content/builds/3.1.0/apache-openjpa/docs/ref_guide_mapping_ext.html - openjpa-site - Git at Google

 <html><head>
       <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
    <title>9.&nbsp; Mapping Extensions</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 3.0 User's Guide"><link rel="up" href="ref_guide_mapping.html" title="Chapter&nbsp;7.&nbsp; Mapping"><link rel="prev" href="ref_guide_mapping_limits.html" title="8.&nbsp; Mapping Limitations"><link rel="next" href="ref_guide_mapping_custom.html" title="10.&nbsp; Custom Mappings"></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">9.&nbsp;
             Mapping Extensions
         </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ref_guide_mapping_limits.html">Prev</a>&nbsp;</td><th width="60%" align="center">Chapter&nbsp;7.&nbsp;
         Mapping
     </th><td width="20%" align="right">&nbsp;<a accesskey="n" href="ref_guide_mapping_custom.html">Next</a></td></tr></table><hr></div><div class="section" id="ref_guide_mapping_ext"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.&nbsp;
             Mapping Extensions
         </h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#ref_guide_mapping_ext_cls">9.1.
                 Class Extensions
             </a></span></dt><dd><dl><dt><span class="section"><a href="ref_guide_mapping_ext.html#subclass-fetch-mode">9.1.1.
                     Subclass Fetch Mode
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-strategy">9.1.2.
                     Strategy
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#discriminator-strategy">9.1.3.
                     Discriminator Strategy
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#version-strategy">9.1.4.
                     Version Strategy
                 </a></span></dt></dl></dd><dt><span class="section"><a href="ref_guide_mapping_ext.html#ref_guide_mapping_ext_field">9.2.
                 Field Extensions
             </a></span></dt><dd><dl><dt><span class="section"><a href="ref_guide_mapping_ext.html#eager-fetch-mode">9.2.1.
                     Eager Fetch Mode
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#nonpolymorphic">9.2.2.
                     Nonpolymorphic
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-criteria">9.2.3.
                     Class Criteria
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#strategy">9.2.4.
                     Strategy
                 </a></span></dt></dl></dd></dl></div>

         <p>
 Mapping extensions allow you to access OpenJPA-specific functionality from your
 mappings. Note that all extensions below are specific to mappings. If you store
 your mappings separately from your persistence metadata, these extensions must
 be specified along with the mapping information, not the persistence metadata
 information.
         </p>
         <div class="section" id="ref_guide_mapping_ext_cls"><div class="titlepage"><div><div><h3 class="title">9.1.&nbsp;
                 Class Extensions
             </h3></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#subclass-fetch-mode">9.1.1.
                     Subclass Fetch Mode
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-strategy">9.1.2.
                     Strategy
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#discriminator-strategy">9.1.3.
                     Discriminator Strategy
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#version-strategy">9.1.4.
                     Version Strategy
                 </a></span></dt></dl></div>

             <p>
 OpenJPA recognizes the following class extensions.
             </p>
             <div class="section" id="subclass-fetch-mode"><div class="titlepage"><div><div><h4 class="title">9.1.1.&nbsp;
                     Subclass Fetch Mode
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e14949"></a>
                 <p>
 This extension specifies how to eagerly fetch subclass state. It overrides the
 global <a class="link" href="ref_guide_conf_jdbc.html#openjpa.jdbc.SubclassFetchMode" title="6.16.&nbsp; openjpa.jdbc.SubclassFetchMode"><code class="literal">
 openjpa.jdbc.SubclassFetchMode</code></a> property. Set the OpenJPA
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/SubclassFetchMode.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.SubclassFetchMode</code>
 </a> annotation to a value from the
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/FetchMode.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.FetchMode</code>
 </a> enum: <code class="literal">JOIN</code>, <code class="literal">PARALLEL</code>, or
 <code class="literal">NONE</code>. See <a class="xref" href="ref_guide_perfpack_eager.html" title="8.&nbsp; Eager Fetching">Section&nbsp;8, &#8220;
             Eager Fetching
         &#8221;</a>
  for a discussion of eager fetching.
                 </p>
             </div>
             <div class="section" id="class-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.2.&nbsp;
                     Strategy
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e14967"></a>
                 <p>
 The <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Strategy.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.Strategy</code></a>
 class annotation allows you to specify a custom mapping strategy for your class.
 See <a class="xref" href="ref_guide_mapping_custom.html" title="10.&nbsp; Custom Mappings">Section&nbsp;10, &#8220;
             Custom Mappings
         &#8221;</a> for information on custom
 mappings.
                 </p>
             </div>
             <div class="section" id="discriminator-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.3.&nbsp;
                     Discriminator Strategy
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e14978"></a>
                 <p>
 The
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/DiscriminatorStrategy.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.DiscriminatorStrategy</code>
 </a> class annotation allows you to specify a custom discriminator strategy.
 See <a class="xref" href="ref_guide_mapping_custom.html" title="10.&nbsp; Custom Mappings">Section&nbsp;10, &#8220;
             Custom Mappings
         &#8221;</a> for information on custom
 mappings.
                 </p>
             </div>
             <div class="section" id="version-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.4.&nbsp;
                     Version Strategy
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e14989"></a>
                 <p>
 The
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/VersionStrategy.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.VersionStrategy</code>
 </a> class annotation allows you to specify a custom version strategy. See
 <a class="xref" href="ref_guide_mapping_custom.html" title="10.&nbsp; Custom Mappings">Section&nbsp;10, &#8220;
             Custom Mappings
         &#8221;</a> for information on custom
 mappings.
                 </p>
             </div>
         </div>
         <div class="section" id="ref_guide_mapping_ext_field"><div class="titlepage"><div><div><h3 class="title">9.2.&nbsp;
                 Field Extensions
             </h3></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#eager-fetch-mode">9.2.1.
                     Eager Fetch Mode
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#nonpolymorphic">9.2.2.
                     Nonpolymorphic
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-criteria">9.2.3.
                     Class Criteria
                 </a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#strategy">9.2.4.
                     Strategy
                 </a></span></dt></dl></div>

