Confluence fails to start due to "ProxyServlet has been compiled by a more recent version of the Java Runtime"
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Summary
In an upgraded or fresh installation of Confluence, the Confluence process fails to start due to "ProxyServlet has been compiled by a more recent version of the Java Runtime"
Environment
Confluence 8.X
Diagnosis
Checking in the Confluence logs (atlassian-confluence.log) files, the following SEVERE error message will appear:
28-Mar-2024 02:22:36.757 SEVERE [Catalina-utility-2] org.apache.catalina.core.StandardContext.loadOnStartup Servlet [dispatcher] in web application [/synchrony-proxy] threw load() exception java.lang.UnsupportedClassVersionError: org/eclipse/jetty/proxy/ProxyServlet had been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class files version up to 52.0 (unable to lad class [org.eclipse.jetty.proxy.ProxyServlet]) (unable to load class [com.atlassian.synchrony.proxy.web.SynchronyProxyServlet])
...
...Cause
The root cause is due to the Java's version incompatibility error below. This is a common error when you are trying to run a code in JDK8 (class 52), when the code was compiled with JDK11 (class 55).
Solution
Confluence 8.X does not support JDK8 as part of their Supported Platforms. You should upgrade as well your JDK version to JDK 11 or JDK 17 to avoid this issue and allow Confluence to start succesfully.