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| <h1 class="title">Event handling in Jena</h1> |
| |
| <h2 id="modelchangedlistener">ModelChangedListener</h2> |
| <p>In Jena it is possible to monitor a <code>Model</code> for changes, so that |
| code can be run after changes are applied without the coding for |
| that Model having to do anything special. We call these changes |
| “events”. This first |
| design and implementation is open for user comment and we may |
| refine or reduce the implementation as more experience is gained with |
| it.</p> |
| <p>To monitor a Model, you must <em>register</em> a <em>ModelChangedListener</em> |
| with that Model:</p> |
| <pre><code> Model m = ModelFactory.createDefaultModel(); |
| ModelChangedListener L = new MyListener(); |
| m.register( L ); |
| </code></pre> |
| <p><em>MyListener</em> must be an implementation of <em>ModelChangedListener</em>, |
| for example:</p> |
| <pre><code> class MyListener implements ModelChangedListener |
| { |
| public void addedStatement( Statement s ) |
| { System.out.println( ">> added statement " + s ); } |
| public void addedStatements( Statement [] statements ) {} |
| public void addedStatements( List statements ) {} |
| public void addedStatements( StmtIterator statements ) {} |
| public void addedStatements( Model m ) {} |
| public void removedStatement( Statement s ) {} |
| public void removedStatements( Statement [] statements ) {} |
| public void removedStatements( List statements ) {} |
| public void removedStatements( StmtIterator statements ) {} |
| public void removedStatements( Model m ) {} |
| } |
| </code></pre> |
| <p>This listener ignores everything except the addition of single |
| statements to <em>m</em>; those it prints out. The listener has a method |
| for each of the ways that statements can be added to a Model:</p> |
| <ul> |
| <li>as a single statement, <code>Model::add(Statement)</code></li> |
| <li>as an element of an array of statements, |
| <code>Model::add(Statement[])</code></li> |
| <li>as an element of a list of statements, <code>Model::add(List)</code></li> |
| <li>as an iterator over statements, <code>Model::add(StmtIterator)</code></li> |
| <li>as part of another Model, <code>Model::add(Model)</code></li> |
| </ul> |
| <p>(Similarly for <em>delete</em>.)</p> |
| <p>The listener method is called when the statement(s) have been added |
| to the Model, if no exceptions have been thrown. It does not matter |
| if the statement was <em>already</em> in the Model or not; it is the act |
| of adding it that fires the listener.</p> |
| <p>There is no guarantee that the statement, array, list, or model |
| that is added or removed is the same one that is passed to the |
| appropriate listener method, and the <em>StmtIterator</em> will never be |
| the same one. However, in the current design:</p> |
| <ul> |
| <li>a single Statement will be .equals to the original Statement</li> |
| <li>a List will be .equals to the original List</li> |
| <li>a Statement[] will be the same length and have .equal elements |
| in the same order</li> |
| <li>a StmtIterator will deliver .equal elements in the same order</li> |
| <li>a Model will contain the same statements</li> |
| </ul> |
| <p>We advise not relying on these ordering properties; instead assume |
| that for any bulk update operation on the model, the listener will |
| be told the method of update and the statements added or removed, |
| but that the order may be different and duplicate statements may |
| have been removed. |
| Note in particular that a Model with any Listeners will have to |
| record the complete contents of any <em>StmtIterator</em> that is added or |
| removed to the model, so that the Model and the Listener can both |
| see all the statements.</p> |
| <p>Finally, there is no guarantee that <em>only</em> Statements etc added |
| through the Model API will be presented to the listener; any |
| Triples added to its underlying Graph will also be presented to the |
| listener as statements.</p> |
| <h2 id="utility-classes">Utility classes</h2> |
| <p>The full Listener API is rather chunky and it can be inconvenient |
| to use, especially for the creation of inline classes. There are |
| four utility classes in <em>org.apache.jena.rdf.listeners:</em></p> |
| <ul> |
| <li><em>NullListener</em>. This class’s methods do nothing. This is useful |
| when you want to subclass and intercept only specific ways of |
| updating a Model.</li> |
| <li><em>ChangedListener</em>. This class only records whether some change |
| has been made, but not what it is. The method <em>hasChanged()</em> |
| returns <em>true</em> if some change has been made since the last call of |
| <em>hasChanged()</em> [or since the listener was created].</li> |
| <li><em>StatementListener</em>. This class translates all bulk update |
| calls (ie the ones other than <em>addedStatement()</em> and |
| <em>removedStatement()</em>) into calls to |
| <em>addedStatement()/removedStatement()</em> for each Statement in the |
| collection. This allows statements to be tracked whether they are |
| added one at a time or in bulk.</li> |
| <li><em>ObjectListener</em>. This class translates all the listener calls |
| into <em>added(Object)</em> or <em>removed(Object)</em> as appropriate; it is |
| left to the user code to distinguish among the types of argument.</li> |
| </ul> |
| <h2 id="when-listeners-are-called">When listeners are called</h2> |
| <p>In the current implementation, listener methods are called |
| immediately the additions or removals are made, in the same thread |
| as the one making the update. If a model has multiple listeners |
| registered, the order in which they are informed about an update is |
| unspecified and may change from update to update. If any listener |
| throws an exception, that exception is thrown through the update |
| call, and other listeners may not be informed of the update. |
| Hence listener code should be brief and exception-free if at all |
| possible.</p> |
| <h2 id="registering-and-unregistering">Registering and unregistering</h2> |
| <p>A listener may be registered with the same model multiple times. If |
| so, it will be invoked as many times as it is registered for each |
| update event on the model.</p> |
| <p>A listener <em>L</em> may be <em>unregistered</em> from a Model using the method |
| <code>unregister(L)</code>. If <em>L</em> is not registered with the model, nothing |
| happens.</p> |
| <p>If a listener is registered multiple times with the same model, |
| each <code>unregister()</code> for that listener will remove just one of the |
| registrations.</p> |
| <h2 id="transactions-and-databases">Transactions and databases</h2> |
| <p>In the current design, listeners are not informed of transaction |
| boundaries, and all events are fed to listeners as soon as they |
| happen.</p> |
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