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<!-- <div class="page-title">
<h3>Fetch Groups<a id="Fetch_Groups"></a></h3>
</div>
-->
<div id="doc-content">
<div class="sect1">
<h2 id="index">Fetch Groups<a id="Fetch_Groups"></a></h2>
<div class="sectionbody">
<div class="paragraph">
<p>When an object is retrieved from the datastore by JDO typically not all
fields are retrieved immediately. This is because for efficiency
purposes only particular field types are retrieved in the initial access
of the object, and then any other objects are retrieved when accessed
(lazy loading). The group of fields that are loaded is called a <strong>fetch
group</strong>. There are 3 types of "fetch groups" to consider</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="#dfg">Default Fetch Group</a> : defined in all JDO specs, containing
the fields of a class that will be retrieved by default (with no user
specification).</p>
</li>
<li>
<p><a href="#static">Named Fetch Groups</a> : defined by the JDO2 specification,
and defined in MetaData (XML/annotations) with the fields of a class
that are part of that fetch group. The definition here is <em>static</em></p>
</li>
<li>
<p><a href="#dynamic">Dynamic Fetch Groups</a> : Programmatic definition of fetch
groups at runtime via an API</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The <strong>fetch group</strong> in use for a class is controled via the <em>FetchPlan</em>
<a href="http://db.apache.org/jdo/api20/apidocs/javax/jdo/FetchPlan.html"><span class="image"><img src="images/javadoc.png" alt="image"></span></a>
interface. To get a handle on the current <em>FetchPlan</em> we do</p>
</div>
<div class="literalblock">
<div class="content">
<pre>FetchPlan fp = pm.getFetchPlan();</pre>
</div>
</div>
<div class="paragraph">
<p><br></p>
</div>
<div class="paragraph">
<p><a id="dfg"></a></p>
</div>
<div class="sect2">
<h3 id="_default_fetch_group_a_id_default_fetch_group_a">Default Fetch Group<a id="Default_Fetch_Group"></a></h3>
<div class="paragraph">
<p>JDO provides an initial fetch group, comprising the fields that will be
retrieved when an object is retrieved if the user does nothing to define
the required behaviour. By default the <em>default fetch group</em> comprises
all fields of the following types :</p>
</div>
<div class="ulist">
<ul>
<li>
<p>primitives : boolean, byte, char, double, float, int, long, short</p>
</li>
<li>
<p>Object wrappers of primitives : Boolean, Byte, Character, Double,
Float, Integer, Long, Short</p>
</li>
<li>
<p>java.lang.String, java.lang.Number, java.lang.Enum</p>
</li>
<li>
<p>java.math.BigDecimal, java.math.BigInteger</p>
</li>
<li>
<p>java.util.Date</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>If you wish to change the <strong>Default Fetch Group</strong> for a class you can
update the Meta-Data for the class as follows (for XML)</p>
</div>
<div class="literalblock">
<div class="content">
<pre>&lt;class name="MyClass"&gt;
...
&lt;field name="fieldX" default-fetch-group="true"/&gt;
&lt;/class&gt;</pre>
</div>
</div>
<div class="paragraph">
<p>or using annotations</p>
</div>
<div class="literalblock">
<div class="content">
<pre>@Persistent(defaultFetchGroup="true")
SomeType fieldX;</pre>
</div>
</div>
<div class="paragraph">
<p>When a <em>PersistenceManager</em> is created it starts with a FetchPlan of the
"default" fetch group. That is, if we call</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Collection fetchGroups = fp.getGroups();</pre>
</div>
</div>
<div class="paragraph">
<p>this will have one group, called "default". At runtime, if you have been
using other fetch groups and want to revert back to the default fetch
group at any time you simply do</p>
</div>
<div class="literalblock">
<div class="content">
<pre>fp.setGroup(FetchPlan.DEFAULT);</pre>
</div>
</div>
<div class="paragraph">
<p><br></p>
</div>
<div class="paragraph">
<p><a id="static"></a></p>
</div>
</div>
<div class="sect2">
<h3 id="_named_fetch_groups_a_id_named_fetch_groups_a">Named Fetch Groups<a id="Named_Fetch_Groups"></a></h3>
<div class="paragraph">
<p>As mentioned above, JDO allows specification of users own fetch groups.
