Gradle: rename mainClassName to mainClass in application block

The `mainClassName` property on the application extension was deprecated
in Gradle 6.4 and removed in Gradle 9.0. Add a recipe that renames it
inside the `application { ... }` block (Groovy and Kotlin DSL) and wire
it into `MigrateToGradle9`.

Author: timtebeek

rewrite-gradle/src/main/java/org/openrewrite/gradle/gradle9/UseMainClassPropertyForApplication.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2026 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.gradle.gradle9;
+
+import lombok.EqualsAndHashCode;
+import lombok.Value;
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Preconditions;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.gradle.IsBuildGradle;
+import org.openrewrite.java.JavaVisitor;
+import org.openrewrite.java.tree.Expression;
+import org.openrewrite.java.tree.J;
+
+@Value
+@EqualsAndHashCode(callSuper = false)
+public class UseMainClassPropertyForApplication extends Recipe {
+
+    private static final String IN_APPLICATION = "IN_APPLICATION";
+
+    String displayName = "Use `mainClass` instead of `mainClassName` in the `application` block";
+
+    String description = "The `mainClassName` property on the `application` extension was deprecated in Gradle 6.4 and removed in Gradle 9.0. " +
+            "Use the `mainClass` property instead. " +
+            "See the [Gradle upgrade guide](https://docs.gradle.org/9.0.0/userguide/upgrading_major_version_9.html) for more information.";
+
+    @Override
+    public TreeVisitor<?, ExecutionContext> getVisitor() {
+        return Preconditions.check(new IsBuildGradle<>(), new JavaVisitor<ExecutionContext>() {
+            @Override
+            public J visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
+                if ("application".equals(method.getSimpleName())) {
+                    getCursor().putMessage(IN_APPLICATION, true);
+                }
+                return super.visitMethodInvocation(method, ctx);
+            }
+
+            @Override
+            public J.Assignment visitAssignment(J.Assignment assignment, ExecutionContext ctx) {
+                if (getCursor().getNearestMessage(IN_APPLICATION) == null) {
+                    return assignment;
+                }
+                Expression variable = assignment.getVariable();
+                if (variable instanceof J.Identifier) {
+                    J.Identifier id = (J.Identifier) variable;
+                    if ("mainClassName".equals(id.getSimpleName())) {
+                        return assignment.withVariable(id.withSimpleName("mainClass"));
+                    }
+                } else if (variable instanceof J.FieldAccess) {
+                    J.FieldAccess fieldAccess = (J.FieldAccess) variable;
+                    if ("mainClassName".equals(fieldAccess.getSimpleName())) {
+                        return assignment.withVariable(
+                                fieldAccess.withName(fieldAccess.getName().withSimpleName("mainClass"))
+                        );
+                    }
+                }
+                return assignment;
+            }
+        });
+    }
+}

rewrite-gradle/src/main/resources/META-INF/rewrite/gradle-9.yml

@@ -25,4 +25,5 @@ recipeList:
       addIfMissing: false
   # Map notation has been deprecated in 9.1+
   - org.openrewrite.gradle.DependencyUseStringNotation
-  - org.openrewrite.gradle.gradle9.UseMainClassProperty
+  - org.openrewrite.gradle.gradle9.UseMainClassProperty
+  - org.openrewrite.gradle.gradle9.UseMainClassPropertyForApplication

rewrite-gradle/src/test/java/org/openrewrite/gradle/gradle9/UseMainClassPropertyForApplicationTest.java

@@ -0,0 +1,140 @@
+/*
+ * Copyright 2026 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.gradle.gradle9;
+
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.openrewrite.DocumentExample;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+
+import static org.openrewrite.gradle.Assertions.buildGradle;
+import static org.openrewrite.gradle.Assertions.buildGradleKts;
+
+class UseMainClassPropertyForApplicationTest implements RewriteTest {
+
+    @Override
+    public void defaults(RecipeSpec spec) {
+        spec.recipe(new UseMainClassPropertyForApplication());
+    }
+
+    @DocumentExample
+    @Test
+    void mainClassNameInApplicationBlock() {
+        rewriteRun(
+          buildGradle(
+            """
+              plugins {
+                  id 'application'
+              }
+
+              application {
+                  mainClassName = "com.example.AppMain"
+              }
+              """,
+            """
+              plugins {
+                  id 'application'
+              }
+
+              application {
+                  mainClass = "com.example.AppMain"
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void mainClassNameInApplicationBlockKotlinDsl() {
+        rewriteRun(
+          buildGradleKts(
+            """
+              plugins {
+                  application
+              }
+
+              application {
+                  mainClassName = "com.example.AppMain"
+              }
+              """,
+            """
+              plugins {
+                  application
+              }
+
+              application {
+                  mainClass = "com.example.AppMain"
+              }
+              """
+          )
+        );
+    }
+
+    @Nested
+    class NoChange {
+        @Test
+        void noChangeWhenAlreadyMigrated() {
+            rewriteRun(
+              buildGradle(
+                """
+                  plugins {
+                      id 'application'
+                  }
+
+                  application {
+                      mainClass = "com.example.AppMain"
+                  }
+                  """
+              )
+            );
+        }
+
+        @Test
+        void noChangeWhenAlreadyMigratedKotlinDsl() {
+            rewriteRun(
+              buildGradleKts(
+                """
+                  plugins {
+                      application
+                  }
+
+                  application {
+                      mainClass = "com.example.AppMain"
+                  }
+                  """
+              )
+            );
+        }
+
+        @Test
+        void noChangeOutsideApplicationBlock() {
+            rewriteRun(
+              buildGradle(
+                """
+                  plugins {
+                      id 'java'
+                  }
+
+                  ext {
+                      mainClassName = "com.example.AppMain"
+                  }
+                  """
+              )
+            );
+        }
+    }
+}