Gradle: rename mainClassName to mainClass in application block (#7540)

* 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`.

* Regenerate recipes.csv

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