These are specified in the MetaData of the class. For example, if we
have the following class</p>
</div>
<div class="literalblock">
<div class="content">
<pre>class MyClass
{
String name;
HashSet coll;
MyOtherClass other;
}</pre>
</div>
</div>
<div class="paragraph">
<p>and we want to have the <span class="underline">other</span> field loaded whenever we
load objects of this class, we define our MetaData as</p>
</div>
<div class="literalblock">
<div class="content">
<pre>&lt;package name="mydomain"&gt;
&lt;class name="MyClass"&gt;
&lt;field name="name"&gt;
&lt;column length="100" jdbc-type="VARCHAR"/&gt;
&lt;/field&gt;
&lt;field name="coll" persistence-modifier="persistent"&gt;
&lt;collection element-type="mydomain.Address"/&gt;
&lt;join/&gt;
&lt;/field&gt;
&lt;field name="other" persistence-modifier="persistent"/&gt;
&lt;fetch-group name="otherfield"&gt;
&lt;field name="other"/&gt;
&lt;/fetch-group&gt;
&lt;/class&gt;
&lt;/package&gt;</pre>
</div>
</div>
<div class="paragraph">
<p>or using annotations</p>
</div>
<div class="literalblock">
<div class="content">
<pre>@PersistenceCapable
@FetchGroup(name="otherfield", members={@Persistent(name="other")})
public class MyClass
{
...
}</pre>
</div>
</div>
<div class="paragraph">
<p>So we have defined a fetch group called "otherfield" that just includes
the field with name <em>other</em>. We can then use this at runtime in our
persistence code.</p>
</div>
<div class="literalblock">
<div class="content">
<pre>PersistenceManager pm = pmf.getPersistenceManager();
pm.getFetchPlan().addGroup("otherfield");
... (load MyClass object)</pre>
</div>
</div>
<div class="paragraph">
<p>By default the <em>FetchPlan</em> will include the default fetch group. We have
changed this above by <span class="underline">adding</span> the fetch group "otherfield",
so when we retrieve an object using this <em>PersistenceManager</em> we will be
retrieving the fields <em>name</em> AND <em>other</em> since they are both in the
current <em>FetchPlan</em>. We can take the above much further than what is
shown by defining nested fetch groups in the MetaData. In addition we
can change the <em>FetchPlan</em> just before any <em>PersistenceManager</em>
operation to control what is fetched during that operation. The user has
full flexibility to add many groups to the current <strong>Fetch Plan</strong>. This
gives much power and control over what will be loaded and when.</p>
</div>
<div class="paragraph">
<p>The <em>FetchPlan</em> applies not just to calls to
<em>PersistenceManager.getObjectById()</em>, but also to
<em>PersistenceManager.newQuery()</em>, <em>PersistenceManager.getExtent()</em>,
<em>PersistenceManager.detachCopy</em> and much more besides.</p>
</div>
<div class="paragraph">
<p>You can read more about <strong>named fetch-groups</strong> and how to use it with
<a href="attach_detach.html"><strong>attach/detach</strong></a></p>
</div>
<div class="paragraph">
<p><a id="dynamic"></a></p>
</div>
</div>
<div class="sect2">
<h3 id="_dynamic_fetch_groups_a_id_dynamic_fetch_groups_a">Dynamic Fetch Groups<a id="Dynamic_Fetch_Groups"></a></h3>
<div class="paragraph">
<p>The mechanism above provides static fetch groups defined in XML or
annotations. That is great when you know in advance what fields you want
to fetch. In some situations you may want to define your fields to fetch
at run time. This became standard in JDO2.2 It operates as follows</p>
</div>
<div class="literalblock">
<div class="content">
<pre>import org.datanucleus.FetchGroup;
// Create a FetchGroup on the PMF called "TestGroup" for MyClass
FetchGroup grp = myPMF.getFetchGroup("TestGroup", MyClass.class);
grp.addMember("field1").addMember("field2");
// Add this group to the fetch plan (using its name)
fp.addGroup("TestGroup");</pre>
</div>
</div>
<div class="paragraph">
<p>So we use the DataNucleus PMF as a way of creating a FetchGroup, and
then register that FetchGroup with the PMF for use by all PMs. We then
enable our FetchGroup for use in the FetchPlan by using its group name
(as we do for a static group). The FetchGroup allows you to add/remove
the fields necessary so you have full API control over the fields to be
fetched.</p>
</div>
<div class="paragraph">
<p><br></p>
</div>
</div>
<div class="sect2">
<h3 id="_fetch_depth_a_id_fetch_depth_a">Fetch Depth<a id="Fetch_Depth"></a></h3>
<div class="paragraph">
<p>The basic fetch group defines which fields are to be fetched. It doesn&#8217;t
explicitly define how far down an object graph is to be fetched. JDO
provides two ways of controlling this.</p>
</div>
<div class="paragraph">
<p>The first is to set the <strong>maxFetchDepth</strong> for the <em>FetchPlan</em>. This value
specifies how far out from the root object the related objects will be
fetched. A positive value means that this number of relationships will
be traversed from the root object. A value of -1 means that no limit
will be placed on the fetching traversal. The default is 1. Let&#8217;s take
an example</p>
</div>
<div class="literalblock">
<div class="content">
<pre>public class MyClass1
{
MyClass2 field1;
...
