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|  | <h1 class="title">Event handling in Jena</h1> | 
|  |  | 
|  |  | 
|  | <main class="d-flex flex-xl-row flex-column"> | 
|  |  | 
|  | <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> | 
|  | <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> | 
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|  | <ul> | 
|  | <li><a href="#modelchangedlistener">ModelChangedListener</a></li> | 
|  | <li><a href="#utility-classes">Utility classes</a></li> | 
|  | <li><a href="#when-listeners-are-called">When listeners are called</a></li> | 
|  | <li><a href="#registering-and-unregistering">Registering and unregistering</a></li> | 
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|  | <article class="flex-column me-lg-4"> | 
|  | <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> | 
|  | <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Model<span style="color:#bbb"> </span>m<span style="color:#bbb"> </span><span style="color:#666">=</span><span style="color:#bbb"> </span>ModelFactory.<span style="color:#b44">createDefaultModel</span>();<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span>ModelChangedListener<span style="color:#bbb"> </span>L<span style="color:#bbb"> </span><span style="color:#666">=</span><span style="color:#bbb"> </span><span style="color:#a2f;font-weight:bold">new</span><span style="color:#bbb"> </span>MyListener();<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span>m.<span style="color:#b44">register</span>(<span style="color:#bbb"> </span>L<span style="color:#bbb"> </span>);<span style="color:#bbb"> | 
|  | </span></span></span></code></pre></div><p><em>MyListener</em> must be an implementation of <em>ModelChangedListener</em>, | 
|  | for example:</p> | 
|  | <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">class</span> <span style="color:#00f">MyListener</span><span style="color:#bbb"> </span><span style="color:#a2f;font-weight:bold">implements</span><span style="color:#bbb"> </span>ModelChangedListener<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span>{<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">addedStatement</span>(<span style="color:#bbb"> </span>Statement<span style="color:#bbb"> </span>s<span style="color:#bbb"> </span>)<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">        </span>{<span style="color:#bbb"> </span>System.<span style="color:#b44">out</span>.<span style="color:#b44">println</span>(<span style="color:#bbb"> </span><span style="color:#b44">">> added statement "</span><span style="color:#bbb"> </span><span style="color:#666">+</span><span style="color:#bbb"> </span>s<span style="color:#bbb"> </span>);<span style="color:#bbb"> </span>}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">addedStatements</span>(<span style="color:#bbb"> </span>Statement<span style="color:#bbb"> </span><span style="color:#666">[]</span><span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">addedStatements</span>(<span style="color:#bbb"> </span>List<span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">addedStatements</span>(<span style="color:#bbb"> </span>StmtIterator<span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">addedStatements</span>(<span style="color:#bbb"> </span>Model<span style="color:#bbb"> </span>m<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">removedStatement</span>(<span style="color:#bbb"> </span>Statement<span style="color:#bbb"> </span>s<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">removedStatements</span>(<span style="color:#bbb"> </span>Statement<span style="color:#bbb"> </span><span style="color:#666">[]</span><span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">removedStatements</span>(<span style="color:#bbb"> </span>List<span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">removedStatements</span>(<span style="color:#bbb"> </span>StmtIterator<span style="color:#bbb"> </span>statements<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb">    </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#0b0;font-weight:bold">void</span><span style="color:#bbb"> </span><span style="color:#00a000">removedStatements</span>(<span style="color:#bbb"> </span>Model<span style="color:#bbb"> </span>m<span style="color:#bbb"> </span>)<span style="color:#bbb"> </span>{}<span style="color:#bbb"> | 
|  | </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span>}<span style="color:#bbb"> | 
|  | </span></span></span></code></pre></div><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> | 
|  |  | 
|  | </article> | 
|  |  | 
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|  | <li><a href="#utility-classes">Utility classes</a></li> | 
|  | <li><a href="#when-listeners-are-called">When listeners are called</a></li> | 
|  | <li><a href="#registering-and-unregistering">Registering and unregistering</a></li> | 
|  | <li><a href="#transactions-and-databases">Transactions and databases</a></li> | 
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