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| <h3>JDO Attach/Detach<a id="JDO_AttachDetach"></a></h3> |
| </div> |
| --> |
| <div id="doc-content"> |
| <div class="sect1"> |
| <h2 id="index">JDO Attach/Detach<a id="JDO_AttachDetach"></a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>JDO provides an interface to the persistence of objects. JDO 1.0 doesn’t |
| provide a way of taking an object that was just persisted and just work |
| on it and update the persisted object later. The user has to copy the |
| fields manually and copy them back to the persisted object later. JDO |
| 2.0 introduces a new way of handling this situation, by <strong>detaching</strong> an |
| object from the persistence graph, allowing it to be worked on in the |
| users application. It can then be <strong>attached</strong> to the persistence graph |
| later. Please refer to <a href="state_transition.html">Object Lifecycle</a> for |
| where this fits in. The first thing to do to use a class with this |
| facility is to tag it as "detachable". This is done by adding the |
| attribute</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre><class name="MyClass" detachable="true"></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This acts as an instruction to the <a href="enhancement.html">enhancement |
| process</a> to add methods necessary to utilise the attach/detach process.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The following code fragment highlights how to use the attach/detach |
| mechanism</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Product working_product=null; |
| Transaction tx=pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| Product prod=new Product(name,description,price); |
| pm.makePersistent(prod); |
| |
| // Detach the product for use |
| working_product = (Product)pm.detachCopy(prod); |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| // Handle the exception |
| } |
| finally |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| } |
| |
| // Work on the detached object in our application |
| working_product.setPrice(new_price); |
| |
| ... |
| |
| // Reattach the updated object |
| tx = pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| Product attached_product = pm.makePersistent(working_product); |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| // Handle the exception |
| } |
| finally |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So we now don’t need to do any manual copying of object fields just |
| using a simple call to detach the object, and then attach it again |
| later. Here are a few things to note with <em>attach/detach</em> :</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Calling <em>detachCopy</em> on an object that is not detachable will return a |
| <strong>transient</strong> instance that is a COPY of the original, so use the COPY |
| thereafter.</p> |
| </li> |
| <li> |
| <p>Calling <em>detachCopy</em> on an object that is detachable will return a |
| <strong>detached</strong> instance that is a COPY of the original, so use this COPY |
| thereafter</p> |
| </li> |
| <li> |
| <p>A <em>detached</em> object retain the id of its datastore entity. Detached |
| objects should be used where you want to update the objects and attach |
| them later (updating the associated object in the datastore. If you want |
| to create copies of the objects in the datastore with their own |
| identities you should use <em>makeTransient</em> instead of <em>detachCopy</em>.</p> |
| </li> |
| <li> |
| <p>Calling <em>detachCopy</em> will detach all fields of that object that are in |
| the current <a href="fetchgroups.html">Fetch Group</a> for that class for that |
| <em>PersistenceManager</em>.</p> |
| </li> |
| <li> |
| <p>By default the fields of the object that will be detached are those in |
| the <em>Default Fetch Group</em>.</p> |
| </li> |
| <li> |
| <p>You should choose your <a href="fetchgroups.html">Fetch Group</a> carefully, |
| bearing in mind which object(s) you want to access whilst detached. |
| Detaching a relation field will detach the related object as well.</p> |
| </li> |
| <li> |
| <p>If you don’t detach a field of an object, you <span class="underline">cannot</span> |
| access the value for that field while the object is detached.</p> |
| </li> |
| <li> |
| <p>If you don’t detach a field of an object, you <span class="underline">can</span> update |
| the value for that field while detached, and thereafter you can access |
| the value for that field.</p> |
| </li> |
| <li> |
| <p>Calling <em>makePersistent</em> will return an (attached) copy of the |
| detached object. It will attach all fields that were originally |
| detached, and will also attach any other fields that were modified |
| whilst detached.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p><a id="detach_all_on_commit"></a></p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_detach_all_on_commit_a_id_detach_all_on_commit_a">Detach All On Commit<a id="Detach_All_On_Commit"></a></h3> |
