Disable consumer POM flattening by default and add an opt-in feature (#11347)

This PR introduces a new feature flag maven.consumer.pom.flatten that allows users to control whether consumer POMs are flattened by removing dependency management sections. This addresses dependency management inheritance scenarios and provides better control over consumer POM generation.

The consumer POM are NOT flattened anymore by default.

Fixes #11346

Author: gnodet

api/maven-api-core/src/main/java/org/apache/maven/api/Constants.java

@@ -463,6 +463,18 @@ public final class Constants {
     @Config(type = "java.lang.Boolean", defaultValue = "true")
     public static final String MAVEN_CONSUMER_POM = "maven.consumer.pom";
 
+    /**
+     * User property for controlling consumer POM flattening behavior.
+     * When set to <code>true</code> (default), consumer POMs are flattened by removing
+     * dependency management and keeping only direct dependencies with transitive scopes.
+     * When set to <code>false</code>, consumer POMs preserve dependency management
+     * like parent POMs, allowing dependency management to be inherited by consumers.
+     *
+     * @since 4.1.0
+     */
+    @Config(type = "java.lang.Boolean", defaultValue = "false")
+    public static final String MAVEN_CONSUMER_POM_FLATTEN = "maven.consumer.pom.flatten";
+
     /**
      * User property for controlling "maven personality". If activated Maven will behave
      * like the previous major version, Maven 3.

api/maven-api-core/src/main/java/org/apache/maven/api/feature/Features.java

@@ -47,6 +47,13 @@ public static boolean consumerPom(@Nullable Map<String, ?> userProperties) {
         return doGet(userProperties, Constants.MAVEN_CONSUMER_POM, !mavenMaven3Personality(userProperties));
     }
 
+    /**
+     * Check if consumer POM flattening is enabled.
+     */
+    public static boolean consumerPomFlatten(@Nullable Map<String, ?> userProperties) {
+        return doGet(userProperties, Constants.MAVEN_CONSUMER_POM_FLATTEN, false);
+    }
+
     /**
      * Check if build POM deployment is enabled.
      */

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java

@@ -33,6 +33,7 @@
 import org.apache.maven.api.Node;
 import org.apache.maven.api.PathScope;
 import org.apache.maven.api.SessionData;
+import org.apache.maven.api.feature.Features;
 import org.apache.maven.api.model.Dependency;
 import org.apache.maven.api.model.DistributionManagement;
 import org.apache.maven.api.model.Model;
@@ -72,6 +73,15 @@ class DefaultConsumerPomBuilder implements PomBuilder {
     @Override
     public Model build(RepositorySystemSession session, MavenProject project, Path src) throws ModelBuilderException {
         Model model = project.getModel().getDelegate();
+        boolean flattenEnabled = Features.consumerPomFlatten(session.getConfigProperties());
+
+        // Check if consumer POM flattening is disabled
+        if (!flattenEnabled) {
+            // When flattening is disabled, treat non-POM projects like parent POMs
+            // Apply only basic transformations without flattening dependency management
+            return buildPom(session, project, src);
+        }
+        // Default behavior: flatten the consumer POM
         String packaging = model.getPackaging();
         String originalPackaging = project.getOriginalModel().getPackaging();
         if (POM_PACKAGING.equals(packaging)) {
@@ -256,6 +266,7 @@ static Model transformNonPom(Model model, MavenProject project) {
             warnNotDowngraded(project);
         }
         model = model.withModelVersion(modelVersion);
+
         return model;
     }
 

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh11084ReactorReaderPreferConsumerPomTest.java

@@ -35,15 +35,15 @@ void partialReactorShouldResolveUsingConsumerPom() throws Exception {
 
         // First build module a to populate project-local-repo with artifacts including consumer POM
         Verifier v1 = newVerifier(testDir.getAbsolutePath());
-        v1.addCliArguments("clean", "package", "-X");
+        v1.addCliArguments("clean", "package", "-X", "-Dmaven.consumer.pom.flatten=true");
         v1.setLogFileName("log-1.txt");
         v1.execute();
         v1.verifyErrorFreeLog();
 