             <p>
 OpenJPA recognizes the following field extensions.
             </p>
             <div class="section" id="eager-fetch-mode"><div class="titlepage"><div><div><h4 class="title">9.2.1.&nbsp;
                     Eager Fetch Mode
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e15003"></a>
                 <p>
 This extension specifies how to eagerly fetch related objects. It overrides the
 global <a class="link" href="ref_guide_conf_jdbc.html#openjpa.jdbc.EagerFetchMode" title="6.4.&nbsp; openjpa.jdbc.EagerFetchMode"><code class="literal">
 openjpa.jdbc.EagerFetchMode</code></a> property. Set the OpenJPA
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/EagerFetchMode.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.EagerFetchMode</code>
 </a> annotation to a value from the
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/FetchMode.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.FetchMode</code>
 </a> enum: <code class="literal">JOIN</code>, <code class="literal">PARALLEL</code>, or
 <code class="literal">NONE</code>. See <a class="xref" href="ref_guide_perfpack_eager.html" title="8.&nbsp; Eager Fetching">Section&nbsp;8, &#8220;
             Eager Fetching
         &#8221;</a>
  for a discussion of eager fetching.
                 </p>
             </div>
             <div class="section" id="nonpolymorphic"><div class="titlepage"><div><div><h4 class="title">9.2.2.&nbsp;
                     Nonpolymorphic
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e15021"></a>
                 <p>
 All fields in Java are polymorphic. If you declare a field of type <code class="literal">T
 </code>, you can assign any subclass of <code class="literal">T</code> to the field as
 well. This is very convenient, but can make relation traversal very inefficient
 under some inheritance strategies. It can even make querying across the field
 impossible. Often, you know that certain fields do not need to be entirely
 polymorphic. By telling OpenJPA about such fields, you can improve the
 efficiency of your relations.
                 </p>
                 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>
                     <p>
 OpenJPA also includes the <code class="literal">type</code> metadata extension for
 narrowing the declared type of a field. See <a class="xref" href="ref_guide_meta_ext.html#type" title="4.2.6.&nbsp; Type">Section&nbsp;4.2.6, &#8220;
                     Type
                 &#8221;</a>.
                     </p>
                 </div>
                 <p>
 OpenJPA defines the following extensions for nonpolymorphic values:
                 </p>
                 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
                         <p>
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Nonpolymorphic.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.Nonpolymorphic</code>
 </a>
                         </p>
                     </li><li class="listitem">
                         <p>
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ElementNonpolymorphic.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.ElementNonpolymorphic</code>
 </a>
                         </p>
                     </li></ul></div>
                 <p>
 The value of these extensions is a constant from the
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/NonpolymorphicType.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.NonpolymorphicType</code>
 </a> enumeration. The default value, <code class="literal">EXACT</code>, indicates
 that the relation will always be of the exact declared type. A value of
 <code class="literal">JOINABLE</code>, on the other hand, means that the relation might
 be to any joinable subclass of the declared type. This value only excludes
 table-per-class subclasses.
                 </p>
             </div>
             <div class="section" id="class-criteria"><div class="titlepage"><div><div><h4 class="title">9.2.3.&nbsp;
                     Class Criteria
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e15049"></a>
                 <a class="indexterm" name="d5e15054"></a>
                 <p>
 This family of boolean extensions determines whether OpenJPA will use the
 expected class of related objects as criteria in the SQL it issues to load a
 relation field. Typically, this is not needed. The foreign key values uniquely
 identify the record for the related object. Under some rare mappings, however,
 you may need to consider both foreign key values and the expected class of the
 related object - for example, if you have an inverse relation that shares the
 foreign key with another inverse relation to an object of a different subclass.
 In these cases, set the proper class criteria extension to <code class="literal">true
 </code> to force OpenJPA to append class criteria to its select SQL.
                 </p>
                 <p>
 OpenJPA defines the following class criteria annotations for field relations and
 array or collection element relations, respectively:
                 </p>
                 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
                         <p>
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ClassCriteria.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.ClassCriteria</code></a>
                         </p>
                     </li><li class="listitem">
                         <p>
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ElementClassCriteria.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.ElementClassCriteria</code>
 </a>
                         </p>
                     </li></ul></div>
             </div>
             <div class="section" id="strategy"><div class="titlepage"><div><div><h4 class="title">9.2.4.&nbsp;
                     Strategy
                 </h4></div></div></div>

