Java 21 is chock-full of great features but that's for naught of you can't actually upgrade, so I've collected all potential upgrade hurdles and we'll go over every issue that you may encounter on the road from Java 17 to 21

nipafx (Nicolai Parlog)

A comprehensive guide covering all potential upgrade hurdles when migrating from Java 17 to Java 21. Topics include API changes (SequencedCollection conflicts, bug fixes in Double.toString and IdentityHashMap), ongoing deprecations (Thread.stop/suspend/resume now throw UnsupportedOperationException, Security Manager, finalization), networking changes (Windows network interface names, URL parsing, HTTP client idle timeout), encoding changes (UTF-8 by default since JDK 18, CLDR v42 date/time formatting), runtime changes (obsolete VM options for biased locking and G1 GC, class loading enforcement, macOS Metal rendering), and JDK tool changes (jar, jlink, jpackage). Also covers best practices: building on multiple Java versions continuously, using the OpenJDK Quality Outreach program, updating third-party bytecode manipulation libraries, and a gradual step-by-step upgrade strategy.

Upgrading From Java 17 To 21: All You Need To Know