PT-13830: Support Java 21 string template syntax

src/main/java/com/puppycrawl/tools/checkstyle/AstTreeStringPrinter.java

@@ -162,7 +162,7 @@ public static String printBranch(DetailAST node) {
      * @param ast the root AST node.
      * @return string AST.
      */
-    private static String printTree(DetailAST ast) {
+    public static String printTree(DetailAST ast) {
         final StringBuilder messageBuilder = new StringBuilder(1024);
         DetailAST node = ast;
         while (node != null) {

src/main/java/com/puppycrawl/tools/checkstyle/JavaAstVisitor.java

@@ -103,6 +103,12 @@ public final class JavaAstVisitor extends JavaLanguageParserBaseVisitor<DetailAs
     /** String representation of the right shift operator. */
     private static final String RIGHT_SHIFT = ">>";
 
+    private static final String QUOTE = "\"";
+
+    private static final String EMBEDDED_EXPRESSION_BEGIN = "\\{";
+
+    private static final String EMBEDDED_EXPRESSION_END = "}";
+
     /**
      * The tokens here are technically expressions, but should
      * not return an EXPR token as their root.
@@ -1533,6 +1539,14 @@ public DetailAstImpl visitPrimaryExp(JavaLanguageParser.PrimaryExpContext ctx) {
         return flattenedTree(ctx);
     }
 
+    @Override
+    public DetailAstImpl visitTemplateExp(JavaLanguageParser.TemplateExpContext ctx) {
+        final DetailAstImpl dot = create(ctx.DOT());
+        dot.addChild(visit(ctx.expr()));
+        dot.addChild(visit(ctx.templateArgument()));
+        return dot;
+    }
+
     @Override
     public DetailAstImpl visitPostfix(JavaLanguageParser.PostfixContext ctx) {
         final DetailAstImpl postfix;
@@ -1742,6 +1756,150 @@ public DetailAstImpl visitPrimitivePrimary(JavaLanguageParser.PrimitivePrimaryCo
         return dot;
     }
 
