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| <h3>Persistence Manager<a id="Persistence_Manager"></a></h3> |
| </div> |
| --> |
| <div id="doc-content"> |
| <div class="sect1"> |
| <h2 id="index">Persistence Manager<a id="Persistence_Manager"></a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Any JDO-enabled application will require at least one |
| <em>PersistenceManager</em> (PM). This is obtained from the |
| <a href="pmf.html">PersistenceManagerFactory</a> for the datastore.</p> |
| </div> |
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| <p>The simplest way of creating a <em>PersistenceManager</em> |
| <a href="api20/apidocs/javax/jdo/PersistenceManager.html"><span class="image"><img src="images/javadoc.png" alt="image"></span></a> |
| is as follows</p> |
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| <div class="literalblock"> |
| <div class="content"> |
| <pre>PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(props); |
| PersistenceManager pm = pmf.getPersistenceManager();</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>A <em>PersistenceManager</em> is the key to all persistence operations in JDO. |
| With it you can persist, update, delete, and retrieve objects from the |
| datastore. A <em>PersistenceManager</em> has a single transaction.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_persist_objects_a_id_persist_objects_a">Persist Objects<a id="Persist_Objects"></a></h3> |
| <div class="paragraph"> |
| <p>To persist an object, the object must first be marked as persistable |
| using <a href="metadata.html">MetaData (XML/Annotations)</a>. Then you would |
| start the PM transaction, and use <em>makePersistent</em> as follows</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>PersistenceManager pm = pmf.getPersistenceManager(); |
| Transaction tx = pm.currentTransaction(); |
| try |
| { |
| // Start the transaction |
| tx.begin(); |
| |
| // Create the object to persist |
| MyClass obj = new MyClass(); |
| |
| // Persist it to the datastore |
| pm.makePersistent(obj); |
| |
| // Commit the transaction, flushing the object to the datastore |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| ... handle exceptions |
| } |
| finally |
| { |
| if (tx.isActive()) |
| { |
| // Error occurred so rollback the transaction |
| tx.rollback(); |
| } |
| pm.close(); |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The <em>makePersistent</em> method of <strong>PersistenceManager</strong> makes the object |
| persistent in the datastore, and updates the 'state' of the object from |
| <em>Transient</em> (at the start) to <em>Hollow</em> (after commit() of the |
| transaction).</p> |
| </div> |
| <div class="paragraph"> |
| <p>When an object is persisted, if it has any other objects referenced from |
| that object they also will be made persistent. This is referred to as |
| <strong>persistence-by-reachability</strong>. The main benefit of this is that if you |
| have an object graph to persist, then you don’t need to call |
| <em>makePersistent()</em> on all objects, instead just using one that can be |
| used to find all of the others. <strong>persistence-by-reachability</strong> is also |
| run at the time of calling <em>commit()</em> on the transaction. This has the |
| effect that if you had called <em>makePersistent()</em> on an object and that |
| had persisted another object, and before commit you had removed the |
| relation to this other object, then at <em>commit()</em> the reachability |
| algorithm will find that this other object is no longer reachable and |
| will remove it from persistence.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_retrieve_objects_a_id_retrieve_objects_a">Retrieve Objects<a id="Retrieve_Objects"></a></h3> |
| <div class="paragraph"> |
| <p>So we’ve made some of our objects persistent, and now we want to |
| retrieve them in our application. Here’s one way of retrieving objects |
| of a particular type.</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>tx = pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| Extent e = pm.getExtent(mydomain.MyClass.class, true); |
| Iterator iter=e.iterator(); |
| while (iter.hasNext()) |
| { |
| MyClass my_obj=(MyClass)iter.next(); |
| ... |
| } |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The <strong>Extent</strong> interface is one of the ways to retrieve your objects. The |
| others use the <strong>Query</strong> interface, allowing more precise filtering over |
| the objects returned.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_update_objects_a_id_update_objects_a">Update Objects<a id="Update_Objects"></a></h3> |
| <div class="paragraph"> |
| <p>To update an object we firstly retrieve it, as above, and then we call |
| any of its mutator methods. For example</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>tx = pm.currentTransaction(); |
| try |
| { |
| tx.begin(); |
| |
| Extent e = pm.getExtent(mydomain.MyClass.class, true); |
| Iterator iter=e.iterator(); |
| while (iter.hasNext()) |
| { |
| MyClass my_obj=(MyClass)iter.next(); |
| my_obj.setValue(25.0); // Change the value |
| ... |
| } |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>When <em>setValue()</em> is called on the persistent object this change is |
| intercepted by JDO and the value change will be automatically sent to |
| the datastore …​ transparently!</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_delete_objects_a_id_delete_objects_a">Delete Objects<a id="Delete_Objects"></a></h3> |
| <div class="paragraph"> |
| <p>So we can persist objects, and retrieve them. Now we want to remove one |
| from persistence.</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>try |
| { |
| tx = pm.currentTransaction(); |
| tx.begin(); |
| |
| ... (code to retrieve object in question) ... |
| |
| pm.deletePersistent(my_obj); |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_making_an_object_transient_a_id_making_an_object_transient_a">Making an object transient<a id="Making_an_object_transient"></a></h3> |
| <div class="paragraph"> |
| <p>As we have seen in the <a href="state_transition.html">JDO States guide</a>, an |
| object can have many possible states. When we want to take an object and |
| work on it, but removing its identity we can make it <strong>transient</strong>. This |
| means that it will retain the values of its fields, yet will no longer |
| be associated with the object in the datastore. We do this as follows</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>try |
| { |
| tx = pm.currentTransaction(); |
| tx.begin(); |
| |
| ... (code to retrieve object in question) ... |
| |
| pm.makeTransient(my_obj); |
| |
| tx.commit(); |
| } |
| catch (Exception e) |
| { |
| if (tx.isActive()) |
| { |
| tx.rollback(); |
| } |
| } |
| |
| ... (code to work on "my_obj")</pre> |
| </div> |
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