Fix for illegal mutation issue with predeclareDeps

plugin-gradle/CHANGES.md

@@ -5,6 +5,8 @@ We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (
 ## [Unreleased]
 ### Added
 - Add `tableTest` format type for standalone `.table` files.
+### Fixed
+- Fix illegal mutation when using predeclared dependencies.
 ### Changes
 - Bump default `tabletest-formatter` version `1.0.1` -> `1.1.1`, now works with Java 17+.
 

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

@@ -32,7 +32,7 @@ public SpotlessExtensionPredeclare(Project project, GradleProvisioner.Policy pol
 		super(project);
 		this.registerDependenciesTask = findRegisterDepsTask().get();
 		SpotlessTaskService taskService = getSpotlessTaskService().get();
-		taskService.isUsingPredeclared = true;
+		taskService.registerDependenciesTask = registerDependenciesTask;
 		taskService.predeclaredProvisioner = policy.dedupingProvisioner(project);
 		taskService.predeclaredP2Provisioner = policy.dedupingP2Provisioner(project);
 		project.afterEvaluate(unused -> toSetup.forEach((name, formatExtension) -> {

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

@@ -56,14 +56,14 @@
  * apply already did).
  */
 public abstract class SpotlessTaskService implements BuildService<BuildServiceParameters.None>, AutoCloseable, OperationCompletionListener {
-	protected boolean isUsingPredeclared;
 	private final Map<String, SpotlessApply> apply = Collections.synchronizedMap(new HashMap<>());
 	private final Map<String, SpotlessTask> source = Collections.synchronizedMap(new HashMap<>());
 	private final Map<String, Provisioner> provisioner = Collections.synchronizedMap(new HashMap<>());
 	private final Map<String, P2Provisioner> p2Provisioner = Collections.synchronizedMap(new HashMap<>());
 
 	@Nullable GradleProvisioner.DedupingProvisioner predeclaredProvisioner;
 	@Nullable GradleProvisioner.DedupingP2Provisioner predeclaredP2Provisioner;
+	@Nullable RegisterDependenciesTask registerDependenciesTask;
 
 	Provisioner provisionerFor(SpotlessExtension spotless) {
 		if (spotless instanceof SpotlessExtensionPredeclare) {
@@ -129,12 +129,9 @@ static void usesServiceTolerateTestFailure(DefaultTask task, Provider<SpotlessTa
 	}
 
 	public void hookSubprojectTask(Project project, SpotlessTask task) {
-		// This check allows isolated projects support by not accessing the root project tasks unless really needed
-		if (!isUsingPredeclared) {
-			return;
+		if (registerDependenciesTask != null) {
+			registerDependenciesTask.hookSubprojectTask(task);
 		}
-
-		project.getRootProject().getTasks().withType(RegisterDependenciesTask.class, registerTask -> registerTask.hookSubprojectTask(task));
 	}
 
 	public static Provider<SpotlessTaskService> registerIfAbsent(Project project, String suffix) {

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

@@ -17,7 +17,6 @@
 
 import java.io.IOException;
 
-import org.assertj.core.api.Assertions;
 import org.gradle.testkit.runner.GradleRunner;
 import org.junit.jupiter.api.Test;
 
@@ -85,7 +84,7 @@ void noRootIsSupported() throws IOException {
 	}
 
 	@Test
-	void predeclaredIsUnsupported() throws IOException {
+	void predeclaredIsSupported() throws IOException {
 		setFile("build.gradle").toLines(
 				"plugins {",
 				"    id 'com.diffplug.spotless'",
@@ -96,7 +95,6 @@ void predeclaredIsUnsupported() throws IOException {
 				" kotlin { ktlint() }",
 				"}");
 		createNSubprojects();
-		Assertions.assertThat(gradleRunner().withArguments("spotlessApply").buildAndFail().getOutput())
-				.containsAnyOf("Cannot access project", "cannot access 'Project.tasks'");
+		gradleRunner().withArguments("spotlessApply").build();
 	}
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2025 DiffPlug
+ * Copyright 2016-2026 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -137,6 +137,36 @@ public void predeclaredOrdering() throws IOException {
 				.contains("Could not find method spotlessPredeclare() for arguments");
 	}
 
+	@Test
+	public void predeclaredDepsRegression() throws IOException {
+		setFile("settings.gradle").toContent("include 'sub'");
+		setFile("build.gradle").toLines(
+				"plugins { id 'com.diffplug.spotless' }",
+				"repositories { mavenCentral() }",
+				"spotless {",
+				"    predeclareDeps()",
+				"    java {",
+				"        target file('test.java')",
+				"        googleJavaFormat('1.17.0')",
+				"    }",
+				"}",
+				"spotlessPredeclare {",
+				"    java { googleJavaFormat('1.17.0') }",
+				"}");
+		setFile("test.java").toResource("java/googlejavaformat/JavaCodeUnformatted.test");
+		setFile("sub/build.gradle").toLines(
+				"plugins { id 'com.diffplug.spotless' }",
+				"repositories { mavenCentral() }",
+				"spotless {",
+				"    java {",
+				"        target file('test.java')",
+				"        googleJavaFormat('1.17.0')",
+				"    }",
+				"}");
+		setFile("sub/test.java").toResource("java/googlejavaformat/JavaCodeUnformatted.test");
+		gradleRunner().withGradleVersion("8.14").withArguments("spotlessApply").build();
+	}
+
 	@Test
 	public void predeclaredUndeclared() throws IOException {
 		setFile("build.gradle").toLines(