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| <h1 class="title">Apache Jena Initialization</h1> |
| |
| <p>Jena has a simple 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. Java initialization can lead to |
| visibility of uninitialized data.</p> |
| <p>The standard initialization sequence is<br> |
| Core -> RIOT -> ARQ -> TDB -> other (including jena text)</p> |
| <p>The sequence from core to TDB should be executed before application |
| components. See below for how to control the order.</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>See <a href="jena-repack.html">notes on repacking Jena code</a> for how to deal |
| with ServiceLoader files in repacked jars.</p> |
| <h2 id="initialization-code">Initialization code</h2> |
| <p>Initialization code is an implementation of <code>JenaSubsystemLifecycle</code>. |
| For use in the default initialization, the class must have a zero-argument constructor</p> |
| <pre><code>public interface JenaSubsystemLifecycle { |
| public void start() ; |
| public void stop() ; |
| default public int level() { return 9999 ; } |
| } |
| </code></pre> |
| <p>The code also 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>20 - RIOT</li> |
| <li>30 - ARQ</li> |
| <li>40 - TDB</li> |
| <li>9999 - other</li> |
| </ul> |
| <h2 id="the-initialization-process">The Initialization Process</h2> |
| <p>The process followed by <code>JenaSystem.init()</code> is to obtain an instance of |
| <code>JenaSubsystemRegistry</code>, ask it to <code>load()</code> initialization code, then call |
| that code in an order based on declared level. The order of invocation |
| of different initialization code within the same level is undefined |
| and may be different from run to run.</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="the-standard-subsystem-registry">The Standard Subsystem Registry</h2> |
| <p>The <code>JenaSubsystemRegistry</code> normally used is based on <code>java.util.ServiceLoader</code>. |
| It looks for class resources |
| <code>META-INF/services/org.apache.jena.sys.JenaSubsystemLifecycle</code> |
| on the classpath during the load step.</p> |
| <p>See the javadoc for <code>java.util.ServiceLoader</code> for more details.</p> |
| <p>See also the javadoc for |
| <a href="/documentation/javadoc/jena/org/apache/jena/system/JenaSystem.html">JenaSystem</a> |
| and the source code.</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> |
| <h2 id="modifying-the-standard-process">Modifying the Standard Process</h2> |
| <p>It is possible, with care, to alter the way |
| that initialization code is discovered.</p> |
| <p>An application can change the <code>JenaSubsystemRegistry</code> instance. |
| This must be done before any Jena code is called anywhere |
| in the current JVM.</p> |
| <pre><code>// Example alternative registry. |
| JenaSubsystemRegistry r = new JenaSubsystemRegistryBasic() { |
| @Override |
| public void load() { |
| if ( JenaSystem.DEBUG_INIT ) |
| System.err.println("Example custom load") ; |
| super.load(); |
| } |
| } ; |
| |
| // Set the sub-system registry |
| JenaSystem.setSubsystemRegistry(r); |
| |
| // Enable output if required |
| // JenaSystem.DEBUG_INIT = true ; |
| |
| // Initialize Jena |
| JenaSystem.init() ; |
| </code></pre> |
| <h2 id="jena-initialization-in-multi-classloader-environments">Jena initialization in multi-classloader environments</h2> |
| <p>In some applications with multiple classloaders, or different classloader strategies, Jena initialization may |
| not work as expected. If the Jena initialization debug information shows that components were not loaded correctly, |
| trying to switch the context class loader may fix the initialization process.</p> |
| <pre><code>ClassLoader contextClassLoader = null; |
| try { |
| contextClassLoader = Thread.currentThread().getContextClassLoader(); |
| Thread.currentThread().setContextClassLoader(getClass().getClassLoader()); |
| JenaSystem.DEBUG_INIT = true; |
| // ... |
| } finally { |
| Thread.currentThread().setContextClassLoader(contextClassLoader); |
| } |
| </code></pre> |
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