Java21可以使用的新特性和旧版本升级到21的指南

Java21新特性

https://docs.oracle.com/en/java/javase/21/core/#Java-Platform%2C-Standard-Edition

升级到Java21指南

https://docs.oracle.com/en/java/javase/21/migrate/index.html#Java-Platform%2C-Standard-Edition

Java8升级到21事项清单

https://docs.oracle.com/en/java/javase/21/migrate/migrating-jdk-8-later-jdk-releases.html#GUID-7744EF96-5899-4FB2-B34E-86D49B2E89B6