Modified: websites/production/db/content/jdo/fetchgroups.html ============================================================================== --- websites/production/db/content/jdo/fetchgroups.html (original) +++ websites/production/db/content/jdo/fetchgroups.html Tue Jan 15 06:29:01 2013 @@ -1,13 +1,219 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd";><html><head><title>Java Data Objects (JDO) - Fetch-Groups</title><style type="text/css" media="all"> - @import url("./style/maven-base.css"); - - @import url("./style/maven-theme.css");</style><link rel="stylesheet" href="./style/print.css" type="text/css" media="print"></link><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"></meta></head><body class="composite"><div id="banner"><table><tr><td nowrap="true"><a href="http://db.apache.org/jdo"; id="projectLogo"><img alt="Java Data Objects (JDO)" src="./images/JDOx120.gif"></img></a><a href="http://db.apache.org/jdo"; 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class="externalLink" title="Coding Standards">Coding Standards</a> + </li> + <li class="none"> + <a href="issuetracking.html" title="Issue Tracking">Issue Tracking</a> + </li> + <li class="none"> + <a href="dependencies.html" title="Dependencies">Dependencies</a> + </li> + </ul> + <a href="http://maven.apache.org/"; title="Built by Maven" class="poweredBy"> + <img class="poweredBy" alt="Built by Maven" src="./images/logos/maven-feather.png" /> + </a> + + + </div> + </div> + <div id="bodyColumn"> + <div id="contentBox"> + + + <div class="section"><h2>Fetch Groups<a name="Fetch_Groups"></a></h2> + <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 <B>fetch group</B>. - There are 3 types of "fetch groups" to consider - </p><ul> + The group of fields that are loaded is called a <b>fetch group</b>. + There are 3 types of "fetch groups" to consider + </p> + <ul> <li><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).</li> <li><a href="#static">Named Fetch Groups</a> : defined by the JDO2 specification, and defined @@ -15,140 +221,165 @@ The definition here is <i>static</i></li> <li><a href="#dynamic">Dynamic Fetch Groups</a> : Programmatic definition of fetch groups at runtime via an API</li> - </ul><p> - The <B>fetch group</B> in use for a class is controled via the <I>FetchPlan</I> - <a href="http://db.apache.org/jdo/api20/apidocs/javax/jdo/FetchPlan.html"; class="newWindow" title="New Window" target="_blank"><IMG SRC="images/javadoc.gif"></IMG></a> - interface. To get a handle on the current <I>FetchPlan</I> we do + </ul> + <p> + The <b>fetch group</b> in use for a class is controled via the <i>FetchPlan</i> + <a class="externalLink" href="http://db.apache.org/jdo/api20/apidocs/javax/jdo/FetchPlan.html"; target="_blank"><img src="images/javadoc.gif" alt="" /></a> + interface. To get a handle on the current <i>FetchPlan</i> we do </p> - <div class="source"><pre>FetchPlan fp = pm.getFetchPlan();</pre></div> - <br></br><a name="dfg"></a><div class="subsection"><a name="Default_Fetch_Group"></a><h3>Default Fetch Group</h3><p> + <div class="source"><pre>FetchPlan fp = pm.getFetchPlan();</pre></div> + <br /> + + <a name="dfg"></a> + <div class="section"><h3>Default Fetch Group<a name="Default_Fetch_Group"></a></h3> + <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 <I>default - fetch group</I> comprises all fields of the following types :- - </p><ul> + is retrieved if the user does nothing to define the required behaviour. By default the <i>default + fetch group</i> comprises all fields of the following types :- + </p> + <ul> <li>primitives : boolean, byte, char, double, float, int, long, short</li> <li>Object wrappers of primitives : Boolean, Byte, Character, Double, Float, Integer, Long, Short</li> <li>java.lang.String, java.lang.Number, java.lang.Enum</li> <li>java.math.BigDecimal, java.math.BigInteger</li> <li>java.util.Date</li> - </ul><p> - If you wish to change the <B>Default Fetch Group</B> for a class you can update the Meta-Data + </ul> + <p> + If you wish to change the <b>Default Fetch Group</b> for a class you can update the Meta-Data for the class as follows (for XML) </p> - <div class="source"><pre> -<class name="MyClass"> + <div class="source"><pre> +<class name="MyClass"> ... - <field name="fieldX" default-fetch-group="true"/> + <field name="fieldX" default-fetch-group="true"/> </class></pre></div> - <p>or using annotations</p> - <div class="source"><pre> -@Persistent(defaultFetchGroup="true") + <p>or using annotations</p> + <div class="source"><pre> +@Persistent(defaultFetchGroup="true") SomeType fieldX;</pre></div> - <p> - When a <I>PersistenceManager</I> is created it starts with a FetchPlan of the "default" fetch group. + <p> + When a <i>PersistenceManager</i> is created it starts with a FetchPlan of the "default" fetch group. That is, if we call </p> - <div class="source"><pre>Collection fetchGroups = fp.getGroups();</pre></div> - <p> - this will have one group, called "default". At runtime, if you have been using other + <div class="source"><pre>Collection fetchGroups = fp.getGroups();</pre></div> + <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 class="source"><pre>fp.setGroup(FetchPlan.DEFAULT);</pre></div> - <br></br></div><a name="static"></a><div class="subsection"><a name="Named_Fetch_Groups"></a><h3>Named Fetch Groups</h3><p> + <div class="source"><pre>fp.setGroup(FetchPlan.DEFAULT);</pre></div> + <br /> + </div> + + <a name="static"></a> + <div class="section"><h3>Named Fetch Groups<a name="Named_Fetch_Groups"></a></h3> + <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 class="source"><pre> + <div class="source"><pre> class MyClass { String name; HashSet coll; MyOtherClass other; }</pre></div> - <p> - and we want to have the <U>other</U> field loaded whenever we load objects of this class, we define + <p> + and we want to have the <u>other</u> field loaded whenever we load objects of this class, we define our MetaData as </p> - <div class="source"><pre> -<package name="mydomain"> - <class name="MyClass"> - <field name="name"> - <column length="100" jdbc-type="VARCHAR"/> + <div class="source"><pre> +<package name="mydomain"> + <class name="MyClass"> + <field name="name"> + <column length="100" jdbc-type="VARCHAR"/> </field> - <field name="coll" persistence-modifier="persistent"> - <collection element-type="mydomain.Address"/> + <field name="coll" persistence-modifier="persistent"> + <collection element-type="mydomain.Address"/> <join/> </field> - <field name="other" persistence-modifier="persistent"/> - <fetch-group name="otherfield"> - <field name="other"/> + <field name="other" persistence-modifier="persistent"/> + <fetch-group name="otherfield"> + <field name="other"/> </fetch-group> </class> </package></pre></div> - <p>or using annotations</p> - <div class="source"><pre> + <p>or using annotations</p> + <div class="source"><pre> @PersistenceCapable -@FetchGroup(name="otherfield", members={@Persistent(name="other")}) +@FetchGroup(name="otherfield", members={@Persistent(name="other")}) public class MyClass { ... }</pre></div> - <p> - So we have defined a fetch group called "otherfield" that just includes the field with name - <I>other</I>. We can then use this at runtime in our persistence code. + <p> + So we have defined a fetch group called "otherfield" that just includes the field with name + <i>other</i>. We can then use this at runtime in our persistence code. </p> - <div class="source"><pre> + <div class="source"><pre> PersistenceManager pm = pmf.getPersistenceManager(); -pm.getFetchPlan().addGroup("otherfield"); +pm.getFetchPlan().addGroup("otherfield"); ... (load MyClass object)</pre></div> - <p> - By default the <I>FetchPlan</I> will include the default fetch group. We have changed this above by - <U>adding</U> the fetch group "otherfield", so when we retrieve an object using this - <I>PersistenceManager</I> we will be retrieving the fields <I>name</I> AND <I>other</I> since they - are both in the current <I>FetchPlan</I>. 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 <I>FetchPlan</I> just - before any <I>PersistenceManager</I> operation to control what is fetched during that operation. - The user has full flexibility to add many groups to the current <B>Fetch Plan</B>. + <p> + By default the <i>FetchPlan</i> will include the default fetch group. We have changed this above by + <u>adding</u> the fetch group "otherfield", so when we retrieve an object using this + <i>PersistenceManager</i> we will be retrieving the fields <i>name</i> AND <i>other</i> since they + are both in the current <i>FetchPlan</i>. 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 <i>FetchPlan</i> just + before any <i>PersistenceManager</i> operation to control what is fetched during that operation. + The user has full flexibility to add many groups to the current <b>Fetch Plan</b>. This gives much power and control over what will be loaded and when. - </p><p> - The <I>FetchPlan</I> applies not just to calls to <I>PersistenceManager.getObjectById()</I>, but also - to <I>PersistenceManager.newQuery()</I>, <I>PersistenceManager.getExtent()</I>, - <I>PersistenceManager.detachCopy</I> and much more besides. - </p><p> - You can read more about <B>named fetch-groups</B> and how to use it with - <a href="attach_detach.html"><B>attach/detach</B></a> - </p></div><a name="dynamic"></a><div class="subsection"><a name="Dynamic_Fetch_Groups"></a><h3>Dynamic Fetch Groups</h3><p> + </p> + <p> + The <i>FetchPlan</i> applies not just to calls to <i>PersistenceManager.getObjectById()</i>, but also + to <i>PersistenceManager.newQuery()</i>, <i>PersistenceManager.getExtent()</i>, + <i>PersistenceManager.detachCopy</i> and much more besides. + </p> + <p> + You can read more about <b>named fetch-groups</b> and how to use it with + <a href="attach_detach.html"><b>attach/detach</b></a> + </p> + </div> + + <a name="dynamic"></a> + <div class="section"><h3>Dynamic Fetch Groups<a name="Dynamic_Fetch_Groups"></a></h3> + <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 class="source"><pre> + <div class="source"><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"); +// 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> - <p> +fp.addGroup("TestGroup");</pre></div> + <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><br></br></div><div class="subsection"><a name="Fetch_Depth"></a><h3>Fetch Depth</h3><p> + </p> + <br /> + </div> + + <div class="section"><h3>Fetch Depth<a name="Fetch_Depth"></a></h3> + <p> The basic fetch group defines which fields are to be fetched. It doesn't explicitly define how far down an object graph is to be fetched. JDO provides two ways of controlling this. - </p><p> - The first is to set the <B>maxFetchDepth</B> for the <I>FetchPlan</I>. This value specifies how + </p> + <p> + The first is to set the <b>maxFetchDepth</b> for the <i>FetchPlan</i>. 