OpenJDK 21 GA is scheduled for 2023/09/19.

The following JEPs are important for us:

• JEP 430: String Templates (Preview)
• JEP 440: Record Patterns
• JEP 441: Pattern Matching for switch
• JEP 443: Unnamed Patterns and Variables (Preview)
• JEP 445: Unnamed Classes and Instance Main Methods (Preview)

In summary: https://javaalmanac.io/jdk/21/

This issue is similar to the previous: #4333

EA Release of JDK21 is available here: https://jdk.java.net/21/

Tasks:

• Add new java language versions 21 and 21-preview.
  • 21 is the new default
  • update also java.md and ant.md
  • and add in KotlinTestingDsl.kt.
• Update asm dependency to 9.5:
  • https://search.maven.org/artifact/org.ow2.asm/asm
  • https://asm.ow2.io/versions.html
  • Update net.sourceforge.pmd.lang.java.symbols.internal.asm.AsmSymbolResolver#ASM_API_V if necessary
• Remove java language version 19-preview
  • update also java.md and ant.md
  • update KotlinTestingDsl.kt
  • remove exclusions in pmd-core/src/main/resources/internal/all-java.xml
• Update net.sourceforge.pmd.lang.java.LanguageVersionTest
  • add 21/21-preview
  • remove 19-preview
• Update implementation for "Pattern Matching for switch" (406, 420, 427, 433, 441)
  • Promote this feature to a standard language feature (was preview before). E.g. remove experimental
  • https://cr.openjdk.org/~gbierman/jep440%2B441/jep440+441-20230612/specs/patterns-switch-record-patterns-jls.html
• Update implementation for "Record Patterns" (405, 432, 440)
  • Promote this feature to a standard language feature (was preview before). E.g. remove experimental
  • https://cr.openjdk.org/~gbierman/jep440%2B441/jep440+441-20230612/specs/patterns-switch-record-patterns-jls.html
• Implement preview feature "String Templates" (430)
  • https://cr.openjdk.org/~gbierman/jep430/jep430-20230526/specs/string-templates-jls.html
• Implement preview feature "Unnamed Patterns and Variables" (443)
  • https://cr.openjdk.org/~abimpoudis/unnamed/jep443-20230322/specs/unnamed-jls.html
• Implement preview feature "Unnamed Classes and Instance Main Methods" (445)
  • https://cr.openjdk.org/~gbierman/jep445/jep445-20230524/specs/unnamed-classes-instance-main-methods-jls.html
• Write release notes
  • removed experimental:
    • ASTRecordPattern
    • ASTPatternList (formerly ASTComponentPatternList)
    • ASTGuard (formerly ASTSwitchGuard)
  • removed classes: TODO
  • deprecated classes: TODO
    • net.sourceforge.pmd.lang.java.ast.ASTPattern#getParenthesisDepth - the method is deprecated, because parenthesized patterns are not allowed with Java 21 anymore.
  • new experimental: TODO
    • ASTTemplateExpression
    • ASTTemplate
    • ASTTemplateFragment
    • ASTUnnamedPattern
• Add JDK21 to the build, so that we integration test with 21, as this is a LTS version
  • integration test in pmd-dist
  • maybe also build PMD with 21, to avoid issues like [java] PMD 7 fails to build under Java 19 #4401
• Update https://github.com/pmd/java-regression-tests
  • new regular language features, so update needed
  • Note: This can only be done, after this PR has been merged
  • Therefore new issue: Update regression tests with Java 21 language features #4642