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+<html><head>
+ <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
+ <title>6. Inheritance</title><base href="display"><link
rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator"
content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html"
title="Apache OpenJPA 2.4 User's Guide"><link rel="up"
href="jpa_overview_mapping.html" title="Chapter 13. Mapping
Metadata"><link rel="prev" href="jpa_overview_mapping_sequence.html"
title="5. Generators"><link rel="next"
href="jpa_overview_mapping_discrim.html" title="7.
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.
+ Inheritance
+ </th></tr><tr><td width="20%" align="left"><a accesskey="p"
href="jpa_overview_mapping_sequence.html">Prev</a> </td><th width="60%"
align="center">Chapter 13.
+ Mapping Metadata
+ </th><td width="20%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_discrim.html">Next</a></td></tr></table><hr></div><div
class="section" id="jpa_overview_mapping_inher"><div
class="titlepage"><div><div><h2 class="title" style="clear: both">6.
+ Inheritance
+ </h2></div></div></div><div class="toc"><dl class="toc"><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="d5e5862"></a>
+ <a class="indexterm" name="d5e5866"></a>
+ <a class="indexterm" name="d5e5869"></a>
+ <a class="indexterm" name="d5e5873"></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="d5e5878"></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 class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <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 class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <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 class="xref" href="ref_guide_mapping_jpa.html" title="7. Additional
JPA Mappings">Section 7, “
+ Additional JPA Mappings
+ ”</a> in the Reference Guide for
+details.
+ </p>
+ </div>
+ <div class="section" id="jpa_overview_mapping_inher_single"><div
class="titlepage"><div><div><h3 class="title">6.1.
+ Single Table
+ </h3></div></div></div><div class="toc"><dl class="toc"><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="d5e5904"></a>
+ <a class="indexterm" name="d5e5908"></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" style="cellpadding: 0; cellspacing: 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" id="jpa_overview_mapping_inher_singleex"><p
class="title"><b>Example 13.5.
+ 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">
+<entity class="org.mag.subcribe.Subscription">
+ <table name="SUB" schema="CNTRCT"/>
+ <inheritance strategy="SINGLE_TABLE"/>
+ ...
+</entity>
+<entity class="org.mag.subscribe.LifetimeSubscription">
+ ...
+</entity>
+</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="d5e5930"></a>
+ <a class="indexterm" name="d5e5933"></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"
id="jpa_overview_mapping_inher_single_adv"><div class="titlepage"><div><div><h4
class="title">6.1.1.
+ Advantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e5939"></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"
id="jpa_overview_mapping_inher_single_disadv"><div
class="titlepage"><div><div><h4 class="title">6.1.2.
+ Disadvantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e5948"></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" id="jpa_overview_mapping_inher_joined"><div
class="titlepage"><div><div><h3 class="title">6.2.
+ Joined
+ </h3></div></div></div><div class="toc"><dl class="toc"><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="d5e5955"></a>
+ <a class="indexterm" name="d5e5959"></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="d5e5966"></a>
+ <a class="indexterm" name="d5e5969"></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" style="cellpadding: 0; cellspacing: 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 class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <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 class="listitem">
+ <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 class="listitem">
+ <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 class="xref" href="jpa_overview_mapping_column.html" title="3.
Column">Section 3, “
+ Column
+ ”</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 class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">referenced-column-name</code>
+ </p>
+ </li><li class="listitem">
+ <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" id="jpa_overview_mapping_inher_joinedex"><p
class="title"><b>Example 13.6.
+ 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">
+<entity class="org.mag.subcribe.Contract">
+ <table schema="CNTRCT"/>
+ <inheritance strategy="JOINED"/>
+ ...
+</entity>
+<entity class="org.mag.subscribe.Subscription.LineItem">
+ <table name="LINE_ITEM" schema="CNTRCT"/>
+ <primary-key-join-column name="ID" referenced-column-name="PK"/>
+ ...
+</entity>
+</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"
id="jpa_overview_mapping_inher_joined_adv"><div class="titlepage"><div><div><h4
class="title">6.2.1.
+ Advantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e6023"></a>
+ <p>
+The joined strategy has the following advantages:
+ </p>
+ <div class="orderedlist"><ol class="orderedlist" type="1"><li
class="listitem">
+ <p>
+ <a class="indexterm" name="d5e6031"></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 class="listitem">
+ <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 class="listitem">
+ <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"
id="jpa_overview_mapping_inher_joined_disadv"><div
class="titlepage"><div><div><h4 class="title">6.2.2.
+ Disadvantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e6040"></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 class="xref" href="ref_guide_perfpack_eager.html" title="8. Eager
Fetching">Section 8, “
+ Eager Fetching
+ ”</a> in the Reference Guide
+describes OpenJPA's options for efficient data loading.
+ </p>
+ </div>
+ </div>
+ </div>
+ <div class="section" id="jpa_overview_mapping_inher_tpc"><div
class="titlepage"><div><div><h3 class="title">6.3.
+ Table Per Class
+ </h3></div></div></div><div class="toc"><dl class="toc"><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="d5e6052"></a>
+ <a class="indexterm" name="d5e6056"></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" style="cellpadding: 0; cellspacing: 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 class="xref"
href="jpa_overview_mapping_field.html#jpa_overview_mapping_embed"
title="8.3. Embedded Mapping">Section 8.3, “
+ Embedded Mapping
+ ”</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" id="jpa_overview_mapping_inher_tpcex"><p
class="title"><b>Example 13.7.