| <div class="paragraph"> |
| <p>JDO2 also provides a mechanism whereby all objects that were enlisted in |
| a transaction are automatically detached when the transaction is |
| committed. You can enable this in one of 3 ways. If you want to use this |
| mode globally for all <em>PersistenceManager</em>s (PMs) from a |
| <em>PersistenceManagerFactory</em> (PMF) you could either set the PMF property |
| "datanucleus.DetachAllOnCommit", or you could create your PMF and call |
| the PMF method <strong>setDetachAllOnCommit(true)</strong>. If instead you wanted to |
| use this mode only for a particular PM, or only for a particular |
| transaction for a particular PM, then you can call the PM method |
| <strong>setDetachAllOnCommit(true)</strong> before the commit of the transaction, and |
| it will apply for all transaction commits thereafter, until turned off |
| (<strong>setDetachAllOnCommit(false)</strong>. Here’s an example</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>// Create a PMF |
| ... |
| |
| // Create an object |
| MyObject my = new MyObject(); |
| |
| PersistenceManager pm = pmf.getPersistenceManager(); |
| Transaction tx = pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| // We want our object to be detached when it's been persisted |
| pm.setDetachAllOnCommit(true); |
| |
| // Persist the object that we created earlier |
| pm.makePersistent(my); |
| |
| tx.commit(); |
| // The object "my" is now in detached state and can be used further |
| } |
| finally |
| { |
| if (tx.isActive) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| <div class="paragraph"> |
| <p><a id="copy_on_attach"></a></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_copy_on_attach_a_id_copy_on_attach_a">Copy On Attach<a id="Copy_On_Attach"></a></h3> |
| <div class="paragraph"> |
| <p>By default when you are attaching a detached object it will return an |
| attached copy of the detached object. JDO2.1 provides a new feature that |
| allows this attachment to just migrate the existing detached object into |
| attached state.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You enable this by setting the <em>PersistenceManagerFactory</em> (PMF) |
| property <strong>datanucleus.CopyOnAttach</strong> to false. Alternatively you can use |
| the methods <em>PersistenceManagerFactory.setCopyOnAttach(boolean flag)</em> or |
| <em>PersistenceManager.setCopyOnAttach(boolean flag)</em>. If we return to the |
| example at the start of this page, this now becomes</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>// Reattach the updated object |
| pm.setCopyOnAttach(false); |
| tx = pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| // working product is currently in detached state |
| |
| pm.makePersistent(working_product); |
| // working_product is now in persistent (attached) state |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| // Handle the exception |
| } |
| finally |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Please note that if you try to attach two detached objects representing |
| the same underlying persistent object within the same transaction (i.e a |
| persistent object with the same identity already exists in the level 1 |
| cache), then a JDOUserException will be thrown.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_serialization_of_detachable_classes_a_id_serialization_of_detachable_classes_a">Serialization of Detachable classes<a id="Serialization_of_Detachable_classes"></a></h3> |
| <div class="paragraph"> |
| <p>During enhancement of Detachable classes, a field called |
| <em>jdoDetachedState</em> is added to the class definition. This field allows |
| reading and changing tracking of detached objects while they are not |
| managed by a PersistenceManager.</p> |
| </div> |
| <div class="paragraph"> |
| <p>When serialization occurs on a Detachable object, the <em>jdoDetachedState</em> |
| field is written to the serialized object stream. On deserialize, this |
| field is written back to the new deserialized instance. This process |
| occurs transparently to the application. However, if deserialization |
| occurs with an un-enhanced version of the class, the detached state is |
| lost.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Serialization and deserialization of Detachable classes and un-enhanced |
| versions of the same class is only possible if the field |
| <em>serialVersionUID</em> is added. It’s recommended during development of the |
| class, to define the <em>serialVersionUID</em> and make the class to implement |
| the <em>java.io.Serializable</em> interface, as the following example:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>class MyClass implements java.io.Serializable |
| { |
| private static final long serialVersionUID = 2765740961462495537L; // any random value here |
| |
| //.... other fields |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| </div> |
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