                 <a class="indexterm" name="d5e15071"></a>
                 <p>
 OpenJPA's
 <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Strategy.html" target="_top">
 <code class="classname">org.apache.openjpa.persistence.jdbc.Strategy</code></a>
 extension allows you to specify a custom mapping
 strategy or value handler for a field. See
 <a class="xref" href="ref_guide_mapping_custom.html" title="10.&nbsp; Custom Mappings">Section&nbsp;10, &#8220;
             Custom Mappings
         &#8221;</a> for information on custom
 mappings.
                 </p>
             </div>
         </div>
     </div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ref_guide_mapping_limits.html">Prev</a>&nbsp;</td><td width="20%" align="center"><a accesskey="u" href="ref_guide_mapping.html">Up</a></td><td width="40%" align="right">&nbsp;<a accesskey="n" href="ref_guide_mapping_custom.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">8.&nbsp;
             Mapping Limitations
         &nbsp;</td><td width="20%" align="center"><a accesskey="h" href="manual.html">Home</a></td><td width="40%" align="right" valign="top">&nbsp;10.&nbsp;
             Custom Mappings
         </td></tr></table></div></body></html>
	<html><head>
	<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
	<title>9.  Mapping Extensions</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 3.0 User's Guide"><link rel="up" href="ref_guide_mapping.html" title="Chapter 7.  Mapping"><link rel="prev" href="ref_guide_mapping_limits.html" title="8.  Mapping Limitations"><link rel="next" href="ref_guide_mapping_custom.html" title="10.  Custom Mappings"></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">9.
	Mapping Extensions
	</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ref_guide_mapping_limits.html">Prev</a> </td><th width="60%" align="center">Chapter 7.
	Mapping
	</th><td width="20%" align="right"> <a accesskey="n" href="ref_guide_mapping_custom.html">Next</a></td></tr></table><hr></div><div class="section" id="ref_guide_mapping_ext"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.
	Mapping Extensions
	</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#ref_guide_mapping_ext_cls">9.1.
	Class Extensions
	</a></span></dt><dd><dl><dt><span class="section"><a href="ref_guide_mapping_ext.html#subclass-fetch-mode">9.1.1.
	Subclass Fetch Mode
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-strategy">9.1.2.
	Strategy
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#discriminator-strategy">9.1.3.
	Discriminator Strategy
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#version-strategy">9.1.4.
	Version Strategy
	</a></span></dt></dl></dd><dt><span class="section"><a href="ref_guide_mapping_ext.html#ref_guide_mapping_ext_field">9.2.
	Field Extensions
	</a></span></dt><dd><dl><dt><span class="section"><a href="ref_guide_mapping_ext.html#eager-fetch-mode">9.2.1.
	Eager Fetch Mode
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#nonpolymorphic">9.2.2.
	Nonpolymorphic
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-criteria">9.2.3.
	Class Criteria
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#strategy">9.2.4.
	Strategy
	</a></span></dt></dl></dd></dl></div>