+    @Override
+    public DetailAstImpl visitTemplateArgument(JavaLanguageParser.TemplateArgumentContext ctx) {
+        final DetailAstImpl templateArgument;
+        if (ctx.template() != null) {
+            templateArgument = visit(ctx.template());
+        }
+        else {
+            templateArgument = flattenedTree(ctx);
+        }
+        return templateArgument;
+    }
+
+    @Override
+    public DetailAstImpl visitStringTemplate(JavaLanguageParser.StringTemplateContext ctx) {
+        final DetailAstImpl begin = buildStringTemplateBeginning(ctx);
+
+        final DetailAstImpl embeddedExp = createImaginary(TokenTypes.EMBEDDED_EXPRESSION);
+        embeddedExp.addChild(visit(ctx.expr()));
+        begin.addChild(embeddedExp);
+
+        ctx.stringTemplateMiddle().stream()
+                .map(this::buildStringTemplateMiddle)
+                .collect(Collectors.toList())
+                .forEach(begin::addChild);
+
+        final DetailAstImpl end = buildStringTemplateEnd(ctx);
+        begin.addChild(end);
+        return begin;
+    }
+
+    private static DetailAstImpl buildStringTemplateBeginning(
+            JavaLanguageParser.StringTemplateContext ctx) {
+
+        final TerminalNode beginContext = ctx.STRING_TEMPLATE_BEGIN();
+        final Token beginToken = beginContext.getSymbol();
+
+        final String stringTemplateBeginText = beginContext.getText();
+        final int childCount = ctx.getChildCount();
+        final int charPositionInLine = beginToken.getCharPositionInLine();
+        final int beginTokenLine = beginToken.getLine();
+
+        final DetailAstImpl begin = buildImaginaryWithDetails(
+                TokenTypes.STRING_TEMPLATE_BEGIN, QUOTE,
+                beginTokenLine, charPositionInLine
+        );
+
+        final int start = QUOTE.length();
+        final int end = stringTemplateBeginText.length() - EMBEDDED_EXPRESSION_BEGIN.length();
+        final String beginContextText = beginContext.getText().substring(start, end);
+
+        final DetailAstImpl beginContent = buildImaginaryWithDetails(
+                TokenTypes.STRING_TEMPLATE_CONTENT, beginContextText,
+                beginTokenLine, charPositionInLine + start
+        );
+        begin.addChild(beginContent);
+
+        final DetailAstImpl embeddedBegin = buildImaginaryWithDetails(
+                TokenTypes.EMBEDDED_EXPRESSION_BEGIN, EMBEDDED_EXPRESSION_BEGIN,
+                beginTokenLine, charPositionInLine + end
+        );
+        begin.addChild(embeddedBegin);
+        return begin;
+    }
+
+    private static DetailAstImpl buildImaginaryWithDetails(
+            int tokenType, String text, int lineNumber, int columnNumber) {
+        final DetailAstImpl imaginary = createImaginary(tokenType);
+        imaginary.setText(text);
+        imaginary.setLineNo(lineNumber);
+        imaginary.setColumnNo(columnNumber);
+        return imaginary;
+    }
+
+    private DetailAstImpl buildStringTemplateMiddle(
+            JavaLanguageParser.StringTemplateMiddleContext middleContext) {
+        final TerminalNode ctx = middleContext.STRING_TEMPLATE_MID();
+        final Token token = ctx.getSymbol();
+        final int charPositionInLine = token.getCharPositionInLine();
+        final int lineNumber = token.getLine();
+        final String text = ctx.getText();
+
+        final DetailAstImpl embeddedEnd = buildImaginaryWithDetails(
+                TokenTypes.EMBEDDED_EXPRESSION_END, EMBEDDED_EXPRESSION_END,
+                lineNumber, charPositionInLine
+        );
+
+        final int startIndex = EMBEDDED_EXPRESSION_END.length();
+        final int endIndex = token.getText().length() - EMBEDDED_EXPRESSION_BEGIN.length();
+        final String contentText = text.substring(startIndex, endIndex);
+
+        final DetailAstImpl content = buildImaginaryWithDetails(
+                TokenTypes.STRING_TEMPLATE_CONTENT, contentText,
+                lineNumber, charPositionInLine + EMBEDDED_EXPRESSION_END.length()
+        );
+        embeddedEnd.addNextSibling(content);
+
+        final DetailAstImpl embeddedBegin = buildImaginaryWithDetails(
+                TokenTypes.EMBEDDED_EXPRESSION_BEGIN, EMBEDDED_EXPRESSION_BEGIN,
+                lineNumber,
+                charPositionInLine + text.length() - EMBEDDED_EXPRESSION_BEGIN.length()
+        );
+        content.addNextSibling(embeddedBegin);
+
+        final DetailAstImpl embeddedExp = createImaginary(TokenTypes.EMBEDDED_EXPRESSION);
+        embeddedExp.addChild(visit(middleContext.expr()));
+        embeddedBegin.addNextSibling(embeddedExp);
+        return embeddedEnd;
+    }
+
+    private static DetailAstImpl buildStringTemplateEnd(
+            JavaLanguageParser.StringTemplateContext ctx) {
+
+        final TerminalNode endContext = ctx.STRING_TEMPLATE_END();
+        final Token endToken = endContext.getSymbol();
+
+        final String stringTemplateEndText = endContext.getText();
+        final int childCount = ctx.getChildCount();
+        final int charPositionInLine = endToken.getCharPositionInLine();
+        final int lineNumber = endToken.getLine();
+
+        final int startIndex = EMBEDDED_EXPRESSION_END.length();
+        final int endIndex = stringTemplateEndText.length() - QUOTE.length();
+        final String endContentContentText = stringTemplateEndText.substring(startIndex, endIndex);
+
+        final DetailAstImpl embeddedEnd = buildImaginaryWithDetails(
+                TokenTypes.EMBEDDED_EXPRESSION_END, EMBEDDED_EXPRESSION_END,
+                lineNumber, charPositionInLine
+        );
+
+        final DetailAstImpl endContent = buildImaginaryWithDetails(
+                TokenTypes.STRING_TEMPLATE_CONTENT, endContentContentText,
+                lineNumber, charPositionInLine + QUOTE.length()
+        );
+        embeddedEnd.addNextSibling(endContent);
+
+        final DetailAstImpl end = buildImaginaryWithDetails(
+                TokenTypes.STRING_TEMPLATE_END, QUOTE,
+                lineNumber,
+                charPositionInLine + stringTemplateEndText.length() - QUOTE.length()
+        );
+        endContent.addNextSibling(end);
+        return embeddedEnd;
+    }
+
     @Override
     public DetailAstImpl visitCreator(JavaLanguageParser.CreatorContext ctx) {
         return flattenedTree(ctx);

