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itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Java runtime</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>Java runtime</h1></header><p>Apache Druid is written in Java and requires a Java runtime. This page provides details about obtaining and configuring
a Java runtime for Druid.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="selecting-a-java-runtime">Selecting a Java runtime<a href="#selecting-a-java-runtime" class="hash-link" aria-label="Direct link to Selecting a Java runtime" title="Direct link to Selecting a Java runtime"></a></h2>
<p>The project team recommends Java 17. Although you can use Java 11, support for it is deprecated.</p>
<p>The project team recommends using an OpenJDK-based Java distribution. There are many free and actively-supported
distributions available, including
<a href="https://docs.aws.amazon.com/corretto/latest/corretto-17-ug/what-is-corretto-17.html" target="_blank" rel="noopener noreferrer">Amazon Corretto</a>,
<a href="https://www.azul.com/downloads/?version=java-17-lts&amp;package=jdk" target="_blank" rel="noopener noreferrer">Azul Zulu</a>, and
<a href="https://adoptium.net/temurin/releases?version=17" target="_blank" rel="noopener noreferrer">Eclipse Temurin</a>.
The project team does not recommend any specific distribution over any other.</p>
<p>Druid relies on the environment variables <code>JAVA_HOME</code> or <code>DRUID_JAVA_HOME</code> to find Java on the machine. You can set
<code>DRUID_JAVA_HOME</code> if there is more than one instance of Java. To verify Java requirements for your environment, run the
<code>bin/verify-java</code> script.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="garbage-collection">Garbage collection<a href="#garbage-collection" class="hash-link" aria-label="Direct link to Garbage collection" title="Direct link to Garbage collection"></a></h2>
<p>In general, the project team recommends using the G1 collector with default settings. This is the default collector in
Java 11 and 17.</p>
<p>Garbage collector selection and tuning is a form of sport in the Java community. There may be situations where adjusting
garbage collection configuration improves or worsens performance. The project team&#x27;s guidance is that most people do
not need to stray away from G1 with default settings.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="strong-encapsulation">Strong encapsulation<a href="#strong-encapsulation" class="hash-link" aria-label="Direct link to Strong encapsulation" title="Direct link to Strong encapsulation"></a></h2>
<p>Java 9 and beyond (including Java 11 and 17) include the capability for
<a href="https://dev.java/learn/strong-encapsulation-(of-jdk-internals)/" target="_blank" rel="noopener noreferrer">strong encapsulation</a> of internal JDK APIs. Druid
uses certain internal JDK APIs, which must be added to <code>--add-exports</code> and <code>--add-opens</code> on the Java command line.</p>
<p>On Java 11, if these parameters are not included, you will see warnings like the following:</p>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="color:#bfc7d5;background-color:#292d3e"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#bfc7d5"><span class="token plain">WARNING: An illegal reflective access operation has occurred</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">WARNING: All illegal access operations will be denied in a future release</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>On Java 17, if these parameters are not included, you will see errors on startup like the following:</p>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="color:#bfc7d5;background-color:#292d3e"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#bfc7d5"><span class="token plain">Exception in thread &quot;main&quot; java.lang.ExceptionInInitializerError</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>Druid&#x27;s out-of-box configuration adds these parameters transparently when you use the bundled <code>bin/start-druid</code> or
similar commands. In this case, there is nothing special you need to do to run successfully. However,
if you have customized your Druid service launching system, you will need to ensure the required Java parameters are
added. There are many ways of doing this. Choose the one that works best for you.</p>
<ol>
<li>
<p>The simplest approach: use Druid&#x27;s bundled <code>bin/start-druid</code> script to launch Druid.</p>
</li>
<li>
<p>If you launch Druid using <code>bin/supervise -c &lt;config&gt;</code>, ensure your config file uses <code>bin/run-druid</code>. This
script uses <code>bin/run-java</code> internally, and automatically adds the proper flags.</p>
</li>
<li>
<p>If you launch Druid using a <code>java</code> command, replace <code>java</code> with <code>bin/run-java</code>. Druid&#x27;s bundled
<code>bin/run-java</code> script automatically adds the proper flags.</p>
</li>
<li>
<p>If you launch Druid without using its bundled scripts, ensure the following parameters are added to your Java
command line:</p>
</li>
</ol>
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="color:#bfc7d5;background-color:#292d3e"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-exports=java.base/jdk.internal.misc=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-exports=java.base/jdk.internal.ref=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=java.base/java.nio=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=java.base/sun.nio.ch=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=java.base/jdk.internal.ref=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=java.base/java.io=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=java.base/java.lang=ALL-UNNAMED \</span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">--add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/docs/34.0.0/operations/web-console"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Web console</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/docs/34.0.0/operations/durable-storage"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Durable storage</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#selecting-a-java-runtime" class="table-of-contents__link toc-highlight">Selecting a Java runtime</a></li><li><a href="#garbage-collection" class="table-of-contents__link toc-highlight">Garbage collection</a></li><li><a href="#strong-encapsulation" class="table-of-contents__link toc-highlight">Strong encapsulation</a></li></ul></div></div></div></div></main></div></div></div><footer class="footer"><div class="container container-fluid"><div class="footer__bottom text--center"><div class="margin-bottom--sm"><img src="/img/favicon.png" class="footer__logo themedComponent_mlkZ themedComponent--light_NVdE"><img src="/img/favicon.png" class="footer__logo themedComponent_mlkZ themedComponent--dark_xIcU"></div><div class="footer__copyright">Copyright © 2025 Apache Software Foundation. Except where otherwise noted, licensed under CC BY-SA 4.0. Apache Druid, Druid, and the Druid logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</div></div></div></footer></div>
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