{"id":2653,"date":"2025-08-21T09:30:27","date_gmt":"2025-08-21T09:30:27","guid":{"rendered":"https:\/\/www.devopsconsulting.in\/blog\/?p=2653"},"modified":"2025-08-21T09:30:28","modified_gmt":"2025-08-21T09:30:28","slug":"how-to-fix-unsupportedclassversionerror-when-running-keycloak","status":"publish","type":"post","link":"https:\/\/www.devopsconsulting.in\/blog\/how-to-fix-unsupportedclassversionerror-when-running-keycloak\/","title":{"rendered":"How to Fix UnsupportedClassVersionError When Running Keycloak"},"content":{"rendered":"\n<p><strong>How to Fix UnsupportedClassVersionError When Running Keycloak<\/strong><\/p>\n\n\n\n<p>When launching self-hosted applications like Keycloak, you might encounter specific errors related to your system&#8217;s environment. One common issue is the <code>java.lang.UnsupportedClassVersionError<\/code>, which can halt the startup process. This error indicates a mismatch between the Java version your application was built with and the version installed on your server.<\/p>\n\n\n\n<p>This article walks you through diagnosing and resolving this error, using a real-world example from a user trying to run Keycloak.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">The Problem: <code>UnsupportedClassVersionError<\/code><\/h2>\n\n\n\n<p>After a fresh installation of Keycloak, a user attempts to launch it using the <code>start-dev<\/code> command:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\">bash<code>root@Abhishek:\/opt\/lampp\/htdocs\/keycloak-26.3.3<em># bin\/kc.sh start-dev<\/em>\n<\/code><\/pre>\n\n\n\n<p>Instead of starting successfully, the system returns the following error:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\">text<code>Error: LinkageError occurred while loading main class io.quarkus.bootstrap.runner.QuarkusEntryPoint\n        java.lang.UnsupportedClassVersionError: io\/quarkus\/bootstrap\/runner\/QuarkusEntryPoint has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0\n<\/code><\/pre>\n\n\n\n<p>This error message is very specific and tells us exactly what is wrong. Let&#8217;s break it down.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Understanding the Error<\/h2>\n\n\n\n<p>The <code>java.lang.UnsupportedClassVersionError<\/code> means the Java Virtual Machine (JVM) is trying to load a class file that was compiled with a newer version of Java than the JVM itself supports.<\/p>\n\n\n\n<p>From the error output, we can identify two key pieces of information:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong><code>class file version 61.0<\/code><\/strong>: This is the version format of the application&#8217;s compiled code (in this case, for Keycloak). This version number corresponds to <strong>Java 17<\/strong>.<\/li>\n\n\n\n<li><strong><code>class file versions up to 55.0<\/code><\/strong>: This is the latest class file version that the currently installed Java runtime can understand. This version number corresponds to <strong>Java 11<\/strong>.<\/li>\n<\/ul>\n\n\n\n<p>The conflict is clear: Keycloak requires Java 17 to run, but the server is using Java 11.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Step-by-Step Solution on Ubuntu<\/h2>\n\n\n\n<p>To fix this, you must upgrade the Java environment on your server to a compatible version. Here\u2019s how to install Java 17 and set it as the default on an Ubuntu system.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Step 1: Install Java 17<\/h2>\n\n\n\n<p>First, you need to install the OpenJDK 17 package. Open your terminal and run the following commands.<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Update your package list:<\/strong> This ensures you are getting the latest available packages from the repository. bash<code>sudo apt update<\/code><\/li>\n\n\n\n<li><strong>Install OpenJDK 17:<\/strong> This command downloads and installs the Java 17 Development Kit (JDK). bash<code>sudo apt install openjdk-17-jdk<\/code><\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\">Step 2: Set Java 17 as the Default Version<\/h2>\n\n\n\n<p>If you have more than one version of Java installed, you must tell the system which one to use by default.<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Run the <code>update-alternatives<\/code> command:<\/strong> bash<code>sudo update-alternatives --config java<\/code><\/li>\n\n\n\n<li>This will display a list of all Java versions installed on your system. You will be prompted to select a version. Enter the number corresponding to the OpenJDK 17 installation and press <code>Enter<\/code>.<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\">Step 3: Verify the New Java Version<\/h2>\n\n\n\n<p>After the installation and configuration are complete, it&#8217;s important to verify that your system is now using the correct Java version.<\/p>\n\n\n\n<p>Run the following command:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\">bash<code>java -version\n<\/code><\/pre>\n\n\n\n<p>The output should now display information indicating you are running OpenJDK version 17.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Step 4: Rerun the Keycloak Script<\/h2>\n\n\n\n<p>With Java 17 now active, you can return to your Keycloak directory and run the startup command again.<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\">bash<code>bin\/kc.sh start-dev\n<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"<p>How to Fix UnsupportedClassVersionError When Running Keycloak When launching self-hosted applications like Keycloak, you might encounter specific errors related to your system&#8217;s environment. One common issue is&#8230; <\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-2653","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How to Fix UnsupportedClassVersionError When Running Keycloak - DevOps Consulting<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.devopsconsulting.in\/blog\/how-to-fix-unsupportedclassversionerror-when-running-keycloak\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to Fix UnsupportedClassVersionError When Running Keycloak - DevOps Consulting\" \/>\n<meta property=\"og:description\" content=\"How to Fix UnsupportedClassVersionError When Running Keycloak When launching self-hosted applications like Keycloak, you might encounter specific errors related to your system&#8217;s environment. 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I\u2019ve had the opportunity to work with Cotocus and continue to contribute to multiple platforms where I share insights across different domains: \u2022 DevOps School \u2013 Tech blogs and tutorials \u2022 Holiday Landmark \u2013 Travel stories and guides \u2022 Stocks Mantra \u2013 Stock market strategies and tips \u2022 My Medic Plus \u2013 Health and fitness guidance \u2022 TrueReviewNow \u2013 Honest product reviews \u2022 Wizbrand \u2013 SEO and digital tools for businesses I\u2019m also exploring the fascinating world of Quantum Computing.\",\"url\":\"https:\\\/\\\/www.devopsconsulting.in\\\/blog\\\/author\\\/abhishek\\\/\"}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"How to Fix UnsupportedClassVersionError When Running Keycloak - DevOps Consulting","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.devopsconsulting.in\/blog\/how-to-fix-unsupportedclassversionerror-when-running-keycloak\/","og_locale":"en_US","og_type":"article","og_title":"How to Fix UnsupportedClassVersionError When Running Keycloak - DevOps Consulting","og_description":"How to Fix UnsupportedClassVersionError When Running Keycloak When launching self-hosted applications like Keycloak, you might encounter specific errors related to your system&#8217;s environment. 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