OpenJDK 17 GA is scheduled for 2021/09/14.
So we should add support for that at the latest with PMD 6.38.0 (August 2021) or earlier.

The following JEPs are important for us:

• JEP 406: Pattern Matching for switch (Preview)
• JEP 409: Sealed Classes

In summary: https://javaalmanac.io/jdk/17/

This issue is similar to the previous: #3079

EA Release of JDK17 is available here: http://jdk.java.net/17/

Tasks:

• Add new java language versions 17 and 17-preview.
  • 17 is the new default
  • also update cli_reference.md
  • also add in KotlinTestingDsl.kt.
• Update asm dependency:
  • https://search.maven.org/artifact/org.ow2.asm/asm
  • https://asm.ow2.io/versions.html
  • Update net.sourceforge.pmd.lang.java.typeresolution.visitors.PMDASMVisitor.ASM_API is necessary
• Promote "Sealed Classes" (409) to a standard feature. Note, there might be some small changes from last preview. Removed experimental annotation
  • "This JEP proposes to finalize Sealed Classes in JDK 17, with no changes from JDK 16."
• Implement "Pattern Matching for switch" (406) as a preview feature, mark AST classes as experimental, if needed.
  • https://openjdk.java.net/jeps/406
  • http://cr.openjdk.java.net/~gbierman/jep406/latest/
• Remove java language version 15-preview
• Write release notes
• Prepare merge for PMD 7 -> [java] Support JDK 17 (LTS) (PMD 7) #3414