}
public class MyClass2
{
MyClass3 field2;
...
}
public class MyClass3
{
MyClass4 field3;
...
}</pre>
</div>
</div>
<div class="paragraph">
<p>and we want to detach <em>field1</em> of instances of <em>MyClass1</em>, down 2 levels
- so detaching the initial "field1" <em>MyClass2</em> object, and its "field2"
<em>MyClass3</em> instance. So we define our fetch-groups like this</p>
</div>
<div class="literalblock">
<div class="content">
<pre>&lt;class name="MyClass1"&gt;
...
&lt;fetch-group name="includingField1"&gt;
&lt;field name="field1"/&gt;
&lt;/fetch-group&gt;
&lt;/class&gt;
&lt;class name="MyClass2"&gt;
...
&lt;fetch-group name="includingField2"&gt;
&lt;field name="field2"/&gt;
&lt;/fetch-group&gt;
&lt;/class&gt;</pre>
</div>
</div>
<div class="paragraph">
<p>and we then define the <strong>maxFetchDepth</strong> as 2, like this</p>
</div>
<div class="literalblock">
<div class="content">
<pre>pm.getFetchPlan().setMaxFetchDepth(2);</pre>
</div>
</div>
<div class="paragraph">
<p>A further refinement to this global fetch depth setting is to control
the fetching of recursive fields. This is performed via a MetaData
setting "recursion-depth". A value of 1 means that only 1 level of
objects will be fetched. A value of -1 means there is no limit on the
amount of recursion. The default is 1. Let&#8217;s take an example</p>
</div>
<div class="literalblock">
<div class="content">
<pre>public class Directory
{
Collection children;
...
}</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>&lt;class name="Directory"&gt;
&lt;field name="children"&gt;
&lt;collection element-type="Directory"/&gt;
&lt;/field&gt;
&lt;fetch-group name="grandchildren"&gt;
&lt;field name="children" recursion-depth="2"/&gt;
&lt;/fetch-group&gt;
...
&lt;/class&gt;</pre>
</div>
</div>
<div class="paragraph">
<p>So when we fetch a Directory, it will fetch 2 levels of the <em>children</em>
field, hence fetching the children and the grandchildren.</p>
</div>
</div>
<div class="sect2">
<h3 id="_fetch_size_a_id_fetch_size_a">Fetch Size<a id="Fetch_Size"></a></h3>
<div class="paragraph">
<p>A FetchPlan can also be used for defining the fetching policy when using
queries. This can be set using</p>
</div>
<div class="literalblock">
<div class="content">
<pre>pm.getFetchPlan().setFetchSize(value);</pre>
</div>
</div>
<div class="paragraph">
<p>The default is <em>FetchPlan.FETCH_SIZE_OPTIMAL</em> which leaves it to
DataNucleus to optimise the fetching of instances. A positive value
defines the number of instances to be fetched. Using
<em>FetchPlan.FETCH_SIZE_GREEDY</em> means that all instances will be fetched
immediately.</p>
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