❯ javac --enable-preview --release 21 src/main/java/HelloWorld.java
Note: src/main/java/HelloWorld.java uses preview features of Java SE 21.
Note: Recompile with -Xlint:preview for details.

/var/tmp $ cat config.xml
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN" "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">

    <module name="NewlineAtEndOfFile">
        <property name="lineSeparator" value="lf"/>
    </module>

    <module name="TreeWalker">
        <module name="UnusedImports"/>
    </module>
</module>


/var/tmp $ cat YOUR_FILE.java
public class HelloWorld {
    public static void main(String[] args) {
        int x = 10;
        int y = 20;
        String result = STR."\{x} + \{y} = \{x + y}";
        System.out.println(result);
    }
}

/var/tmp $ RUN_LOCALE="-Duser.language=en -Duser.country=US"
/var/tmp $ java $RUN_LOCALE -jar checkstyle-X.XX-all.jar -c config.xml YOUR_FILE.java
> java $RUN_LOCALE -jar checkstyle-10.12.4-all.jar -c checkstyle.xml src/main/java/HelloWorld.java 

Starting audit...
com.puppycrawl.tools.checkstyle.api.CheckstyleException: Exception was thrown while processing src/main/java/HelloWorld.java
        at com.puppycrawl.tools.checkstyle.Checker.processFiles(Checker.java:309)
        at com.puppycrawl.tools.checkstyle.Checker.process(Checker.java:226)
        at com.puppycrawl.tools.checkstyle.Main.runCheckstyle(Main.java:415)
        at com.puppycrawl.tools.checkstyle.Main.runCli(Main.java:338)
        at com.puppycrawl.tools.checkstyle.Main.execute(Main.java:195)
        at com.puppycrawl.tools.checkstyle.Main.main(Main.java:130)
Caused by: com.puppycrawl.tools.checkstyle.api.CheckstyleException: IllegalStateException occurred while parsing file /home/xxx/VCS/own/checkstyle-java21-reproducer/src/main/java/HelloWorld.java.
        at com.puppycrawl.tools.checkstyle.JavaParser.parse(JavaParser.java:105)
        at com.puppycrawl.tools.checkstyle.TreeWalker.processFiltered(TreeWalker.java:152)
        at com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck.process(AbstractFileSetCheck.java:101)
        at com.puppycrawl.tools.checkstyle.Checker.processFile(Checker.java:337)
        at com.puppycrawl.tools.checkstyle.Checker.processFiles(Checker.java:296)
        ... 5 more
Caused by: java.lang.IllegalStateException: 6:32: mismatched input '}' expecting ';'
        at com.puppycrawl.tools.checkstyle.JavaParser$CheckstyleErrorListener.syntaxError(JavaParser.java:255)
        at org.antlr.v4.runtime.ProxyErrorListener.syntaxError(ProxyErrorListener.java:41)
        at org.antlr.v4.runtime.Parser.notifyErrorListeners(Parser.java:544)
        at org.antlr.v4.runtime.DefaultErrorStrategy.reportInputMismatch(DefaultErrorStrategy.java:327)
        at org.antlr.v4.runtime.DefaultErrorStrategy.reportError(DefaultErrorStrategy.java:139)
        at com.puppycrawl.tools.checkstyle.CheckstyleParserErrorStrategy.recoverInline(CheckstyleParserErrorStrategy.java:38)
        at org.antlr.v4.runtime.Parser.match(Parser.java:208)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.blockStatement(JavaLanguageParser.java:6190)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.block(JavaLanguageParser.java:6101)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.methodBody(JavaLanguageParser.java:2940)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.methodDeclaration(JavaLanguageParser.java:2898)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.memberDeclaration(JavaLanguageParser.java:2745)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.classBodyDeclaration(JavaLanguageParser.java:2671)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.classBody(JavaLanguageParser.java:2477)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.classDeclaration(JavaLanguageParser.java:1096)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.types(JavaLanguageParser.java:753)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.typeDeclaration(JavaLanguageParser.java:667)
        at com.puppycrawl.tools.checkstyle.grammar.java.JavaLanguageParser.compilationUnit(JavaLanguageParser.java:414)
        at com.puppycrawl.tools.checkstyle.JavaParser.parse(JavaParser.java:99)
        ... 9 more
Caused by: org.antlr.v4.runtime.InputMismatchException
        ... 23 more
Checkstyle ends with 1 errors.