+ 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">
+<entity class="org.mag.Magazine">
+ <table name="MAG"/>
+ <inheritance strategy="TABLE_PER_CLASS"/>
+ ...
+</entity>
+<entity class="org.mag.Tabloid">
+ <table name="TABLOID"/>
+ ...
+</entity>
+</pre>
+ </div></div><br class="example-break">
+ <div class="section" id="jpa_overview_mapping_inher_tpc_adv"><div
class="titlepage"><div><div><h4 class="title">6.3.1.
+ Advantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e6091"></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"
id="jpa_overview_mapping_inher_tpc_disadv"><div class="titlepage"><div><div><h4
class="title">6.3.2.
+ Disadvantages
+ </h4></div></div></div>
+
+ <a class="indexterm" name="d5e6098"></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 class="xref"
href="ref_guide_mapping_limits.html#ref_guide_mapping_limits_tpc"
title="8.1. Table Per Class">Section 8.1, “
+ Table Per Class
+ ”</a> in the Reference Guide
+describes the limitations OpenJPA places on table-per-class mapping.
+ </p>
+ </div>
+ </div>
+ </div>
+ <div class="section" id="jpa_overview_mapping_inher_together"><div
class="titlepage"><div><div><h3 class="title">6.4.
+ 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" style="cellpadding: 0; cellspacing: 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" id="jpa_overview_mapping_inher_togetherex"><p
class="title"><b>Example 13.8.
+ 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">
+<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">
+ <mapped-superclass class="org.mag.subscribe.Document">
+ <attributes>
+ <id name="id">
+ <generated-value strategy="IDENTITY"/>
+ </id>
+ ...
+ </attributes>
+ </mapped-superclass>
+ <entity class="org.mag.Magazine">
+ <table name="MAG"/>
+ <id-class="org.mag.Magazine.MagazineId"/>
+ <attributes>
+ <id name="isbn">
+ <column length="9"/>
+ </id>
+ <id name="title"/>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.Article">
+ <table name="ART">
+ <unique-constraint>
+ <column-name>TITLE</column-name>
+ </unique-constraint>
+ </table>
+ <sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/>
+ <attributes>
+ <id name="id">
+ <generated-value strategy="SEQUENCE"
generator="ArticleSeq"/>
+ </id>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.pub.Company">
+ <table name="COMP"/>
+ <attributes>
+ <id name="id">
+ <column name="CID"/>
+ </id>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.pub.Author">
+ <table name="AUTH"/>
+ <attributes>
+ <id name="id">
+ <column name="AID" column-definition="INTEGER64"/>
+ <generated-value strategy="TABLE" generator="AuthorGen"/>
+ <table-generator name="AuthorGen" table="AUTH_GEN"
+ pk-column-name="PK" value-column-name="AID"/>
+ </id>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.subcribe.Contract">
+ <table schema="CNTRCT"/>
+ <inheritance strategy="JOINED"/>
+ <attributes>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.subcribe.Subscription">
+ <table name="SUB" schema="CNTRCT"/>
+ <inheritance strategy="SINGLE_TABLE"/>
+ <attributes>
+ <id name="id">
+ <generated-value strategy="IDENTITY"/>
+ </id>
+ ...
+ </attributes>
+ </entity>
+ <entity class="org.mag.subscribe.Subscription.LineItem">
+ <table name="LINE_ITEM" schema="CNTRCT"/>
+ <primary-key-join-column name="ID" referenced-column-name="PK"/>
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.LifetimeSubscription"
name="Lifetime">
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.TrialSubscription" name="Trial">
+ ...
+ </entity>
+</entity-mappings>
+</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> </td><td width="20%"
align="center"><a accesskey="u" href="jpa_overview_mapping.html">Up</a></td><td
width="40%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_discrim.html">Next</a></td></tr><tr><td width="40%"
align="left" valign="top">5.
+ Generators
+ </td><td width="20%" align="center"><a accesskey="h"
href="manual.html">Home</a></td><td width="40%" align="right"
valign="top"> 7.
+ Discriminator
+ </td></tr></table></div></body></html>
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+<html><head>
+ <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
+ <title>5. Generators</title><base href="display"><link
rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator"
content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html"
title="Apache OpenJPA 2.4 User's Guide"><link rel="up"
href="jpa_overview_mapping.html" title="Chapter 13. Mapping
Metadata"><link rel="prev" href="jpa_overview_mapping_id.html" title="4.
Identity Mapping"><link rel="next" href="jpa_overview_mapping_inher.html"
title="6. Inheritance"></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">5.
+ Generators
+ </th></tr><tr><td width="20%" align="left"><a accesskey="p"
href="jpa_overview_mapping_id.html">Prev</a> </td><th width="60%"
align="center">Chapter 13.
+ Mapping Metadata
+ </th><td width="20%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_inher.html">Next</a></td></tr></table><hr></div><div
class="section" id="jpa_overview_mapping_sequence"><div
class="titlepage"><div><div><h2 class="title" style="clear: both">5.
+ Generators
+ </h2></div></div></div><div class="toc"><dl class="toc"><dt><span
class="section"><a
href="jpa_overview_mapping_sequence.html#jpa_overview_mapping_sequence_seqgen">5.1.
+ Sequence Generator
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_mapping_sequence.html#jpa_overview_mapping_sequence_tablegen">5.2.
