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| <h1 class="title">Apache Jena Initialization</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> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#initialization-code">Initialization code</a></li> |
| <li><a href="#the-initialization-process">The Initialization Process</a></li> |
| <li><a href="#debugging">Debugging</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>Jena has an initialization sequence that is |
| used to setup components available at runtime.</p> |
| <p>Application code is welcome to also use this mechanism. This |
| must be done with care. During Jena initialization, there can be |
| visibility of uninitialized data in class static members.</p> |
| <p>The standard initialization sequence is<br> |
| Core -> RIOT -> ARQ -> TDB -> other (including jena text)</p> |
| <p>The sequence from 0 to level 500 is the Jena platform |
| initialization. Application may use the jena initialization mechanism and it is |
| recommended to place initialization code above level 500.</p> |
| <p>Initialization occurs when <code>JenaSystem.init()</code> is first called. Jena ensures that this |
| is done when the application first uses any Jena code by using class |
| initializers.</p> |
| <p>Application can call <code>JenaSystem.init()</code>.</p> |
| <p>See <a href="jena-repack.html">notes on repacking Jena code</a> for how to deal |
| with <code>ServiceLoader</code> files in repacked jars.</p> |
| <h2 id="initialization-code">Initialization code</h2> |
| <p>Initialization code is an implementation of <code>JenaSubsystemLifecycle</code> which |
| itself extends <code>SubsystemLifecycle</code>.</p> |
| <p>For use in the default initialization, the class must have a zero-argument |
| constructor and implement:</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:#bbb"> </span><span style="color:#a2f;font-weight:bold">public</span><span style="color:#bbb"> </span><span style="color:#a2f;font-weight:bold">interface</span> <span style="color:#00f">JenaSubsystemLifecycle</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">start</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">stop</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">default</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">int</span><span style="color:#bbb"> </span><span style="color:#00a000">level</span>()<span style="color:#bbb"> </span>{<span style="color:#bbb"> </span><span style="color:#a2f;font-weight:bold">return</span><span style="color:#bbb"> </span>9999<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>The code should supply a level, indicating its place in the order of initialization. |
| The levels used by Jena are:</p> |
| <ul> |
| <li>0 - reserved</li> |
| <li>10 - Used by jena-core</li> |
| <li>15 - CLI Commands registry</li> |
| <li>20 - RIOT</li> |
| <li>30 - ARQ</li> |
| <li>40 - Text indexing</li> |
| <li>40 - TDB1</li> |
| <li>42 - TDB2</li> |
| <li>60 - Additional HTTP configuration</li> |
| <li>60 - RDFPatch</li> |
| <li>96 - SHACL</li> |
| <li>96 - ShEx</li> |
| <li>101 - Fuseki</li> |
| <li>9999 - Default.</li> |
| </ul> |
| <p>Levels up to 500 are considered to be “Jena system level”, Application code |
| should use level above 500.</p> |
| <p>Fuseki initialization includes <a href="/documentation/fuseki2/fuseki-modules">Fuseki |
| Modules</a> which uses <code>SubsystemLifecycle</code> |
| with a different Java interface.</p> |
| <h2 id="the-initialization-process">The Initialization Process</h2> |
| <p>The process followed by <code>JenaSystem.init()</code> is to load all java <code>ServiceLoader</code> |
| registered <code>JenaSubsystemLifecycle</code>, sort into level order, then call <code>init</code> on |
| each initialization object. Initialization code at the same level may be called |
| in any order and that order may be different between runs.</p> |
| <p>Only the first call of <code>JenaSystem.init()</code> causes the process to run. |
| Any subsequent calls are cheap, so calling <code>JenaSystem.init()</code> |
| when in doubt about the initialization state is safe.</p> |
| <p>Overlapping concurrent calls to <code>JenaSystem.init()</code> are thread-safe. |
| On a return from <code>JenaSystem.init()</code>, Jena has been initialized at some point.</p> |
| <h2 id="debugging">Debugging</h2> |
| <p>There is a flag <code>JenaSystem.DEBUG_INIT</code> to help with development. It is not |
| intended for runtime logging.</p> |
| <p>Jena components print their initialization beginning and end points on |
| <code>System.err</code> to help track down ordering issues.</p> |
| |
| </article> |
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| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#initialization-code">Initialization code</a></li> |
| <li><a href="#the-initialization-process">The Initialization Process</a></li> |
| <li><a href="#debugging">Debugging</a></li> |
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