	<p>
	Mapping extensions allow you to access OpenJPA-specific functionality from your
	mappings. Note that all extensions below are specific to mappings. If you store
	your mappings separately from your persistence metadata, these extensions must
	be specified along with the mapping information, not the persistence metadata
	information.
	</p>
	<div class="section" id="ref_guide_mapping_ext_cls"><div class="titlepage"><div><div><h3 class="title">9.1.
	Class Extensions
	</h3></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#subclass-fetch-mode">9.1.1.
	Subclass Fetch Mode
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-strategy">9.1.2.
	Strategy
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#discriminator-strategy">9.1.3.
	Discriminator Strategy
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#version-strategy">9.1.4.
	Version Strategy
	</a></span></dt></dl></div>

	<p>
	OpenJPA recognizes the following class extensions.
	</p>
	<div class="section" id="subclass-fetch-mode"><div class="titlepage"><div><div><h4 class="title">9.1.1.
	Subclass Fetch Mode
	</h4></div></div></div>

	<a class="indexterm" name="d5e14949"></a>
	<p>
	This extension specifies how to eagerly fetch subclass state. It overrides the
	global <a class="link" href="ref_guide_conf_jdbc.html#openjpa.jdbc.SubclassFetchMode" title="6.16.  openjpa.jdbc.SubclassFetchMode"><code class="literal">
	openjpa.jdbc.SubclassFetchMode</code></a> property. Set the OpenJPA
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/SubclassFetchMode.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.SubclassFetchMode</code>
	</a> annotation to a value from the
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/FetchMode.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.FetchMode</code>
	</a> enum: <code class="literal">JOIN</code>, <code class="literal">PARALLEL</code>, or
	<code class="literal">NONE</code>. See <a class="xref" href="ref_guide_perfpack_eager.html" title="8.  Eager Fetching">Section 8, “
	Eager Fetching
	”</a>
	for a discussion of eager fetching.
	</p>
	</div>
	<div class="section" id="class-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.2.
	Strategy
	</h4></div></div></div>

	<a class="indexterm" name="d5e14967"></a>
	<p>
	The <a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Strategy.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.Strategy</code></a>
	class annotation allows you to specify a custom mapping strategy for your class.
	See <a class="xref" href="ref_guide_mapping_custom.html" title="10.  Custom Mappings">Section 10, “
	Custom Mappings
	”</a> for information on custom
	mappings.
	</p>
	</div>
	<div class="section" id="discriminator-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.3.
	Discriminator Strategy
	</h4></div></div></div>

	<a class="indexterm" name="d5e14978"></a>
	<p>
	The
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/DiscriminatorStrategy.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.DiscriminatorStrategy</code>
	</a> class annotation allows you to specify a custom discriminator strategy.
	See <a class="xref" href="ref_guide_mapping_custom.html" title="10.  Custom Mappings">Section 10, “
	Custom Mappings
	”</a> for information on custom
	mappings.
	</p>
	</div>
	<div class="section" id="version-strategy"><div class="titlepage"><div><div><h4 class="title">9.1.4.
	Version Strategy
	</h4></div></div></div>

	<a class="indexterm" name="d5e14989"></a>
	<p>
	The
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/VersionStrategy.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.VersionStrategy</code>
	</a> class annotation allows you to specify a custom version strategy. See
	<a class="xref" href="ref_guide_mapping_custom.html" title="10.  Custom Mappings">Section 10, “
	Custom Mappings
	”</a> for information on custom
	mappings.
	</p>
	</div>
	</div>
	<div class="section" id="ref_guide_mapping_ext_field"><div class="titlepage"><div><div><h3 class="title">9.2.
	Field Extensions
	</h3></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="ref_guide_mapping_ext.html#eager-fetch-mode">9.2.1.
	Eager Fetch Mode
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#nonpolymorphic">9.2.2.
	Nonpolymorphic
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#class-criteria">9.2.3.
	Class Criteria
	</a></span></dt><dt><span class="section"><a href="ref_guide_mapping_ext.html#strategy">9.2.4.
	Strategy
	</a></span></dt></dl></div>

	<p>
	OpenJPA recognizes the following field extensions.
	</p>
	<div class="section" id="eager-fetch-mode"><div class="titlepage"><div><div><h4 class="title">9.2.1.
	Eager Fetch Mode
	</h4></div></div></div>