+ Table Generator
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_mapping_sequence.html#jpa_overview_mapping_sequence_genex">5.3.
+ Example
+ </a></span></dt></dl></div>
+
+ <a class="indexterm" name="d5e5656"></a>
+ <a class="indexterm" name="d5e5659"></a>
+ <p>
+One aspect of identity mapping not covered in the previous section is JPA's
+ability to automatically assign a value to your numeric identity fields using
+<span class="emphasis"><em>generators</em></span>. We discussed the available
generator types in
+<a class="xref" href="jpa_overview_meta_field.html#jpa_overview_meta_id"
title="2.3. Id">Section 2.3, “
+ Id
+ ”</a>. Now we show you how to define
+named generators.
+ </p>
+ <div class="section" id="jpa_overview_mapping_sequence_seqgen"><div
class="titlepage"><div><div><h3 class="title">5.1.
+ Sequence Generator
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e5669"></a>
+ <a class="indexterm" name="d5e5672"></a>
+ <p>
+Most databases allow you to create native sequences. These are database
+structures that generate increasing numeric values. The <code
class="classname">
+SequenceGenerator</code> annotation represents a named database sequence.
+You can place the annotation on any package, entity class, persistent field
+declaration (if your entity uses field access), or getter method for a
+persistent property (if your entity uses property access). <code
class="classname">
+SequenceGenerator</code> has the following properties:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5680"></a>
+<code class="literal">String name</code>: The generator name. This property is
required.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5686"></a>
+<code class="literal">String sequenceName</code>: The name of the database
sequence. If
+you do not specify the database sequence, your vendor will choose an
appropriate
+default.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5692"></a>
+<code class="literal">int initialValue</code>: The initial sequence value.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5698"></a>
+<code class="literal">int allocationSize</code>: The number of values to
allocate in
+memory for each trip to the database. Allocating values in memory allows the
JPA
+runtime to avoid accessing the database for every sequence request.
+This number also specifies the amount that the sequence value is incremented
+each time the sequence is accessed. Defaults to 50.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5704"></a>
+ <code class="literal">String schema</code>: The sequence's
schema. If you do not name a
+schema, JPA uses the default schema for the database connection.
+ </p>
+ </li></ul></div>
+ <div class="note" style="margin-left: 0.5in; margin-right:
0.5in;"><h3 class="title">Note</h3>
+ <p>
+OpenJPA allows you to use one of OpenJPA's built-in generator
+implementations in the <code class="literal">sequenceName</code> property. You
can also
+set the <code class="literal">sequenceName</code> to <code
class="literal">system</code> to use the
+system sequence defined by the <a class="link"
href="ref_guide_conf_openjpa.html#openjpa.Sequence" title="5.67.
openjpa.Sequence"><code class="literal">
+openjpa.Sequence</code></a> configuration property. See the Reference
+Guide's <a class="xref" href="ref_guide_sequence.html" title="6.
Generators">Section 6, “
+ Generators
+ ”</a> for details.
+ </p>
+ </div>
+ <p>
+The XML element for a sequence generator is <code
class="literal">sequence-generator
+</code>. Its attributes mirror the above annotation's properties:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">sequence-name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">initial-value</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">allocation-size</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">schema</code>
+ </p>
+ </li></ul></div>
+ <p>
+To use a sequence generator, set your <code
class="classname">GeneratedValue</code>
+annotation's <code class="literal">strategy</code> property to <code
class="literal">
+GenerationType.SEQUENCE</code>, and its <code class="literal">generator</code>
property
+to the sequence generator's declared name. Or equivalently, set your <code
class="literal">
+generated-value</code> XML element's <code class="literal">strategy</code>
attribute to
+<code class="literal">SEQUENCE</code> and its <code
class="literal">generator</code> attribute to
+the generator name.
+ </p>
+ </div>
+ <div class="section" id="jpa_overview_mapping_sequence_tablegen"><div
class="titlepage"><div><div><h3 class="title">5.2.
+ Table Generator
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e5745"></a>
+ <a class="indexterm" name="d5e5748"></a>
+ <p>
+A <code class="classname">TableGenerator</code> refers to a database table
used to store
+increasing sequence values for one or more entities. As with <code
class="classname">
+SequenceGenerator</code>, you can place the <code
class="classname">TableGenerator
+</code> annotation on any package, entity class, persistent field
+declaration (if your entity uses field access), or getter method for a
+persistent property (if your entity uses property access). <code
class="classname">
+TableGenerator</code> has the following properties:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5758"></a>
+<code class="literal">String name</code>: The generator name. This property is
required.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5764"></a>
+<code class="literal">String table</code>: The name of the generator table. If
left
+unspecified, your vendor will choose a default table.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5770"></a>
+<code class="literal">String schema</code>: The named table's schema.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5776"></a>
+<code class="literal">String catalog</code>: The named table's catalog.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5782"></a>
+<code class="literal">String pkColumnName</code>: The name of the primary key
column in
+the generator table. If unspecified, your implementation will choose a default.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5788"></a>
+<code class="literal">String valueColumnName</code>: The name of the column
that holds
+the sequence value. If unspecified, your implementation will choose a default.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5794"></a>
+<code class="literal">String pkColumnValue</code>: The primary key column
value of the
+row in the generator table holding this sequence value. You can use the same
+generator table for multiple logical sequences by supplying different <code
class="literal">
+pkColumnValue</code> s. If you do not specify a value, the implementation
+will supply a default.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5801"></a>
+<code class="literal">int initialValue</code>: The value of the generator's
first issued
+number.
