Move isolated project tests to separate file and fix ktlint incompatibility

Author: inorichi

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/BaseKotlinExtension.java

@@ -160,7 +160,7 @@ private KtlintConfig(
 				Map<String, Object> editorConfigOverride,
 				List<String> customRuleSets) throws IOException {
 			Objects.requireNonNull(version);
-			File defaultEditorConfig = getProject().getRootProject().getLayout().getProjectDirectory().file(".editorconfig").getAsFile();
+			File defaultEditorConfig = new File(getProject().getRootProject().getProjectDir(), ".editorconfig");
 			FileSignature editorConfigPath = defaultEditorConfig.exists() ? FileSignature.signAsList(defaultEditorConfig) : null;
 			this.version = version;
 			this.editorConfigPath = editorConfigPath;

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/IsolatedProjectTest.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright 2026 DiffPlug
+ *
+ * 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
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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 com.diffplug.gradle.spotless;
+
+import java.io.IOException;
+
+import org.assertj.core.api.Assertions;
+import org.gradle.testkit.runner.GradleRunner;
+import org.junit.jupiter.api.Test;
+
+import com.diffplug.common.base.StringPrinter;
+
+class IsolatedProjectTest extends GradleIntegrationHarness {
+	private static final int N = 10;
+
+	@Override
+	public GradleRunner gradleRunner() throws IOException {
+		setFile("gradle.properties").toContent("org.gradle.unsafe.isolated-projects=true");
+		return super.gradleRunner();
+	}
+
+	private void createNSubprojects() throws IOException {
+		for (int i = 0; i < N; i++) {
+			createSubproject(Integer.toString(i));
+		}
+		String settings = StringPrinter.buildString(printer -> {
+			for (int i = 0; i < N; i++) {
+				printer.println("include '" + i + "'");
+			}
+		});
+		setFile("settings.gradle").toContent(settings);
+	}
+
+	void createSubproject(String name) throws IOException {
+		setFile(name + "/build.gradle").toLines(
+				"plugins {",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"repositories { mavenCentral() }",
+				"spotless {",
+				"    kotlin {",
+				"		 target file('Test.kt')",
+				"        ktlint()",
+				"    }",
+				"}");
+		setFile(name + "/Test.kt").toResource("kotlin/ktlint/basic.dirty");
+	}
+
+	@Test
+	void rootIsSupported() throws IOException {
+		setFile("build.gradle").toLines(
+				"plugins {",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"repositories { mavenCentral() }",
+				"spotless {",
+				"    kotlin {",
+				"		 target file('Test.kt')",
+				"        ktlint()",
+				"    }",
+				"}");
+		setFile("Test.kt").toResource("kotlin/ktlint/basic.dirty");
+		createNSubprojects();
+		gradleRunner().withArguments("spotlessApply").build();
+	}
+
+	@Test
+	void noRootIsSupported() throws IOException {
+		setFile("build.gradle").toLines();
+		createNSubprojects();
+		gradleRunner().withArguments("spotlessApply").build();
+	}
+
+	@Test
+	void predeclaredIsUnsupported() throws IOException {
+		setFile("build.gradle").toLines(
+				"plugins {",
+				"    id 'com.diffplug.spotless'",
+				"}",
+				"repositories { mavenCentral() }",
+				"spotless { predeclareDeps() }",
+				"spotlessPredeclare {",
+				" kotlin { ktlint() }",
+				"}");
+		createNSubprojects();
+		Assertions.assertThat(gradleRunner().withArguments("spotlessApply").buildAndFail().getOutput())
+				.containsAnyOf("Cannot access project", "cannot access 'Project.tasks'");
+	}
+}

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/MultiProjectTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2026 DiffPlug
+ * Copyright 2016-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -151,38 +151,4 @@ public void predeclaredUndeclared() throws IOException {
 		Assertions.assertThat(gradleRunner().withArguments("spotlessApply").buildAndFail().getOutput())
 				.contains("Could not find method spotlessPredeclare() for arguments");
 	}
-
-	@Test
-	void nonPredeclaredSupportsIsolatedProjects() throws IOException {
-		setFile("build.gradle").toLines(
-				"plugins {",
-				"    id 'com.diffplug.spotless'",
-				"}",
-				"repositories { mavenCentral() }",
-				"",
-				"spotless {",
-				"    java {",
-				"        target file('test.java')",
-				"        googleJavaFormat('1.17.0')",
-				"    }",
-				"}");
-		createNSubprojects();
-		gradleRunner().withArguments("spotlessApply", "-Dorg.gradle.unsafe.isolated-projects=true").build();
-	}
-
-	@Test
-	void predeclaredRequiresNonIsolatedProjects() throws IOException {
-		setFile("build.gradle").toLines(
-				"plugins {",
-				"    id 'com.diffplug.spotless'",
-				"}",
-				"repositories { mavenCentral() }",
-				"spotless { predeclareDeps() }",
-				"spotlessPredeclare {",
-				" java { googleJavaFormat('1.17.0') }",
-				"}");
-		createNSubprojects();
-		Assertions.assertThat(gradleRunner().withArguments("spotlessApply", "-Dorg.gradle.unsafe.isolated-projects=true").buildAndFail().getOutput())
-				.containsAnyOf("Cannot access project", "cannot access 'Project.tasks'");
-	}
 }