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| <h3>Object Retrieval</h3> |
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| <div id="doc-content"> |
| <div class="sect1"> |
| <h2 id="Object_Retrieval">Object Retrieval</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Any application will require access to persisted objects. |
| The JDO API provides several ways to do this</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="get_object_by_id">Retrieve an object from its identity</h3> |
| <div class="paragraph"> |
| <p>The simplest form of object retrieval is where we have the identity. |
| This is simply</p> |
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| <div class="listingblock"> |
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| <pre class="CodeRay highlight"><code data-lang="java">Person per = (Person)pm.getObjectById(identity);</code></pre> |
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| <p>or</p> |
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| <div class="content"> |
| <pre class="CodeRay highlight"><code data-lang="java">Person per = pm.getObjectById(Person.class, identity);</code></pre> |
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| <p>If the object is in the JDO cache (Level 1 or Level 2) then it is retrieved from there, |
| otherwise the JDO implementation goes to the datastore. |
| When the object is retrieved its fields are populated according to its <a href="fetchgroups.html">Fetch Plan</a>.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="extent">Retrieve an object based on its Extent</h3> |
| <div class="paragraph"> |
| <p>An <strong>Extent</strong> is a collection of objects of a particular type of object that have been persisted. |
| When you define the MetaData for a class you can define if the class requires an Extent. |
| The default is true. You access the Extent as follows</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="CodeRay highlight"><code data-lang="java">Extent ex = pm.getExtent(MyClass.class, <span class="predefined-constant">true</span>); |
| <span class="predefined-type">Iterator</span> iter = ex.iterator(); |
| <span class="keyword">while</span> (iter.hasNext()) |
| { |
| MyClass obj = (MyClass)iter.next(); |
| ... |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The second argument in the <em>getExtent</em> call is whether to include instances of subclasses. |
| An Extent is useful where you want to restrict a Query to query over just that set of objects. |
| It can also be used where you just want to retrieve all persisted objects of a type (as an alternative to using a Query).</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="query">Retrieve an object based on a query criteria</h3> |
| <div class="paragraph"> |
| <p>Where we want to retrieve all objects based on some criteria (e.g all objects of class A where field 'x' of A is a certain value) we need to |
| use a query language and the <a href="query_api.html">JDO Query API</a>. |
| The JDO API provides the <a href="jdoql.html">JDOQL</a> object-based query language where you express your query in terms of classes and fields you are using. |
| Dependent on your JDO provider and the datastore being used you may also be able to use a (native) query language such as <a href="query_sql.html">SQL</a>, expressing |
| your query in terms of datastore tables/columns.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To give an example of a JDOQL query</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="CodeRay highlight"><code data-lang="java"><span class="predefined-type">Query</span> q = pm.newQuery(MyClass.class, <span class="string"><span class="delimiter">"</span><span class="content">field1 < value</span><span class="delimiter">"</span></span>); |
| q.declareParameters(<span class="string"><span class="delimiter">"</span><span class="content">int value</span><span class="delimiter">"</span></span>); |
| <span class="predefined-type">List</span> results = q.execute(<span class="integer">205</span>); |
| <span class="predefined-type">Iterator</span> iter = results.iterator(); |
| <span class="keyword">while</span> (iter.hasNext()) |
| { |
| MyClass obj = (MyClass)iter.next(); |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>If the objects found by the query are in the JDO cache then they are |
| retrieved from there, otherwise the JDO implementation goes to the datastore. |
| When the objects are retrieved their fields are populated according to the Fetch Group.</p> |
| </div> |
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