+ </p>
+ </li><li class="listitem">
+ <p>
+ <a class="indexterm" name="d5e5807"></a>
+<code class="literal">int allocationSize</code>: The number of values to
allocate in
+memory for each trip to the database. Allocating values in memory allows the
JPA
+runtime to avoid accessing the database for every sequence request.
+This number also specifies the amount that the sequence value is incremented
+each time the generator table is updated. Defaults to 50.
+ </p>
+ </li></ul></div>
+ <p>
+The XML equivalent is the <code class="literal">table-generator</code>
element. This
+element's attributes correspond exactly to the above annotation's properties:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">table</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">schema</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">catalog</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">pk-column-name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">value-column-name</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">pk-column-value</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">initial-value</code>
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">allocation-size</code>
+ </p>
+ </li></ul></div>
+ <p>
+To use a table generator, set your <code
class="classname">GeneratedValue</code>
+annotation's <code class="literal">strategy</code> property to <code
class="literal">
+GenerationType.TABLE</code>, and its <code class="literal">generator</code>
property to
+the table generator's declared name. Or equivalently, set your <code
class="literal">
+generated-value</code> XML element's <code class="literal">strategy</code>
attribute to
+<code class="literal">TABLE</code> and its <code
class="literal">generator</code> attribute to the
+generator name.
+ </p>
+ </div>
+ <div class="section" id="jpa_overview_mapping_sequence_genex"><div
class="titlepage"><div><div><h3 class="title">5.3.
+ Example
+ </h3></div></div></div>
+
+ <p>
+Let's take advantage of generators in our entity model. Here are our updated
+mappings.
+ </p>
+ <div class="example" id="jpa_overview_mapping_sequenceex"><p
class="title"><b>Example 13.4.
+ Generator 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(generate=GenerationType.IDENTITY)
+ private long id;
+
+ ...
+}
+
+@Entity
+@Table(schema="CNTRCT")
+public class Contract
+ extends Document {
+
+ ...
+}
+
+@Entity
+@Table(name="SUB", schema="CNTRCT")
+public class Subscription {
+
+ @Id
+ @GeneratedValue(strategy=GenerationType.IDENTITY)
+ private long id;
+
+ ...
+
+ @Entity
+ @Table(name="LINE_ITEM", schema="CNTRCT")
+ 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 for <code class="literal">Article</code> and <code
class="literal">Author</code>
+expressed in XML form:
+ </p>
+<pre class="programlisting">
+<entity class="org.mag.Article">
+ <table name="ART">
+ <unique-constraint>
+ <column-name>TITLE</column-name>
+ </unique-constraint>
+ </table>
+ <sequence-generator name="ArticleSeq" sequence-name="ART_SEQ"/>
+ <attributes>
+ <id name="id">
+ <generated-value strategy="SEQUENCE" generator="ArticleSeq"/>
+ </id>
+ ...
+ </attributes>
+</entity>
+<entity class="org.mag.pub.Author">
+ <table name="AUTH"/>
+ <attributes>
+ <id name="id">
+ <column name="AID" column-definition="INTEGER64"/>
+ <generated-value strategy="TABLE" generator="AuthorGen"/>
+ <table-generator name="AuthorGen" table="AUTH_GEN"
+ pk-column-name="PK" value-column-name="AID"/>
+ </id>
+ ...
+ </attributes>
+</entity>
+</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_id.html">Prev</a> </td><td width="20%"
align="center"><a accesskey="u" href="jpa_overview_mapping.html">Up</a></td><td
width="40%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_inher.html">Next</a></td></tr><tr><td width="40%"
align="left" valign="top">4.
+ Identity Mapping
+ </td><td width="20%" align="center"><a accesskey="h"
href="manual.html">Home</a></td><td width="40%" align="right"
valign="top"> 6.
+ Inheritance
+ </td></tr></table></div></body></html>
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+<html><head>
+ <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
+ <title>2. Unique Constraints</title><base href="display"><link
rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator"
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title="Apache OpenJPA 2.4 User's Guide"><link rel="up"
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Metadata"><link rel="prev" href="jpa_overview_mapping.html"
title="Chapter 13. Mapping Metadata"><link rel="next"
href="jpa_overview_mapping_column.html" title="3. Column"></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">2.
+ Unique Constraints
+ </th></tr><tr><td width="20%" align="left"><a accesskey="p"
href="jpa_overview_mapping.html">Prev</a> </td><th width="60%"
align="center">Chapter 13.
+ Mapping Metadata
+ </th><td width="20%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_column.html">Next</a></td></tr></table><hr></div><div
class="section" id="jpa_overview_mapping_unq"><div
class="titlepage"><div><div><h2 class="title" style="clear: both">2.
+ Unique Constraints
+ </h2></div></div></div>
+
+ <a class="indexterm" name="d5e5471"></a>
+ <a class="indexterm" name="d5e5475"></a>
+ <p>
+Unique constraints ensure that the data in a column or combination of columns
is
+unique for each row. A table's primary key, for example, functions as an
+implicit unique constraint. In JPA, you represent other unique
+constraints with an array of <code class="classname"> UniqueConstraint</code>
+annotations within the table annotation. The unique constraints you define are
+used during table creation to generate the proper database constraints, and may
+also be used at runtime to order <code class="literal">INSERT</code>, <code
class="literal">UPDATE
+</code>, and <code class="literal">DELETE</code> statements. For example,
suppose there
+is a unique constraint on the columns of field <code class="literal">F</code>.