src/main/java/com/puppycrawl/tools/checkstyle/api/TokenTypes.java

@@ -6469,6 +6469,21 @@ public final class TokenTypes {
     public static final int RECORD_PATTERN_COMPONENTS =
             JavaLanguageLexer.RECORD_PATTERN_COMPONENTS;
 
+    public static final int STRING_TEMPLATE_BEGIN =
+            JavaLanguageLexer.STRING_TEMPLATE_BEGIN;
+    public static final int STRING_TEMPLATE_MID =
+            JavaLanguageLexer.STRING_TEMPLATE_MID;
+    public static final int STRING_TEMPLATE_END =
+            JavaLanguageLexer.STRING_TEMPLATE_END;
+    public static final int STRING_TEMPLATE_CONTENT =
+            JavaLanguageLexer.STRING_TEMPLATE_CONTENT;
+    public static final int EMBEDDED_EXPRESSION_BEGIN =
+            JavaLanguageLexer.EMBEDDED_EXPRESSION_BEGIN;
+    public static final int EMBEDDED_EXPRESSION =
+            JavaLanguageLexer.EMBEDDED_EXPRESSION;
+    public static final int EMBEDDED_EXPRESSION_END =
+            JavaLanguageLexer.EMBEDDED_EXPRESSION_END;
+
     /** Prevent instantiation. */
     private TokenTypes() {
     }

src/main/resources/com/puppycrawl/tools/checkstyle/grammar/java/JavaLanguageLexer.g4

@@ -110,7 +110,11 @@ tokens {
 
     LITERAL_NON_SEALED, LITERAL_SEALED, LITERAL_PERMITS,
     PERMITS_CLAUSE, PATTERN_DEF, LITERAL_WHEN,
-    RECORD_PATTERN_DEF, RECORD_PATTERN_COMPONENTS
+    RECORD_PATTERN_DEF, RECORD_PATTERN_COMPONENTS,
+
+    STRING_TEMPLATE_BEGIN, STRING_TEMPLATE_MID, STRING_TEMPLATE_END,
+    STRING_TEMPLATE_CONTENT, EMBEDDED_EXPRESSION_BEGIN, EMBEDDED_EXPRESSION,
+    EMBEDDED_EXPRESSION_END
 }
 
 @header {
@@ -148,6 +152,10 @@ import com.puppycrawl.tools.checkstyle.grammar.CrAwareLexerSimulator;
 
     /** Tracks the starting column of a block comment. */
     int startCol = -1;
+
+    private boolean isTextBlockMode() {
+        return _modeStack.size() > 0 && _modeStack.peek() == TextBlock;
+    }
 }
 
 // Keywords and restricted identifiers
@@ -242,7 +250,14 @@ LITERAL_FALSE:           'false';
 
 CHAR_LITERAL:            '\'' (EscapeSequence | ~['\\\r\n]) '\'';
 
