Reduce unnecessary immutable object allocations in model building

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/DependencyManagementInjector.java

@@ -32,10 +32,11 @@ public interface DependencyManagementInjector {
     /**
      * Merges default values from the dependency management section of the given model into itself.
      *
-     * @param model The model into which to merge the values specified by its dependency management sections, must not
-     *            be <code>null</code>.
+     * @param model The model to read from, must not be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model injectManagement(Model model, ModelBuilderRequest request, ModelProblemCollector problems);
+    void injectManagement(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/InheritanceAssembler.java

@@ -36,10 +36,15 @@ public interface InheritanceAssembler {
      *
      * @param child The child model into which to merge the values inherited from the parent, must not be
      *            <code>null</code>.
+     * @param builder The builder to write inherited values into, must not be {@code null}.
      * @param parent The (read-only) parent model from which to inherit the values, may be <code>null</code>.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model assembleModelInheritance(
-            Model child, Model parent, ModelBuilderRequest request, ModelProblemCollector problems);
+    void assembleModelInheritance(
+            Model child,
+            Model.Builder builder,
+            Model parent,
+            ModelBuilderRequest request,
+            ModelProblemCollector problems);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/ModelNormalizer.java

@@ -33,19 +33,23 @@ public interface ModelNormalizer {
     /**
      * Merges duplicate elements like multiple declarations of the same build plugin in the specified model.
      *
-     * @param model The model whose duplicate elements should be merged, must not be {@code null}.
+     * @param model The model to read from, must not be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model mergeDuplicates(Model model, ModelBuilderRequest request, ModelProblemCollector problems);
+    void mergeDuplicates(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems);
 
     /**
      * Sets default values in the specified model that for technical reasons cannot be set directly in the Modello
      * definition.
      *
-     * @param model The model in which to set the default values, must not be {@code null}.
+     * @param model The model to read from, must not be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model injectDefaultValues(Model model, ModelBuilderRequest request, ModelProblemCollector problems);
+    void injectDefaultValues(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/ModelPathTranslator.java

@@ -34,10 +34,11 @@ public interface ModelPathTranslator {
      * Resolves the well-known paths of the specified model against the given base directory. Paths within plugin
      * configuration are not processed.
      *
-     * @param model The model whose paths should be resolved, may be {@code null}.
+     * @param model The model to read from, may be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param basedir The base directory to resolve relative paths against, may be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @since 4.0.0
      */
-    Model alignToBaseDirectory(Model model, Path basedir, ModelBuilderRequest request);
+    void alignToBaseDirectory(Model model, Model.Builder builder, Path basedir, ModelBuilderRequest request);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/ModelUrlNormalizer.java

@@ -32,8 +32,9 @@ public interface ModelUrlNormalizer {
     /**
      * Normalizes the well-known URLs of the specified model.
      *
-     * @param model The model whose URLs should be normalized, may be {@code null}.
+     * @param model The model to read from, may be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      */
-    Model normalize(Model model, ModelBuilderRequest request);
+    void normalize(Model model, Model.Builder builder, ModelBuilderRequest request);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/PluginConfigurationExpander.java

@@ -32,9 +32,11 @@ public interface PluginConfigurationExpander {
     /**
      * Merges values from general build plugin configuration into the individual plugin executions of the given model.
      *
-     * @param model The model whose build plugin configuration should be expanded, must not be <code>null</code>.
+     * @param model The model to read from, must not be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model expandPluginConfiguration(Model model, ModelBuilderRequest request, ModelProblemCollector problems);
+    void expandPluginConfiguration(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/PluginManagementInjector.java

@@ -32,10 +32,11 @@ public interface PluginManagementInjector {
     /**
      * Merges default values from the plugin management section of the given model into itself.
      *
-     * @param model The model into which to merge the values specified by its plugin management section, must not be
-     *            <code>null</code>.
+     * @param model The model to read from, must not be {@code null}.
+     * @param builder The model builder to write changes to, must not be {@code null}.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model injectManagement(Model model, ModelBuilderRequest request, ModelProblemCollector problems);
+    void injectManagement(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems);
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/services/model/ProfileInjector.java