In the
+same transaction, you remove an object <code class="literal">A</code> and
persist a new
+object <code class="literal">B</code>, both with the same <code
class="literal">F</code> value. The
+JPA runtime must ensure that the SQL deleting <code class="literal">A</code>
+is sent to the database before the SQL inserting <code
class="literal">B</code> to avoid a
+unique constraint violation.
+ </p>
+ <p>
+<code class="classname">UniqueConstraint</code> has these properties:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">String name</code>: The name of the constraint. OpenJPA
will choose
+a name if you do not provide one, or will create an anonymous constraint.
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">String[] columnNames</code>: The names of the columns the
+constraint spans.
+ </p>
+ </li></ul></div>
+ <p>
+In XML, unique constraints are represented by nesting <code class="literal">
+unique-constraint</code> elements within the <code class="literal">
table</code>
+element. Each <code class="literal">unique-constraint</code> element in turn
nests
+<code class="literal">column-name</code> text elements to enumerate the
constraint's
+columns.
+ </p>
+ <div class="example" id="jpa_overview_mapping_unq_attrex"><p
class="title"><b>Example 13.2.
+ Defining a Unique Constraint
+ </b></p><div class="example-contents">
+
+ <p>
+The following defines a unique constraint on the <code class="literal">
TITLE</code>
+column of the <code class="literal">ART</code> table:
+ </p>
+<pre class="programlisting">
+@Entity
+@Table(name="ART", uniqueConstraints=@UniqueConstraint(name="TITLE_CNSTR",
columnNames="TITLE"))
+public class Article {
+ ...
+}
+</pre>
+ <p>
+The same metadata expressed in XML form:
+ </p>
+<pre class="programlisting">
+<entity class="org.mag.Article">
+ <table name="ART">
+ <unique-constraint>
+ <name>TITLE_CNSTR</name>
+ <column-name>TITLE</column-name>
+ </unique-constraint>
+ </table>
+ ...
+</entity>
+</pre>
+ </div></div><br class="example-break">
+ </div><div class="navfooter"><hr><table width="100%" summary="Navigation
footer"><tr><td width="40%" align="left"><a accesskey="p"
href="jpa_overview_mapping.html">Prev</a> </td><td width="20%"
align="center"><a accesskey="u" href="jpa_overview_mapping.html">Up</a></td><td
width="40%" align="right"> <a accesskey="n"
href="jpa_overview_mapping_column.html">Next</a></td></tr><tr><td width="40%"
align="left" valign="top">Chapter 13.
+ Mapping Metadata
+ </td><td width="20%" align="center"><a accesskey="h"
href="manual.html">Home</a></td><td width="40%" align="right"
valign="top"> 3.
+ Column
+ </td></tr></table></div></body></html>
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+<html><head>
+ <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
+ <title>Chapter 5. Metadata</title><base href="display"><link
rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator"
content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html"
title="Apache OpenJPA 2.4 User's Guide"><link rel="up" href="jpa_overview.html"
title="Part 2. Java Persistence API"><link rel="prev"
href="jpa_overview_pc_conclusion.html" title="4. Conclusions"><link
rel="next" href="jpa_overview_meta_field.html" title="2. Field and
Property Metadata"></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">Chapter 5.
+ Metadata
+ </th></tr><tr><td width="20%" align="left"><a accesskey="p"
href="jpa_overview_pc_conclusion.html">Prev</a> </td><th width="60%"
align="center">Part 2. Java Persistence API</th><td width="20%"
align="right"> <a accesskey="n"
href="jpa_overview_meta_field.html">Next</a></td></tr></table><hr></div><div
class="chapter" id="jpa_overview_meta"><div class="titlepage"><div><div><h2
class="title">Chapter 5.
+ Metadata
+ </h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl
class="toc"><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_class">1.
+ Class Metadata
+ </a></span></dt><dd><dl><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_entity">1.1.
+ Entity
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_idclass">1.2.
+ Id Class
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_embeddablesuper">1.3.
+ Mapped Superclass
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_embeddable">1.4.
+ Embeddable
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_entity_listeners">1.5.
+ EntityListeners
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_classex">1.6.
+ Example
+ </a></span></dt></dl></dd><dt><span class="section"><a
href="jpa_overview_meta_field.html">2.
+ Field and Property Metadata
+ </a></span></dt><dd><dl><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_explicit_access">2.1.
+ Explicit Access
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_transient">2.2.
+ Transient
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_id">2.3.
+ Id
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_gen">2.4.
+ Generated Value
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_embedid">2.5.
+ Embedded Id
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_version">2.6.
+ Version
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_basic">2.7.
+ Basic
+ </a></span></dt><dd><dl><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_fetch">2.7.1.
+ Fetch Type
+ </a></span></dt></dl></dd><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_embedded">2.8.
+ Embedded
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_manytoone">2.9.
+ Many To One
+ </a></span></dt><dd><dl><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_cascade">2.9.1.
+ Cascade Type
+ </a></span></dt></dl></dd><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_onetomany">2.10.
+ One To Many
+ </a></span></dt><dd><dl><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_mappedby">2.10.1.
+ Bidirectional Relations
+ </a></span></dt></dl></dd><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_onetoone">2.11.
