the table-per-class strategy described below, the
joined strategy is often the slowest of the inheritance models. Retrieving any
subclass requires one or more database joins, and storing subclasses requires
multiple <code class="literal">INSERT</code> or <code class="literal">UPDATE</code> statements. This
is only the case when persistence operations are performed on subclasses; if
most operations are performed on the least-derived persistent superclass, then
this mapping is very fast.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
When executing a select against a hierarchy that uses joined subclass table
inheritance, you must consider how to load subclass state.
<a href="ref_guide_perfpack_eager.html" title="7.&nbsp; Eager Fetching">Section&nbsp;7, &#8220;
Eager Fetching
&#8221;</a> in the Reference Guide
describes OpenJPA's options for efficient data loading.
</p></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="jpa_overview_mapping_inher_tpc"></a>6.3.&nbsp;
Table Per Class
</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_tpc_adv">6.3.1.
Advantages
</a></span></dt><dt><span class="section"><a href="jpa_overview_mapping_inher.html#jpa_overview_mapping_inher_tpc_disadv">6.3.2.
Disadvantages
</a></span></dt></dl></div><a class="indexterm" name="d0e10685"></a><a class="indexterm" name="d0e10692"></a><p>
Like the <code class="literal">JOINED</code> strategy, the <code class="literal">
InheritanceType.TABLE_PER_CLASS</code> strategy uses a different table for
each class in the hierarchy. Unlike the <code class="literal">JOINED</code> strategy,
however, each table includes all state for an instance of the corresponding
class. Thus to load a subclass instance, the JPA implementation must
only read from the subclass table; it does not need to join to superclass
tables.
</p><div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" cellspacing="0" cellpadding="0" width="189"><tr><td><img src="img/inher-tpc.png"></td></tr></table></div><p>
Suppose that our sample model's <code class="classname">Magazine</code> class has a
subclass <code class="classname">Tabloid</code>. The classes are mapped using the
table-per-class strategy, as in the diagram above. In a table-per-class mapping,
<code class="classname"> Magazine</code>'s table <code class="literal">MAG</code> contains all
state declared in the base <code class="classname">Magazine</code> class. <code class="classname">
Tabloid</code> maps to a separate table, <code class="literal"> TABLOID</code>. This
table contains not only the state declared in the <code class="classname">Tabloid</code>
subclass, but all the base class state from <code class="classname">Magazine</code> as
well. Thus the <code class="literal">TABLOID</code> table would contain columns for
<code class="literal">isbn</code>, <code class="literal">title</code>, and other <code class="classname">
Magazine</code> fields. These columns would default to the names used in
<code class="classname">Magazine</code>'s mapping metadata.
<a href="jpa_overview_mapping_field.html#jpa_overview_mapping_embed" title="8.3.&nbsp; Embedded Mapping">Section&nbsp;8.3, &#8220;
Embedded Mapping
&#8221;</a> will show you how to use
<code class="literal">AttributeOverride</code>s and <code class="literal">AssociationOverride</code>
s to override superclass field mappings.
</p><div class="example"><a name="jpa_overview_mapping_inher_tpcex"></a><p class="title"><b>Example&nbsp;12.7.&nbsp;
Table Per Class Mapping
</b></p><div class="example-contents"><pre class="programlisting">
@Entity
@Table(name="MAG")
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Magazine {
...
}
@Entity
@Table(name="TABLOID")
public class Tabloid
extends Magazine {
...
}
</pre><p>
And the same classes in XML:
</p><pre class="programlisting">
&lt;entity class="org.mag.Magazine"&gt;
&lt;table name="MAG"/&gt;
&lt;inheritance strategy="TABLE_PER_CLASS"/&gt;
...
&lt;/entity&gt;
&lt;entity class="org.mag.Tabloid"&gt;
&lt;table name="TABLOID"/&gt;
...
&lt;/entity&gt;
</pre></div></div><br class="example-break"><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_tpc_adv"></a>6.3.1.&nbsp;
Advantages
</h4></div></div></div><a class="indexterm" name="d0e10776"></a><p>
The table-per-class strategy is very efficient when operating on instances of a
known class. Under these conditions, the strategy never requires joining to
superclass or subclass tables. Reads, joins, inserts, updates, and deletes are
all efficient in the absence of polymorphic behavior. Also, as in the joined
strategy, adding additional classes to the hierarchy does not require modifying
existing class tables.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="jpa_overview_mapping_inher_tpc_disadv"></a>6.3.2.&nbsp;
Disadvantages
</h4></div></div></div><a class="indexterm" name="d0e10788"></a><p>
Polymorphic relations to non-leaf classes in a table-per-class hierarchy have
many limitations. When the concrete subclass is not known, the related object
could be in any of the subclass tables, making joins through the relation
impossible. This ambiguity also affects identity lookups and queries; these
operations require multiple SQL <code class="literal">SELECT</code>s (one for each
possible subclass), or a complex <code class="literal">UNION</code>.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
<a href="ref_guide_mapping_limits.html#ref_guide_mapping_limits_tpc" title="8.1.&nbsp; Table Per Class">Section&nbsp;8.1, &#8220;
Table Per Class
&#8221;</a> in the Reference Guide
describes the limitations OpenJPA places on table-per-class mapping.