@@ -37,24 +37,34 @@ public interface ProfileInjector {
      * and model completely decoupled by injecting deep copies rather than the original objects from the profile.
      *
      * @param model The model into which to merge the values defined by the profile, must not be <code>null</code>.
+     * @param builder The builder to write profile-injected values into, must not be {@code null}.
      * @param profile The (read-only) profile whose values should be injected, may be <code>null</code>.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    default Model injectProfile(
-            Model model, Profile profile, ModelBuilderRequest request, ModelProblemCollector problems) {
-        return injectProfiles(model, List.of(profile), request, problems);
+    default void injectProfile(
+            Model model,
+            Model.Builder builder,
+            Profile profile,
+            ModelBuilderRequest request,
+            ModelProblemCollector problems) {
+        injectProfiles(model, builder, List.of(profile), request, problems);
     }
 
     /**
-     * Merges values from the specified profile into the given model. Implementations are expected to keep the profile
+     * Merges values from the specified profiles into the given model. Implementations are expected to keep the profile
      * and model completely decoupled by injecting deep copies rather than the original objects from the profile.
      *
-     * @param model The model into which to merge the values defined by the profile, must not be <code>null</code>.
+     * @param model The model into which to merge the values defined by the profiles, must not be <code>null</code>.
+     * @param builder The builder to write profile-injected values into, must not be {@code null}.
      * @param profiles The (read-only) list of profiles whose values should be injected, must not be <code>null</code>.
      * @param request The model building request that holds further settings, must not be {@code null}.
      * @param problems The container used to collect problems that were encountered, must not be {@code null}.
      */
-    Model injectProfiles(
-            Model model, List<Profile> profiles, ModelBuilderRequest request, ModelProblemCollector problems);
+    void injectProfiles(
+            Model model,
+            Model.Builder builder,
+            List<Profile> profiles,
+            ModelBuilderRequest request,
+            ModelProblemCollector problems);
 }

impl/maven-impl/src/main/java/org/apache/maven/impl/DefaultModelUrlNormalizer.java

@@ -46,12 +46,11 @@ public DefaultModelUrlNormalizer(UrlNormalizer urlNormalizer) {
     }
 
     @Override
-    public Model normalize(Model model, ModelBuilderRequest request) {
+    public void normalize(Model model, Model.Builder builder, ModelBuilderRequest request) {
         if (model == null) {
-            return null;
+            return;
         }
 
-        Model.Builder builder = Model.newBuilder(model);
         builder.url(normalize(model.getUrl()));
 
         Scm scm = model.getScm();
@@ -70,8 +69,6 @@ public Model normalize(Model model, ModelBuilderRequest request) {
                 builder.distributionManagement(dist.withSite(site.withUrl(normalize(site.getUrl()))));
             }
         }
-
-        return builder.build();
     }
 
     private String normalize(String url) {

impl/maven-impl/src/main/java/org/apache/maven/impl/DefaultPluginConfigurationExpander.java

@@ -45,22 +45,48 @@
 public class DefaultPluginConfigurationExpander implements PluginConfigurationExpander {
 
     @Override
-    public Model expandPluginConfiguration(Model model, ModelBuilderRequest request, ModelProblemCollector problems) {
+    public void expandPluginConfiguration(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems) {
         Build build = model.getBuild();
         if (build != null) {
-            build = build.withPlugins(expandPlugin(build.getPlugins()));
+            List<Plugin> expandedPlugins = expandPlugin(build.getPlugins());
             PluginManagement pluginManagement = build.getPluginManagement();
-            if (pluginManagement != null) {
-                build = build.withPluginManagement(
-                        pluginManagement.withPlugins(expandPlugin(pluginManagement.getPlugins())));
+            List<Plugin> expandedMgmtPlugins =
+                    pluginManagement != null ? expandPlugin(pluginManagement.getPlugins()) : null;
+
+            boolean buildModified = expandedPlugins != build.getPlugins()
+                    || (expandedMgmtPlugins != null && expandedMgmtPlugins != pluginManagement.getPlugins());
+
+            if (buildModified) {
+                Build.Builder bb = Build.newBuilder(build);
+                if (expandedPlugins != build.getPlugins()) {
+                    bb.plugins(expandedPlugins);
+                }
+                if (expandedMgmtPlugins != null && expandedMgmtPlugins != pluginManagement.getPlugins()) {
+                    bb.pluginManagement(PluginManagement.newBuilder(pluginManagement)
+                            .plugins(expandedMgmtPlugins)
+                            .build());
+                }
+                build = bb.build();
             }
-            model = model.withBuild(build);
         }
         Reporting reporting = model.getReporting();
+        List<ReportPlugin> expandedReportPlugins = null;
         if (reporting != null) {
-            expandReport(reporting.getPlugins());
+            expandedReportPlugins = expandReport(reporting.getPlugins());
+        }
+        boolean modelModified = build != model.getBuild()
+                || (expandedReportPlugins != null && expandedReportPlugins != reporting.getPlugins());
+        if (modelModified) {
+            if (build != model.getBuild()) {
+                builder.build(build);
+            }
+            if (expandedReportPlugins != null && expandedReportPlugins != reporting.getPlugins()) {
+                builder.reporting(Reporting.newBuilder(reporting)
+                        .plugins(expandedReportPlugins)
+                        .build());
+            }
         }
-        return model.withBuild(build);
     }
 
     private List<Plugin> expandPlugin(List<Plugin> oldPlugins) {

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultDependencyManagementInjector.java