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's take an example </p> - <div class="source"><pre> + <div class="source"><pre> public class MyClass1 { MyClass2 field1; @@ -166,64 +397,86 @@ public class MyClass3 MyClass4 field3; ... }</pre></div> - <p> - and we want to detach <I>field1</I> of instances of <I>MyClass1</I>, down 2 levels - so detaching - the initial "field1" <I>MyClass2</I> object, and its "field2" <I>MyClass3</I> instance. So we + <p> + and we want to detach <i>field1</i> of instances of <i>MyClass1</i>, down 2 levels - so detaching + the initial "field1" <i>MyClass2</i> object, and its "field2" <i>MyClass3</i> instance. So we define our fetch-groups like this </p> - <div class="source"><pre> -<class name="MyClass1"> + <div class="source"><pre> +<class name="MyClass1"> ... - <fetch-group name="includingField1"> - <field name="field1"/> + <fetch-group name="includingField1"> + <field name="field1"/> </fetch-group> </class> -<class name="MyClass2"> +<class name="MyClass2"> ... - <fetch-group name="includingField2"> - <field name="field2"/> + <fetch-group name="includingField2"> + <field name="field2"/> </fetch-group> </class></pre></div> - <p> - and we then define the <B>maxFetchDepth</B> as 2, like this + <p> + and we then define the <b>maxFetchDepth</b> as 2, like this </p> - <div class="source"><pre>pm.getFetchPlan().setMaxFetchDepth(2);</pre></div> - <p> + <div class="source"><pre>pm.getFetchPlan().setMaxFetchDepth(2);</pre></div> + <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 + 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's take an example </p> - <div class="source"><pre> + <div class="source"><pre> public class Directory { Collection children; ... }</pre></div> - - <div class="source"><pre> -<class name="Directory"> - <field name="children"> - <collection element-type="Directory"/> + <div class="source"><pre> +<class name="Directory"> + <field name="children"> + <collection element-type="Directory"/> </field> - <fetch-group name="grandchildren"> - <field name="children" recursion-depth="2"/> + <fetch-group name="grandchildren"> + <field name="children" recursion-depth="2"/> </fetch-group> ... </class></pre></div> - <p> - So when we fetch a Directory, it will fetch 2 levels of the <I>children</I> field, hence fetching + <p> + So when we fetch a Directory, it will fetch 2 levels of the <i>children</i> field, hence fetching the children and the grandchildren. - </p></div><div class="subsection"><a name="Fetch_Size"></a><h3>Fetch Size</h3><p> + </p> + </div> + + <div class="section"><h3>Fetch Size<a name="Fetch_Size"></a></h3> + <p> A FetchPlan can also be used for defining the fetching policy when using queries. This can be set using </p> - <div class="source"><pre>pm.getFetchPlan().setFetchSize(value);</pre></div> - <p> - The default is <I>FetchPlan.FETCH_SIZE_OPTIMAL</I> which leaves it to DataNucleus to optimise the fetching + <div class="source"><pre>pm.getFetchPlan().setFetchSize(value);</pre></div> + <p> + The default is <i>FetchPlan.FETCH_SIZE_OPTIMAL</i> which leaves it to DataNucleus to optimise the fetching of instances. A positive value defines the number of instances to be fetched. Using - <I>FetchPlan.FETCH_SIZE_GREEDY</I> means that all instances will be fetched immediately. - </p></div></div></div></div><div class="clear"><hr></hr></div><table border="0" width="100%" cellpadding="0" cellspacing="0"><tr><td width="100%"><img height="1" border="0" width="100%" src="./images/header_line.gif"></img></td></tr></table><div id="footer"><div class="xright"><a href="http://www.apache.org/foundation/sponsorship.html";>Be an Apache Software Foundation sponsor</a><br><a href="http://www.apache.org/foundation/thanks.html";>Thanks to ASF sponsors!</a></br></div><div class="xleft"> - Documentation published: <br></br> - © 2005-2011 <a href="http://www.apache.org/";>Apache Software Foundation</a></div></div></body></html> \ No newline at end of file + <i>FetchPlan.FETCH_SIZE_GREEDY</i> means that all instances will be fetched immediately. + </p> + </div> + </div> + + + </div> + </div> + <div class="clear"> + <hr/> + </div> + <div id="footer"> + <div class="xright"> + Copyright © 2005-2013. + All Rights Reserved. + + </div> + <div class="clear"> + <hr/> + </div> + </div> + </body> +</html>