-STRING_LITERAL:          '"' (EscapeSequence | ~["\\\r\n])* '"';
+fragment StringFragment: (EscapeSequence | ~["\\\r\n])*;
+
+STRING_LITERAL:          '"' StringFragment '"';
+
+// explore if we can gather StringFragment and emit a token
+STRING_TEMPLATE_BEGIN:   '"'  StringFragment '\\' '{';
+STRING_TEMPLATE_MID:     '}' StringFragment '\\' '{';
+STRING_TEMPLATE_END:     '}' StringFragment '"';
 
 TEXT_BLOCK_LITERAL_BEGIN: '"' '"' '"' -> pushMode(TextBlock);
 
@@ -403,7 +418,20 @@ fragment Letter
 // Text block lexical mode
 mode TextBlock;
     TEXT_BLOCK_CONTENT
-        : ( TwoDoubleQuotes
+        : TextBlockFragment
+        ;
+
+    TEXT_BLOCK_TEMPLATE_BEGIN
+        : TextBlockFragment '\\' '{' -> pushMode(DEFAULT_MODE);
+
+    TEXT_BLOCK_TEMPLATE_MID
+        : '}' TextBlockFragment '\\' '{';
+
+    TEXT_BLOCK_TEMPLATE_END
+        : '}' TextBlockFragment TEXT_BLOCK_LITERAL_END -> popMode;
+
+    fragment TextBlockFragment
+        :    ( TwoDoubleQuotes
           | OneDoubleQuote
           | Newline
           | ~'"'

src/main/resources/com/puppycrawl/tools/checkstyle/grammar/java/JavaLanguageParser.g4

@@ -688,6 +688,7 @@ expression
 
 expr
     : primary                                                              #primaryExp
+    | expr DOT templateArgument                                            #templateExp
     | expr bop=DOT id                                                      #refOp
     | expr bop=DOT id LPAREN expressionList? RPAREN                        #methodCall
     | expr bop=DOT LITERAL_THIS                                            #thisExp
@@ -757,6 +758,24 @@ primary
       DOT LITERAL_CLASS                                                    #primitivePrimary
     ;
 
+templateArgument
+    : template
+    | STRING_LITERAL
+    | TEXT_BLOCK_LITERAL_BEGIN TEXT_BLOCK_CONTENT TEXT_BLOCK_LITERAL_END
+    ;
+
+template
+    : stringTemplate
+    ;
+
+stringTemplate
+    : STRING_TEMPLATE_BEGIN expr? stringTemplateMiddle* STRING_TEMPLATE_END
+    ;
+
+stringTemplateMiddle
+    : STRING_TEMPLATE_MID expr?
+    ;
+
 classType
     : (classOrInterfaceType[false] DOT)? annotations[false] id typeArguments?
     ;

src/test/java/com/puppycrawl/tools/checkstyle/grammar/java21/Java21AstRegressionTest.java

@@ -0,0 +1,41 @@
+///////////////////////////////////////////////////////////////////////////////////////////////
+// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
+// Copyright (C) 2001-2023 the original author or authors.
+//
+// This library is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Lesser General Public
+// License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+///////////////////////////////////////////////////////////////////////////////////////////////
+
+package com.puppycrawl.tools.checkstyle.grammar.java21;
+
+import org.junit.jupiter.api.Test;
+
+import com.puppycrawl.tools.checkstyle.AbstractTreeTestSupport;
+
+public class Java21AstRegressionTest extends AbstractTreeTestSupport {
+
+    @Override
+    protected String getPackageLocation() {
+        return "com/puppycrawl/tools/checkstyle/grammar/java21";
+    }
+
+    @Test
+    public void testBasicStringTemplate() throws Exception {
+        verifyAst(
+                getNonCompilablePath(
+                        "ExpectedStringTemplateBasic.txt"),
+                getNonCompilablePath(
+                        "InputStringTemplateBasic.java"));
+    }
+}