+ One To One
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_manytomany">2.12.
+ Many To Many
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_orderby">2.13.
+ Order By
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_mapkey">2.14.
+ Map Key
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_field.html#jpa_overview_meta_fielddefaults">2.15.
+ Persistent Field Defaults
+ </a></span></dt></dl></dd><dt><span class="section"><a
href="jpa_overview_meta_xml.html">3.
+ XML Schema
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta_complete.html">4.
+ Conclusion
+ </a></span></dt></dl></div>
+
+ <a class="indexterm" name="d5e996"></a>
+ <a class="indexterm" name="d5e998"></a>
+ <p>
+JPA requires that you accompany each persistent class with persistence
metadata.
+This metadata serves three primary purposes:
+ </p>
+ <div class="orderedlist"><ol class="orderedlist" type="1"><li
class="listitem">
+ <p>
+To identify persistent classes.
+ </p>
+ </li><li class="listitem">
+ <p>
+To override default JPA behavior.
+ </p>
+ </li><li class="listitem">
+ <p>
+To provide the JPA implementation with information that it cannot glean from
+simply reflecting on the persistent class.
+ </p>
+ </li></ol></div>
+ <p>
+ <a class="indexterm" name="d5e1011"></a>
+Persistence metadata is specified using either the Java annotations defined in
+the <code class="literal">javax.persistence</code> package, XML mapping files,
or a
+mixture of both. In the latter case, XML declarations override conflicting
+annotations. If you choose to use XML metadata, the XML files must be available
+at development and runtime, and must be discoverable via either of two
+strategies:
+ </p>
+ <div class="orderedlist"><ol class="orderedlist" type="1"><li
class="listitem">
+ <p>
+In a resource named <code class="filename">orm.xml</code> placed in a <code
class="filename">
+META-INF</code> directory within a directory in your classpath or within a
+jar archive containing your persistent classes.
+ </p>
+ </li><li class="listitem">
+ <p>
+Declared in your <a class="link"
href="jpa_overview_persistence.html#jpa_overview_persistence_xml"
title="1. persistence.xml"><code class="filename">
+persistence.xml</code></a> configuration file. In this case, each XML
+metadata file must be listed in a <code class="literal">mapping-file</code>
element whose
+content is either a path to the given file or a resource location available to
+the class' class loader.
+ </p>
+ </li></ol></div>
+ <p>
+We describe the standard metadata annotations and XML equivalents throughout
+this chapter. The full schema for XML mapping files is available in
+<a class="xref" href="jpa_overview_meta_xml.html" title="3. XML
Schema">Section 3, “
+ XML Schema
+ ”</a>. JPA also standardizes relational
+mapping metadata and named query metadata, which we discuss in
+<a class="xref" href="jpa_overview_mapping.html" title="Chapter 13.
Mapping Metadata">Chapter 13, <i>
+ Mapping Metadata
+ </i></a> and
+<a class="xref" href="jpa_overview_query.html#jpa_overview_query_named"
title="1.11. Named Queries">Section 1.11, “
+ Named Queries
+ ”</a> respectively.
+ </p>
+ <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3
class="title">Note</h3>
+ <p>
+OpenJPA defines many useful annotations beyond the standard set. See
+<a class="xref" href="ref_guide_meta_jpa.html" title="3. Additional JPA
Metadata">Section 3, “
+ Additional JPA Metadata
+ ”</a> and
+<a class="xref" href="ref_guide_meta_ext.html" title="4. Metadata
Extensions">Section 4, “
+ Metadata Extensions
+ ”</a>
+in the Reference Guide for details. There are currently no XML equivalents for
+these extension annotations.
+ </p>
+ </div>
+ <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3
class="title">Note</h3>
+ <p>
+Persistence metadata may be used to validate the contents of your entities
prior to communicating
+with the database. This differs from mapping meta data which is primarily used
for schema generation.
+For example if you indicate that a relationship is not optional (e.g.
@Basic(optional=false)) OpenJPA
+will validate that the variable in your entity is not null <span
class="emphasis"><em>before</em></span> inserting a row
+in the database.
+ </p>
+ </div>
+ <div class="mediaobject"><table border="0" summary="manufactured viewport
for HTML img" style="cellpadding: 0; cellspacing: 0;" width="369"><tr><td><img
src="img/jpa-meta-model.png"></td></tr></table></div>
+ <p>
+Through the course of this chapter, we will create the persistent object model
+above.
+ </p>
+ <div class="section" id="jpa_overview_meta_class"><div
class="titlepage"><div><div><h2 class="title" style="clear: both">1.
+ Class Metadata
+ </h2></div></div></div><div class="toc"><dl class="toc"><dt><span
class="section"><a href="jpa_overview_meta.html#jpa_overview_meta_entity">1.1.
+ Entity
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_idclass">1.2.
+ Id Class
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_embeddablesuper">1.3.
+ Mapped Superclass
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_embeddable">1.4.
+ Embeddable
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_entity_listeners">1.5.
+ EntityListeners
+ </a></span></dt><dt><span class="section"><a
href="jpa_overview_meta.html#jpa_overview_meta_classex">1.6.
+ Example
+ </a></span></dt></dl></div>
+
+ <p>
+The following metadata annotations and XML elements apply to persistent class
+declarations.
+ </p>
+ <div class="section" id="jpa_overview_meta_entity"><div
class="titlepage"><div><div><h3 class="title">1.1.