         // Now build only module b; ReactorReader should pick consumer POM from project-local-repo
         Verifier v2 = newVerifier(testDir.getAbsolutePath());
         v2.setLogFileName("log-2.txt");
-        v2.addCliArguments("clean", "compile", "-f", "b", "-X");
+        v2.addCliArguments("clean", "compile", "-f", "b", "-X", "-Dmaven.consumer.pom.flatten=true");
         v2.execute();
         v2.verifyErrorFreeLog();
     }

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh11162ConsumerPomScopesTest.java

@@ -44,6 +44,7 @@ void testConsumerPomFiltersScopes() throws Exception {
 
         Verifier verifier = newVerifier(basedir.toString());
         verifier.addCliArgument("install");
+        verifier.addCliArgument("-Dmaven.consumer.pom.flatten=true");
         verifier.execute();
         verifier.verifyErrorFreeLog();
 

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh11346DependencyManagementOverrideTest.java

@@ -0,0 +1,121 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.maven.it;
+
+import java.io.File;
+import java.util.List;
+
+import org.apache.maven.api.Constants;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * This is a test set for dependency management override scenarios when
+ * consumer POM flattening is disabled (maven.consumer.pom.flatten=false).
+ *
+ * Scenario:
+ * - A 1.0 depends on B 1.0 and manages C to 1.2
+ * - B 1.0 has no dependencies
+ * - B 2.0 depends on C 1.1
+ * - D depends on A 1.0 and manages B to 2.0
+ *
+ * Question: Does D depend on C, and which version?
+ *
+ * Expected behavior when flattening is disabled: D should get C 1.2 (from A's dependency management),
+ * not C 1.1 (from B 2.0's dependency), because A's dependency
+ * management applies to D's transitive dependencies.
+ *
+ * @see <a href="https://github.com/apache/maven/issues/11346">gh-11346</a>
+ */
+public class MavenITgh11346DependencyManagementOverrideTest extends AbstractMavenIntegrationTestCase {
+
+    /**
+     * Verify that when consumer POM flattening is disabled, dependency management
+     * from intermediate dependencies applies to the consumer's transitive dependencies.
+     * This test uses -Dmaven.consumer.pom.flatten=false to enable dependency management
+     * inheritance from transitive dependencies.
+     *
+     * @throws Exception in case of failure
+     */
+    @Test
+    public void testDependencyManagementOverride() throws Exception {
+        File testDir = extractResources("/gh-11346-dependency-management-override");
+
+        Verifier verifier = newVerifier(testDir.getAbsolutePath());
+        verifier.deleteArtifacts("org.apache.maven.its.mng.depman");
+        // Test with dependency manager transitivity disabled instead of consumer POM flattening
+        verifier.addCliArgument("-D" + Constants.MAVEN_CONSUMER_POM_FLATTEN + "=false");
+        verifier.addCliArgument("verify");
+        verifier.execute();
+        verifier.verifyErrorFreeLog();
+
+        // Check module D's classpath
+        List<String> dClasspath = verifier.loadLines("module-d/target/classpath.txt");
+
+        // D should have A 1.0
+        assertTrue(dClasspath.contains("module-a-1.0.jar"), "D should depend on A 1.0: " + dClasspath);
+
+        // D should have B 2.0 (managed by D)
+        assertTrue(dClasspath.contains("module-b-2.0.jar"), "D should depend on B 2.0 (managed by D): " + dClasspath);
+        assertFalse(dClasspath.contains("module-b-1.0.jar"), "D should not depend on B 1.0: " + dClasspath);
+
+        // D should have C 1.2 (from A's dependency management)
+        // A's dependency management of C to 1.2 should apply to D
+        assertTrue(
+                dClasspath.contains("module-c-1.2.jar"),
+                "D should depend on C 1.2 (A's dependency management should apply): " + dClasspath);
+        assertFalse(
+                dClasspath.contains("module-c-1.1.jar"),
+                "D should not depend on C 1.1 (should be managed to 1.2): " + dClasspath);
+    }
+
+    @Test
+    public void testDependencyManagementOverrideNoTransitive() throws Exception {
+        File testDir = extractResources("/gh-11346-dependency-management-override");
+
+        Verifier verifier = newVerifier(testDir.getAbsolutePath());
+        verifier.deleteArtifacts("org.apache.maven.its.mng.depman");
+        // Test with dependency manager transitivity disabled instead of consumer POM flattening
+        verifier.addCliArgument("-D" + Constants.MAVEN_CONSUMER_POM_FLATTEN + "=false");
+        verifier.addCliArgument("-D" + Constants.MAVEN_RESOLVER_DEPENDENCY_MANAGER_TRANSITIVITY + "=false");
+        verifier.addCliArgument("verify");
+        verifier.execute();
+        verifier.verifyErrorFreeLog();
+
+        // Check module D's classpath
+        List<String> dClasspath = verifier.loadLines("module-d/target/classpath.txt");
+
+        // D should have A 1.0
+        assertTrue(dClasspath.contains("module-a-1.0.jar"), "D should depend on A 1.0: " + dClasspath);
+
+        // D should have B 2.0 (managed by D)
+        assertTrue(dClasspath.contains("module-b-2.0.jar"), "D should depend on B 2.0 (managed by D): " + dClasspath);
+        assertFalse(dClasspath.contains("module-b-1.0.jar"), "D should not depend on B 1.0: " + dClasspath);
+
+        // D should have C 1.1 as the resolver is not transitive
+        assertFalse(
+                dClasspath.contains("module-c-1.2.jar"),
+                "D should depend on C 1.2 (A's dependency management should apply): " + dClasspath);
+        assertTrue(
+                dClasspath.contains("module-c-1.1.jar"),
+                "D should not depend on C 1.1 (should be managed to 1.2): " + dClasspath);
+    }
+}