</p></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="jpa_overview_mapping_inher_together"></a>6.4.&nbsp;
Putting it All Together
</h3></div></div></div><p>
Now that we have covered JPA's inheritance strategies, we can update our mapping
document with inheritance information. Here is the complete model:
</p><div class="mediaobject"><table border="0" summary="manufactured viewport for HTML img" cellspacing="0" cellpadding="0" width="341"><tr><td><img src="img/jpa-inher-all.png"></td></tr></table></div><p>
And here is the corresponding mapping metadata:
</p><div class="example"><a name="jpa_overview_mapping_inher_togetherex"></a><p class="title"><b>Example&nbsp;12.8.&nbsp;
Inheritance Mapping
</b></p><div class="example-contents"><pre class="programlisting">
package org.mag;
@Entity
@IdClass(Magazine.MagazineId.class)
@Table(name="MAG")
public class Magazine {
@Column(length=9)
@Id private String isbn;
@Id private String title;
...
public static class MagazineId {
...
}
}
@Entity
@Table(name="ART", uniqueConstraints=@Unique(columnNames="TITLE"))
@SequenceGenerator(name="ArticleSeq", sequenceName="ART_SEQ")
public class Article {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ArticleSeq")
private long id;
...
}
package org.mag.pub;
@Entity
@Table(name="COMP")
public class Company {
@Column(name="CID")
@Id private long id;
...
}
@Entity
@Table(name="AUTH")
public class Author {
@Id
@GeneratedValue(strategy=GenerationType.TABLE, generator="AuthorGen")
@TableGenerator(name="AuthorGen", table="AUTH_GEN", pkColumnName="PK",
valueColumnName="AID")
@Column(name="AID", columnDefinition="INTEGER64")
private long id;
...
}
@Embeddable
public class Address {
...
}
package org.mag.subscribe;
@MappedSuperclass
public abstract class Document {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private long id;
...
}
@Entity
@Table(schema="CNTRCT")
@Inheritance(strategy=InheritanceType.JOINED)
public class Contract
extends Document {
...
}
@Entity
@Table(name="SUB", schema="CNTRCT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class Subscription {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private long id;
...
@Entity
@Table(name="LINE_ITEM", schema="CNTRCT")
@PrimaryKeyJoinColumn(name="ID", referencedColumnName="ID")
public static class LineItem
extends Contract {
...
}
}
@Entity(name="Lifetime")
public class LifetimeSubscription
extends Subscription {
...
}
@Entity(name="Trial")
public class TrialSubscription
extends Subscription {
...
}
</pre><p>
The same metadata expressed in XML form:
</p><pre class="programlisting">
&lt;entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd"
version="1.0"&gt;
&lt;mapped-superclass class="org.mag.subscribe.Document"&gt;
&lt;attributes&gt;
&lt;id name="id"&gt;
&lt;generated-value strategy="IDENTITY"/&gt;
&lt;/id&gt;
...
&lt;/attributes&gt;
&lt;/mapped-superclass&gt;
&lt;entity class="org.mag.Magazine"&gt;
&lt;table name="MAG"/&gt;
&lt;id-class="org.mag.Magazine.MagazineId"/&gt;
&lt;attributes&gt;
&lt;id name="isbn"&gt;
&lt;column length="9"/&gt;
&lt;/id&gt;
&lt;id name="title"/&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.Article"&gt;
&lt;table name="ART"&gt;
&lt;unique-constraint&gt;
&lt;column-name&gt;TITLE&lt;/column-name&gt;
&lt;/unique-constraint&gt;
&lt;/table&gt;
&lt;sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/&gt;
&lt;attributes&gt;
&lt;id name="id"&gt;
&lt;generated-value strategy="SEQUENCE" generator="ArticleSeq"/&gt;
&lt;/id&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Company"&gt;
&lt;table name="COMP"/&gt;
&lt;attributes&gt;
&lt;id name="id"&gt;
&lt;column name="CID"/&gt;
&lt;/id&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.pub.Author"&gt;
&lt;table name="AUTH"/&gt;
&lt;attributes&gt;
&lt;id name="id"&gt;
&lt;column name="AID" column-definition="INTEGER64"/&gt;
&lt;generated-value strategy="TABLE" generator="AuthorGen"/&gt;
&lt;table-generator name="AuthorGen" table="AUTH_GEN"
pk-column-name="PK" value-column-name="AID"/&gt;
&lt;/id&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.subcribe.Contract"&gt;
&lt;table schema="CNTRCT"/&gt;
&lt;inheritance strategy="JOINED"/&gt;
&lt;attributes&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.subcribe.Subscription"&gt;
&lt;table name="SUB" schema="CNTRCT"/&gt;
&lt;inheritance strategy="SINGLE_TABLE"/&gt;
&lt;attributes&gt;
&lt;id name="id"&gt;
&lt;generated-value strategy="IDENTITY"/&gt;
&lt;/id&gt;
...
&lt;/attributes&gt;
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.Subscription.LineItem"&gt;
&lt;table name="LINE_ITEM" schema="CNTRCT"/&gt;
&lt;primary-key-join-column name="ID" referenced-column-name="PK"/&gt;
...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"&gt;
...
&lt;/entity&gt;
&lt;entity class="org.mag.subscribe.TrialSubscription" name="Trial"&gt;
...
&lt;/entity&gt;
&lt;/entity-mappings&gt;
</pre></div></div><br class="example-break"></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="jpa_overview_mapping_sequence.html">Prev</a>&nbsp;</td><td width="20%" align="center"><a accesskey="u" href="jpa_overview_mapping.html">Up</a></td><td width="40%" align="right">&nbsp;<a accesskey="n" href="jpa_overview_mapping_discrim.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.&nbsp;
Generators
&nbsp;</td><td width="20%" align="center"><a accesskey="h" href="manual.html">Home</a></td><td width="40%" align="right" valign="top">&nbsp;7.&nbsp;
Discriminator
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