@@ -46,16 +46,17 @@ public class DefaultDependencyManagementInjector implements DependencyManagement
     private ManagementModelMerger merger = new ManagementModelMerger();
 
     @Override
-    public Model injectManagement(Model model, ModelBuilderRequest request, ModelProblemCollector problems) {
-        return merger.mergeManagedDependencies(model);
+    public void injectManagement(
+            Model model, Model.Builder builder, ModelBuilderRequest request, ModelProblemCollector problems) {
+        merger.mergeManagedDependencies(model, builder);
     }
 
     /**
      * ManagementModelMerger
      */
     protected static class ManagementModelMerger extends MavenModelMerger {
 
-        public Model mergeManagedDependencies(Model model) {
+        public void mergeManagedDependencies(Model model, Model.Builder builder) {
             DependencyManagement dependencyManagement = model.getDependencyManagement();
             if (dependencyManagement != null) {
                 Map<Object, Dependency> dependencies = new HashMap<>();
@@ -86,10 +87,9 @@ public Model mergeManagedDependencies(Model model) {
                         Dependency dependency = dependencies.get(key);
                         newDeps.add(dependency);
                     }
-                    return Model.newBuilder(model).dependencies(newDeps).build();
+                    builder.dependencies(newDeps);
                 }
             }
-            return model;
         }
 
         @Override

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java

@@ -37,7 +37,6 @@
 import org.apache.maven.api.services.ModelBuilderRequest;
 import org.apache.maven.api.services.ModelProblemCollector;
 import org.apache.maven.api.services.model.InheritanceAssembler;
-import org.apache.maven.model.v4.MavenMerger;
 
 /**
  * Handles inheritance of model values.
@@ -52,25 +51,32 @@ public class DefaultInheritanceAssembler implements InheritanceAssembler {
 
     private static final String CHILD_DIRECTORY_PROPERTY = "project.directory";
 
-    private final MavenMerger merger;
+    private final InheritanceModelMerger merger;
 
     @Inject
     public DefaultInheritanceAssembler() {
         this(new InheritanceModelMerger());
     }
 
-    public DefaultInheritanceAssembler(MavenMerger merger) {
+    public DefaultInheritanceAssembler(InheritanceModelMerger merger) {
         this.merger = merger;
     }
 
     @Override
-    public Model assembleModelInheritance(
-            Model child, Model parent, ModelBuilderRequest request, ModelProblemCollector problems) {
+    public void assembleModelInheritance(
+            Model child,
+            Model.Builder builder,
+            Model parent,
+            ModelBuilderRequest request,
+            ModelProblemCollector problems) {
+        if (parent == null) {
+            return;
+        }
         Map<Object, Object> hints = new HashMap<>();
         String childPath = child.getProperties().getOrDefault(CHILD_DIRECTORY_PROPERTY, child.getArtifactId());
         hints.put(CHILD_DIRECTORY, childPath);
         hints.put(MavenModelMerger.CHILD_PATH_ADJUSTMENT, getChildPathAdjustment(child, parent, childPath));
-        return merger.merge(child, parent, false, hints);
+        merger.mergeIntoBuilder(builder, child, parent, false, hints);
     }
 
     /**
@@ -140,6 +146,15 @@ private String getChildPathAdjustment(Model child, Model parent, String childDir
      */
     protected static class InheritanceModelMerger extends MavenModelMerger {
 
+        public void mergeIntoBuilder(
+                Model.Builder builder, Model target, Model source, boolean sourceDominant, Map<?, ?> hints) {
+            Map<Object, Object> context = new HashMap<>();
+            if (hints != null) {
+                context.putAll(hints);
+            }
+            mergeModel(builder, target, source, sourceDominant, context);
+        }
+
         @Override
         protected String extrapolateChildUrl(String parentUrl, boolean appendPath, Map<Object, Object> context) {
             Object childDirectory = context.get(CHILD_DIRECTORY);