I upgraded confluence from 6.13.0 to 7.6.2 followed this page https://confluence.atlassian.com/doc/upgrading-confluence-manually-255363437.html
It is all OK to start confluence using the original JDK 8.
After finished the upgrade, I changed the JDK from 8 to 14, confluence can NOT start, logs/catalina.out has following error message:
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.apache.felix.framework.ext.ClassPathExtenderFactory$DefaultClassLoaderExtender (file:/home/tzc/atlassian/atlassian-confluence-7.6.2/confluence/WEB-INF/lib/org.apache.felix.framework-5.6.12.jar) to method java.net.URLClassLoader.addURL(java.net.URL)
WARNING: Please consider reporting this to the maintainers of org.apache.felix.framework.ext.ClassPathExtenderFactory$DefaultClassLoaderExtender
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
04-Aug-2020 11:04:10.598 SEVERE [Catalina-utility-1] org.apache.catalina.core.StandardContext.startInternal One or more Filters failed to start. Full details will be found in the appropriate container log file
04-Aug-2020 11:04:10.598 SEVERE [Catalina-utility-1] org.apache.catalina.core.StandardContext.startInternal Context [] startup failed due to previous errors
WARNING: Cannot update an inactive framework.