+ Entity
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e1044"></a>
+ <a class="indexterm" name="d5e1047"></a>
+ <a class="indexterm" name="d5e1050"></a>
+ <p>
+The <code class="classname">Entity</code> annotation denotes an entity class.
All entity
+classes must have this annotation. The <code class="classname">Entity</code>
annotation
+takes one optional property:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">String name</code>: Name used to refer to the entity in
queries.
+Must not be a reserved literal in JPQL. Defaults to the unqualified name of the
+entity class.
+ </p>
+ </li></ul></div>
+ <p>
+The equivalent XML element is <code class="literal">entity</code>. It has the
following
+attributes:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">class</code>: The entity class. This attribute is
required.
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">name</code>: Named used to refer to the class in
queries. See the
+name property above.
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">access</code>: The access type to use for the class.
Must either be
+<code class="literal">FIELD</code> or <code class="literal">PROPERTY</code>.
For details on access
+types, see <a class="xref" href="jpa_overview_meta_field.html" title="2.
Field and Property Metadata">Section 2, “
+ Field and Property Metadata
+ ”</a>.
+ </p>
+ </li></ul></div>
+ <div class="note" style="margin-left: 0.5in; margin-right:
0.5in;"><h3 class="title">Note</h3>
+ <p>
+OpenJPA uses a process called <span
class="emphasis"><em>enhancement</em></span> to modify the
+bytecode of entities for transparent lazy loading and immediate dirty tracking.
+See <a class="xref" href="ref_guide_pc_enhance.html" title="2.
Enhancement">Section 2, “
+ Enhancement
+ ”</a> in the Reference Guide for
+details on enhancement.
+ </p>
+ </div>
+ </div>
+ <div class="section" id="jpa_overview_meta_idclass"><div
class="titlepage"><div><div><h3 class="title">1.2.
+ Id Class
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e1081"></a>
+ <a class="indexterm" name="d5e1083"></a>
+ <a class="indexterm" name="d5e1086"></a>
+ <p>
+As we discussed in <a class="xref"
href="jpa_overview_pc_identity.html#jpa_overview_pc_identitycls"
title="2.1. Identity Class">Section 2.1, “
+ Identity Class
+ ”</a>,
+entities with multiple identity fields must use an <span class="emphasis"><em>
identity class
+</em></span> to encapsulate their persistent identity. The <code
class="classname">IdClass
+</code> annotation specifies this class. It accepts a single <code
class="classname">
+java.lang.Class</code> value.
+ </p>
+ <p>
+The equivalent XML element is <code class="literal">id-class</code>, which has
a single
+attribute:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">class</code>: Set this required attribute to the name of
the
+identity class.
+ </p>
+ </li></ul></div>
+ </div>
+ <div class="section" id="jpa_overview_meta_embeddablesuper"><div
class="titlepage"><div><div><h3 class="title">1.3.
+ Mapped Superclass
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e1102"></a>
+ <a class="indexterm" name="d5e1104"></a>
+ <a class="indexterm" name="d5e1107"></a>
+ <p>
+A <span class="emphasis"><em>mapped superclass</em></span> is a non-entity
class that can define
+persistent state and mapping information for entity subclasses. Mapped
+superclasses are usually abstract. Unlike true entities, you cannot query a
+mapped superclass, pass a mapped superclass instance to any <code
class="classname">
+EntityManager</code> or <code class="classname">Query</code> methods, or
declare a
+persistent relation with a mapped superclass target. You denote a mapped
+superclass with the <code class="classname">MappedSuperclass</code> marker
annotation.
+ </p>
+ <p>
+The equivalent XML element is <code class="literal">mapped-superclass</code>.
It expects
+the following attributes:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">class</code>: The entity class. This attribute is
required.
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">access</code>: The access type to use for the class.
Must either be
+<code class="literal">FIELD</code> or <code class="literal">PROPERTY</code>.
For details on access
+types, see <a class="xref" href="jpa_overview_meta_field.html" title="2.
Field and Property Metadata">Section 2, “
+ Field and Property Metadata
+ ”</a>.
+ </p>
+ </li></ul></div>
+ <div class="note" style="margin-left: 0.5in; margin-right:
0.5in;"><h3 class="title">Note</h3>
+ <p>
+OpenJPA allows you to query on mapped superclasses. A query on a mapped
+superclass will return all matching subclass instances. OpenJPA also allows you
+to declare relations to mapped superclass types; however, you cannot query
+across these relations.
+ </p>
+ </div>
+ </div>
+ <div class="section" id="jpa_overview_meta_embeddable"><div
class="titlepage"><div><div><h3 class="title">1.4.
+ Embeddable
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e1131"></a>
+ <a class="indexterm" name="d5e1133"></a>
+ <a class="indexterm" name="d5e1136"></a>
+ <p>
+The <code class="classname">Embeddable</code> annotation designates an
embeddable
+persistent class. Embeddable instances are stored as part of the record of
their
+owning instance. All embeddable classes must have this annotation.
+ </p>
+ <p>
+A persistent class can either be an entity or an embeddable class, but not
both.
+ </p>
+ <p>
+The equivalent XML element is <code class="literal">embeddable</code>. It
understands the
+following attributes:
+ </p>
+ <div class="itemizedlist"><ul class="itemizedlist"
style="list-style-type: disc; "><li class="listitem">
+ <p>
+<code class="literal">class</code>: The entity class. This attribute is
required.
