Add support for guard clauses in Java 21 switch expressions

core/pom.xml

@@ -226,7 +226,7 @@
     <profile>
       <id>jdk11</id>
       <activation>
-        <jdk>(,17)</jdk>
+        <jdk>[11,17)</jdk>
       </activation>
       <build>
         <plugins>
@@ -236,13 +236,40 @@
             <configuration>
               <excludes>
                 <exclude>**/Java17InputAstVisitor.java</exclude>
+                <exclude>**/Java21InputAstVisitor.java</exclude>
               </excludes>
             </configuration>
           </plugin>
           <plugin>
             <artifactId>maven-javadoc-plugin</artifactId>
             <configuration>
               <excludePackageNames>com.google.googlejavaformat.java.java17</excludePackageNames>
+              <excludePackageNames>com.google.googlejavaformat.java.java21</excludePackageNames>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
+    <profile>
+      <id>jdk17</id>
+      <activation>
+        <jdk>[17,21)</jdk>
+      </activation>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-compiler-plugin</artifactId>
+            <configuration>
+              <excludes>
+                <exclude>**/Java21InputAstVisitor.java</exclude>
+              </excludes>
+            </configuration>
+          </plugin>
+          <plugin>
+            <artifactId>maven-javadoc-plugin</artifactId>
+            <configuration>
+              <excludePackageNames>com.google.googlejavaformat.java.java21</excludePackageNames>
             </configuration>
           </plugin>
         </plugins>

core/src/main/java/com/google/googlejavaformat/java/Formatter.java

@@ -151,7 +151,17 @@ public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOExcept
     OpsBuilder builder = new OpsBuilder(javaInput, javaOutput);
     // Output the compilation unit.
     JavaInputAstVisitor visitor;
-    if (Runtime.version().feature() >= 17) {
+    if (Runtime.version().feature() >= 21) {
+      try {
+        visitor =
+            Class.forName("com.google.googlejavaformat.java.java21.Java21InputAstVisitor")
+                .asSubclass(JavaInputAstVisitor.class)
+                .getConstructor(OpsBuilder.class, int.class)
+                .newInstance(builder, options.indentationMultiplier());
+      } catch (ReflectiveOperationException e) {
+        throw new LinkageError(e.getMessage(), e);
+      }
+    } else if (Runtime.version().feature() >= 17) {
       try {
         visitor =
             Class.forName("com.google.googlejavaformat.java.java17.Java17InputAstVisitor")

core/src/main/java/com/google/googlejavaformat/java/java21/Java21InputAstVisitor.java

@@ -0,0 +1,76 @@
+package com.google.googlejavaformat.java.java21;
+
+import static com.google.common.collect.Iterables.getOnlyElement;
+
+import com.google.googlejavaformat.OpsBuilder;
+import com.google.googlejavaformat.java.java17.Java17InputAstVisitor;
+import com.sun.source.tree.*;
+import java.util.List;
+
+public class Java21InputAstVisitor extends Java17InputAstVisitor {
+
+  public Java21InputAstVisitor(OpsBuilder builder, int indentMultiplier) {
+    super(builder, indentMultiplier);
+  }
+
+  @Override
+  public Void visitCase(CaseTree node, Void unused) {
+    sync(node);
+    markForPartialFormat();
+    builder.forcedBreak();
+    List<? extends CaseLabelTree> labels = node.getLabels();
+    boolean isDefault =
+        labels.size() == 1 && getOnlyElement(labels).getKind().name().equals("DEFAULT_CASE_LABEL");
+    if (isDefault) {
+      token("default", plusTwo);
+    } else {
+      token("case", plusTwo);
+      builder.open(labels.size() > 1 ? plusFour : ZERO);
+      builder.space();
+      boolean first = true;
+      for (Tree expression : labels) {
+        if (!first) {
+          token(",");
+          builder.breakOp(" ");
+        }
+        scan(expression, null);
+        first = false;
+      }
+      builder.close();
+    }
+    if (node.getGuard() != null) {
+      builder.space();
+      token("when");
+      builder.space();
+      scan(node.getGuard(), null);
+    }
+    switch (node.getCaseKind()) {
+      case STATEMENT:
+        token(":");
+        builder.open(plusTwo);
+        visitStatements(node.getStatements());
+        builder.close();
+        break;
+      case RULE:
+        builder.space();
+        token("-");
+        token(">");
+        builder.space();
+        if (node.getBody().getKind() == Tree.Kind.BLOCK) {
+          // Explicit call with {@link CollapseEmptyOrNot.YES} to handle empty case blocks.
+          visitBlock(
+              (BlockTree) node.getBody(),
+              CollapseEmptyOrNot.YES,
+              AllowLeadingBlankLine.NO,
+              AllowTrailingBlankLine.NO);
+        } else {
+          scan(node.getBody(), null);
+        }
+        builder.guessToken(";");
+        break;
+      default:
+        throw new AssertionError(node.getCaseKind());
+    }
+    return null;
+  }
+}