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITmng5102MixinsTest.java

@@ -48,7 +48,7 @@ public void testWithPath() throws Exception {
         verifier.setAutoclean(false);
         verifier.deleteDirectory("target");
         verifier.deleteArtifacts("org.apache.maven.its.mng5102");
-        verifier.addCliArgument("install");
+        verifier.addCliArguments("install", "-Dmaven.consumer.pom.flatten=true");
         verifier.execute();
         verifier.verifyErrorFreeLog();
 

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITmng6656BuildConsumer.java

@@ -64,7 +64,7 @@ public void testPublishedPoms() throws Exception {
         verifier.setAutoclean(false);
         verifier.addCliArgument("-Dchangelist=MNG6656");
 
-        verifier.addCliArgument("install");
+        verifier.addCliArguments("install", "-Dmaven.consumer.pom.flatten=true");
         verifier.execute();
         verifier.verifyErrorFreeLog();
 

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITmng6957BuildConsumer.java

@@ -62,7 +62,7 @@ public void testPublishedPoms() throws Exception {
 
         Verifier verifier = newVerifier(testDir.getAbsolutePath());
         verifier.setAutoclean(false);
-        verifier.addCliArgument("-Dchangelist=MNG6957");
+        verifier.addCliArguments("-Dchangelist=MNG6957", "-Dmaven.consumer.pom.flatten=true");
 
         verifier.addCliArgument("install");
         verifier.execute();

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITmng8414ConsumerPomWithNewFeaturesTest.java

@@ -46,7 +46,7 @@ void testNotPreserving() throws Exception {
                 extractResources("/mng-8414-consumer-pom-with-new-features").toPath();
 
         Verifier verifier = newVerifier(basedir.toString(), null);
-        verifier.addCliArguments("package");
+        verifier.addCliArguments("package", "-Dmaven.consumer.pom.flatten=true");
         verifier.execute();
         verifier.verifyErrorFreeLog();
 
@@ -78,7 +78,7 @@ void testPreserving() throws Exception {
 
         Verifier verifier = newVerifier(basedir.toString(), null);
         verifier.setLogFileName("log-preserving.txt");
-        verifier.addCliArguments("-f", "pom-preserving.xml", "package");
+        verifier.addCliArguments("-f", "pom-preserving.xml", "package", "-Dmaven.consumer.pom.flatten=true");
         verifier.execute();
         verifier.verifyErrorFreeLog();