After a lengthy preparation period, this year the rollout of Java 21 will start for AEM as a Cloud Service. While the public documentation contains all relevant information (and I don’t want to reiterate them here), I want to make a few things more clear.
First, this is the update of the Java version used to run AEM as a Cloud Service. This version can be different from the Java version which is used to build the application. As Java versions are backwards compatible and can read binaries created by older versions, it is entirely possible to run the AEM CS instance with Java 21, but still build the application with Java 11. Of course this restricts you to the language features of Java 11 and for example you cannot use Records, but besides that there is no negative impact at all.
This scenario is fully supported; but at some point you need to update your build version to a newer Java version, as freshly added APIs might use Java features which are not available in Java 11. And as a personal recommendation I would suggest to switch also your build time Java version to Java 21.
This change of the runtime Java version should in most cases be totally invisible for you; at least as long as you don’t use or add 3rd-party libraries, which need to support new Java versions explicitly; the most prominent libraries in the AEM context are Groovy (often as part of the Groovy console) and the ASM library (a library which allows to create and modify Java bytecode). If you deploy one of these into your AEM instance, make sure that you update these to a version which supports Java 21.
Things on a content management system 