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| <h3>Object Retrieval<a id="Object_Retrieval"></a></h3> |
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| --> |
| <div id="doc-content"> |
| <div class="sect1"> |
| <h2 id="index">Object Retrieval<a id="Object_Retrieval"></a></h2> |
| <div class="sectionbody"> |
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| <p>JDO provides persistence of objects. The logical next step after |
| persisting objects is to retrieve them for use in your application. |
| There are several ways to do this</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_retrieve_an_object_from_its_identity_a_id_retrieve_an_object_from_its_identity_a">Retrieve an object from its identity<a id="Retrieve_an_object_from_its_identity"></a></h3> |
| <div class="paragraph"> |
| <p>The simplest form of object retrieval is where we have the identity. |
| This is simply</p> |
| </div> |
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| <div class="content"> |
| <pre>Object obj = pm.getObjectById(identity);</pre> |
| </div> |
| </div> |
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| <p>If the object is in the JDO cache 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 Fetch Group.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_retrieve_an_object_based_on_its_extent_a_id_retrieve_an_object_based_on_its_extent_a">Retrieve an object based on its Extent<a id="Retrieve_an_object_based_on_its_Extent"></a></h3> |
| <div class="paragraph"> |
| <p>A persistable class can be persisted with an <strong>Extent</strong> of all instances |
| of that type. You can use this to retrieve objects of the required type, |
| like this</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Extent ex = pm.getExtent(MyClass.class, true); |
| Iterator iter = ex.iterator(); |
| while (iter.hasNext()) |
| { |
| MyClass obj = (MyClass)iter.next(); |
| ... |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The second argument in the <em>getExtent</em> call is whether to include |
| instances of subclasses.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_retrieve_an_object_based_on_a_criteria_a_id_retrieve_an_object_based_on_a_criteria_a">Retrieve an object based on a criteria<a id="Retrieve_an_object_based_on_a_criteria"></a></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. JDO2 provides 2 options here. JDOQL is |
| object-based and allows you to express your query in terms of the |
| classes and fields you are using. SQL is datastore-based and allows you |
| to express your query in terms of the datastore tables and columns.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To give an example of a JDOQL query</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Query q = pm.newQuery(MyClass.class, "field1 < value"); |
| q.declareParameters("int value"); |
| List results = q.execute(205); |
| Iterator iter = results.iterator(); |
| while (iter.hasNext()) |
| { |
| MyClass obj = (MyClass)iter.next(); |
| }</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> |
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