+ </p>
+ </li><li class="listitem">
+ <p>
+<code class="literal">access</code>: The access type to use for the class.
Must either be
+<code class="literal">FIELD</code> or <code class="literal">PROPERTY</code>.
For details on access
+types, see <a class="xref" href="jpa_overview_meta_field.html" title="2.
Field and Property Metadata">Section 2, “
+ Field and Property Metadata
+ ”</a>.
+ </p>
+ </li></ul></div>
+ <div class="note" style="margin-left: 0.5in; margin-right:
0.5in;"><h3 class="title">Note</h3>
+ <p>
+OpenJPA allows a persistent class to be both an entity and an embeddable class.
+Instances of the class will act as entities when persisted explicitly or
assigned
+to non-embedded fields of entities. Instances will act as embedded values when
+assigned to embedded fields of entities.
+ </p>
+ <p>
+To signal that a class is both an entity and an embeddable class in OpenJPA,
+simply add both the <code class="literal">@Entity</code> and the <code
class="literal">@Embeddable
+</code> annotations to the class.
+ </p>
+ </div>
+ </div>
+ <div class="section" id="jpa_overview_meta_entity_listeners"><div
class="titlepage"><div><div><h3 class="title">1.5.
+ EntityListeners
+ </h3></div></div></div>
+
+ <a class="indexterm" name="d5e1161"></a>
+ <a class="indexterm" name="d5e1163"></a>
+ <a class="indexterm" name="d5e1165"></a>
+ <a class="indexterm" name="d5e1168"></a>
+ <p>
+An entity may list its lifecycle event listeners in the <code
class="classname">
+EntityListeners</code> annotation. This value of this annotation is an
+array of the listener <code class="classname">Class</code> es for the entity.
The
+equivalent XML element is <code class="literal">entity-listeners</code>. For
more details
+on entity listeners, see <a class="xref" href="jpa_overview_pc_callbacks.html"
title="3. Lifecycle Callbacks">Section 3, “
+ Lifecycle Callbacks
+ ”</a>.
+ </p>
+ </div>
+ <div class="section" id="jpa_overview_meta_classex"><div
class="titlepage"><div><div><h3 class="title">1.6.
+ Example
+ </h3></div></div></div>
+
+ <p>
+Here are the class declarations for our persistent object model, annotated with
+the appropriate persistence metadata. Note that <code
class="classname">Magazine</code>
+declares an identity class, and that <code class="classname">Document</code>
and
+<code class="classname">Address</code> are a mapped superclass and an
embeddable class,
+respectively. <code class="classname">LifetimeSubscription</code> and <code
class="classname">
+TrialSubscription</code> override the default entity name to supply a
+shorter alias for use in queries.
+ </p>
+ <div class="example" id="jpa_overview_meta_classlisting"><p
class="title"><b>Example 5.1.
+ Class Metadata
+ </b></p><div class="example-contents">
+
+<pre class="programlisting">
+package org.mag;
+
+@Entity
+@IdClass(Magazine.MagazineId.class)
+public class Magazine {
+ ...
+
+ public static class MagazineId {
+ ...
+ }
+}
+
+@Entity
+public class Article {
+ ...
+}
+
+
+package org.mag.pub;
+
+@Entity
+public class Company {
+ ...
+}
+
+@Entity
+public class Author {
+ ...
+}
+
+@Embeddable
+public class Address {
+ ...
+}
+
+
+package org.mag.subscribe;
+
+@MappedSuperclass
+public abstract class Document {
+ ...
+}
+
+@Entity
+public class Contract
+ extends Document {
+ ...
+}
+
+@Entity
+public class Subscription {
+ ...
+
+ @Entity
+ 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 equivalent declarations in XML:
+ </p>
+<pre class="programlisting">
+<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">
+ <mapped-superclass class="org.mag.subscribe.Document">
+ ...
+ </mapped-superclass>
+ <entity class="org.mag.Magazine">
+ <id-class class="org.mag.Magazine$MagazineId"/>
+ ...
+ </entity>
+ <entity class="org.mag.Article">
+ ...
+ </entity>
+ <entity class="org.mag.pub.Company">
+ ...
+ </entity>
+ <entity class="org.mag.pub.Author">
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.Contract">
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.LineItem">
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.LifetimeSubscription"
name="Lifetime">
+ ...
+ </entity>
+ <entity class="org.mag.subscribe.TrialSubscription" name="Trial">
+ ...
+ </entity>
+ <embeddable class="org.mag.pub.Address">
+ ...
+ </embeddable>
+</entity-mappings>
+</pre>
+ </div></div><br class="example-break">
+ </div>
+ </div>
+
+
+
+</div><div class="navfooter"><hr><table width="100%" summary="Navigation
footer"><tr><td width="40%" align="left"><a accesskey="p"
href="jpa_overview_pc_conclusion.html">Prev</a> </td><td width="20%"
align="center"><a accesskey="u" href="jpa_overview.html">Up</a></td><td
width="40%" align="right"> <a accesskey="n"
href="jpa_overview_meta_field.html">Next</a></td></tr><tr><td width="40%"
align="left" valign="top">4.
+ Conclusions
+ </td><td width="20%" align="center"><a accesskey="h"
href="manual.html">Home</a></td><td width="40%" align="right"
valign="top"> 2.
+ Field and Property Metadata
+ </td></tr></table></div></body></html>
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