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| <h3>JDO 3.0 Overview<a id="JDO_3.0_Overview"></a></h3> |
| </div> |
| --> |
| <div id="doc-content"> |
| <div class="sect1"> |
| <h2 id="index">JDO 3.0 Overview<a id="JDO_3.0_Overview"></a></h2> |
| <div class="sectionbody"> |
| <div class="sect2"> |
| <h3 id="_background_a_id_background_a">Background<a id="Background"></a></h3> |
| <div class="paragraph"> |
| <p>Java Data Objects (JDO) is a specification begun in 2000, with 2 major |
| releases JDO1 (2002 under JSR0012) and JDO2 (2006 under JSR0243). It was |
| placed under Apache in 2005 and is the rare example of a specification |
| that has undergone continual improvement during its lifetime, for the |
| last 4 years being developed totally in the open, accepting input from |
| everyone.</p> |
| </div> |
| <div class="paragraph"> |
| <p>JDO 3.0 was started in October 2008, and encompasses additions to the |
| specification in the areas of a metadata API, an enhancer API, addition |
| of cancel/timeout control to queries, and addition of control to the |
| locking of objects when read.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_metadata_api_a_id_metadata_api_a">Metadata API<a id="Metadata_API"></a></h3> |
| <div class="paragraph"> |
| <p>To persist Java objects you need to specify which classes are |
| persistable, and how they are persisted. This was traditionally handled |
| using XML configuration. With the advent of JDK1.5, annotations were |
| added as another possible way of defining such information. JDO 3.0 |
| takes this further and provides a <strong>Metadata API</strong>, allowing runtime |
| definition. This is of particular use for systems that don’t know at |
| application startup which classes should be persistable, maybe because |
| the class hasn’t been written yet.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To demonstrate the Metadata API, lets assume that we have a |
| PersistenceManagerFactory created for our datastore. So we request a new |
| Metadata object.</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(props); |
| JDOMetaData jdomd = pmf.newMetadata();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we can now start defining the metadata for the package/class(es) we |
| want to persist. The Metadata is structured in a similar way to the XML |
| DTD/XSD. So let’s add a class</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>ClassMetadata cmd = jdomd.newClassMetadata("test.Client"); |
| cmd.setTable("CLIENT").setDetachable(true).setIdentityType(javax.jdo.annotations.IdentityType.DATASTORE); |
| cmd.setPersistenceModifier(javax.jdo.metadata.ClassPersistenceModifier.PERSISTENCE_CAPABLE);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we have a class <em>test.Client</em> using datastore-identity, that is |
| detachable, and is persisted to a table <em>CLIENT</em>. As you can see, you |
| can chain setters for convenient coding.</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>InheritanceMetadata inhmd = cmd.newInheritanceMetadata(); |
| inhmd.setStrategy(javax.jdo.annotations.InheritanceStrategy.NEW_TABLE); |
| DiscriminatorMetadata dmd = inhmd.newDiscriminatorMetadata(); |
| dmd.setColumn("disc").setValue("Client").setStrategy(javax.jdo.annotations.DiscriminatorStrategy.VALUE_MAP); |
| dmd.setIndexed(Indexed.TRUE); |
| |
| VersionMetadata vermd = cmd.newVersionMetadata(); |
| vermd.setStrategy(javax.jdo.annotations.VersionStrategy.VERSION_NUMBER).setColumn("version"); |
| vermd.setIndexed(Indexed.TRUE);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we will use "new-table" inheritance for this class, and it will have |
| a discriminator stored in column <em>disc</em> of type "value-map". The class |
| will also be versioned, using column <em>version</em>, that is indexed. All of |
| this was for the class as a whole, so let’s look at the |
| fields/properties of the class.</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>FieldMetadata fmd = cmd.newFieldMetadata("name"); |
| fmd.setNullValue(javax.jdo.annotations.NullValue.DEFAULT).setColumn("name").setIndexed(true).setUnique(true);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we have a field <em>name</em> that is persisted into column <em>name</em>, and is |
| unique and indexed. The API metadata components all follow the DTD as |
| stated earlier, so if our field was a collection we could then define |
| <em>CollectionMetadata</em> below it.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The only thing left to do is register the metadata with the |
| PersistenceManagerFactory, like this</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>pmf.registerMetadata(jdomd);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>and any contact with the class will now persist according to this API.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You can similarly browse already registered metadata using</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>ComponentMetadata compmd = pmf.getMetadata("mydomain.MyClass");</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Note that you cannot change already registered metadata with JDO 3.0.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You can view the Javadocs for the Metadata API |
| <a href="http://db.apache.org/jdo/api30/apidocs/javax/jdo/metadata/package-summary.html">here</a>.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_enhancer_api_a_id_enhancer_api_a">Enhancer API<a id="Enhancer_API"></a></h3> |
| <div class="paragraph"> |
| <p>JDO implementations typically (but aren’t compelled to) include a |
| bytecode enhancement step, allowing for efficient change detection of |
| objects. While the Metadata API above is very useful, if we just define |
| metadata for a class we still need to enhance the class using this |
| metadata. This is where the <strong>Enhancer API</strong> comes in. To start we need to |
| get a JDOEnhancer</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>JDOEnhancer enhancer = JDOHelper.getEnhancer();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>and now that we have the enhancer and want to enhance our class above so |
| we need to register our new metadata with it (generate the metadata as |
| shown above)</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>enhancer.registerMetadata(jdomd);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Now we can handle the enhancement using a separate class loader if |
| required (for example if the classes were defined dynamically, e.g by |
| ASM)</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>enhancer.setClassLoader(myClassLoader);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Finally we select what to enhance, and perform the enhancement</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>String[] classes = {"test.Client"}; |
| enhancer.addClasses(classes); |
| enhancer.enhance();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So the class is now enhanced and is ready for use.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You can view the Javadocs for the Enhancer API |
| <a href="http://db.apache.org/jdo/api30/apidocs/index.html">here</a>.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_query_cancel_timeout_api_a_id_query_canceltimeout_api_a">Query Cancel/Timeout API<a id="Query_CancelTimeout_API"></a></h3> |
| <div class="paragraph"> |
| <p>On occasions a query may be inefficient, or may suffer from problems in |
| the underlying datastore, and so we don’t want to affect the |
| application. In this case it would make sense to have control over a |
| timeout for the query, or be able to cancel it. JDO 3.0 introduces the |
| Query cancel/timeout control, via the following new methods to |
| <em>javax.jdo.Query</em></p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>void setTimeoutMillis(Integer interval); |
| Integer getTimeoutMillis(); |
| void cancelAll(); |
| void cancel(Thread thread);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we have the ability to cancel a query as required, or just let it |
| timeout.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_control_of_read_objects_locking_a_id_control_of_read_objects_locking_a">Control of read objects locking<a id="Control_of_read_objects_locking"></a></h3> |
| <div class="paragraph"> |
| <p>When we are using datastore (pessimistic) transactions it often doesn’t |
| make sense to just lock all objects read in the transaction. For this |
| reason JDO 3.0 introduces control over which objects are locked and |
| which aren’t.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In metadata for each class you can specify the "serialize-read" setting. |
| True will mean that objects of this type will be locked when read.</p> |
| </div> |
| <div class="paragraph"> |
| <p>On a Transaction you can override the metadata settings via the |
| following method</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>void setSerializeRead(Boolean serialize); |
| Boolean getSerializeRead();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>On a Query you can override the metadata and Transaction settings via |
| the following method</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>void setSerializeRead(Boolean serialize); |
| Boolean getSerializeRead();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This concludes our simple overview of JDO3. We hope you enjoy using it</p> |
| </div> |
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