	<a class="indexterm" name="d5e15003"></a>
	<p>
	This extension specifies how to eagerly fetch related objects. It overrides the
	global <a class="link" href="ref_guide_conf_jdbc.html#openjpa.jdbc.EagerFetchMode" title="6.4.  openjpa.jdbc.EagerFetchMode"><code class="literal">
	openjpa.jdbc.EagerFetchMode</code></a> property. Set the OpenJPA
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/EagerFetchMode.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.EagerFetchMode</code>
	</a> annotation to a value from the
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/FetchMode.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.FetchMode</code>
	</a> enum: <code class="literal">JOIN</code>, <code class="literal">PARALLEL</code>, or
	<code class="literal">NONE</code>. See <a class="xref" href="ref_guide_perfpack_eager.html" title="8.  Eager Fetching">Section 8, “
	Eager Fetching
	”</a>
	for a discussion of eager fetching.
	</p>
	</div>
	<div class="section" id="nonpolymorphic"><div class="titlepage"><div><div><h4 class="title">9.2.2.
	Nonpolymorphic
	</h4></div></div></div>

	<a class="indexterm" name="d5e15021"></a>
	<p>
	All fields in Java are polymorphic. If you declare a field of type <code class="literal">T
	</code>, you can assign any subclass of <code class="literal">T</code> to the field as
	well. This is very convenient, but can make relation traversal very inefficient
	under some inheritance strategies. It can even make querying across the field
	impossible. Often, you know that certain fields do not need to be entirely
	polymorphic. By telling OpenJPA about such fields, you can improve the
	efficiency of your relations.
	</p>
	<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>
	<p>
	OpenJPA also includes the <code class="literal">type</code> metadata extension for
	narrowing the declared type of a field. See <a class="xref" href="ref_guide_meta_ext.html#type" title="4.2.6.  Type">Section 4.2.6, “
	Type
	”</a>.
	</p>
	</div>
	<p>
	OpenJPA defines the following extensions for nonpolymorphic values:
	</p>
	<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
	<p>
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Nonpolymorphic.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.Nonpolymorphic</code>
	</a>
	</p>
	</li><li class="listitem">
	<p>
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ElementNonpolymorphic.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.ElementNonpolymorphic</code>
	</a>
	</p>
	</li></ul></div>
	<p>
	The value of these extensions is a constant from the
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/NonpolymorphicType.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.NonpolymorphicType</code>
	</a> enumeration. The default value, <code class="literal">EXACT</code>, indicates
	that the relation will always be of the exact declared type. A value of
	<code class="literal">JOINABLE</code>, on the other hand, means that the relation might
	be to any joinable subclass of the declared type. This value only excludes
	table-per-class subclasses.
	</p>
	</div>
	<div class="section" id="class-criteria"><div class="titlepage"><div><div><h4 class="title">9.2.3.
	Class Criteria
	</h4></div></div></div>

	<a class="indexterm" name="d5e15049"></a>
	<a class="indexterm" name="d5e15054"></a>
	<p>
	This family of boolean extensions determines whether OpenJPA will use the
	expected class of related objects as criteria in the SQL it issues to load a
	relation field. Typically, this is not needed. The foreign key values uniquely
	identify the record for the related object. Under some rare mappings, however,
	you may need to consider both foreign key values and the expected class of the
	related object - for example, if you have an inverse relation that shares the
	foreign key with another inverse relation to an object of a different subclass.
	In these cases, set the proper class criteria extension to <code class="literal">true
	</code> to force OpenJPA to append class criteria to its select SQL.
	</p>
	<p>
	OpenJPA defines the following class criteria annotations for field relations and
	array or collection element relations, respectively:
	</p>
	<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
	<p>
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ClassCriteria.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.ClassCriteria</code></a>
	</p>
	</li><li class="listitem">
	<p>
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/ElementClassCriteria.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.ElementClassCriteria</code>
	</a>
	</p>
	</li></ul></div>
	</div>
	<div class="section" id="strategy"><div class="titlepage"><div><div><h4 class="title">9.2.4.
	Strategy
	</h4></div></div></div>

	<a class="indexterm" name="d5e15071"></a>
	<p>
	OpenJPA's
	<a class="ulink" href="../../apidocs/org/apache/openjpa/persistence/jdbc/Strategy.html" target="_top">
	<code class="classname">org.apache.openjpa.persistence.jdbc.Strategy</code></a>
	extension allows you to specify a custom mapping
	strategy or value handler for a field. See
	<a class="xref" href="ref_guide_mapping_custom.html" title="10.  Custom Mappings">Section 10, “
	Custom Mappings
	”</a> for information on custom
	mappings.
	</p>
	</div>
	</div>
	</div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ref_guide_mapping_limits.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ref_guide_mapping.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ref_guide_mapping_custom.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">8.
	Mapping Limitations
	</td><td width="20%" align="center"><a accesskey="h" href="manual.html">Home</a></td><td width="40%" align="right" valign="top"> 10.
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