Enh: Allow validator factory to abstain

maven-resolver-impl/src/test/java/org/eclipse/aether/internal/impl/DefaultRepositorySystemReentrancyTest.java

@@ -100,7 +100,7 @@ public void validateArtifact(org.eclipse.aether.artifact.Artifact artifact) {
                         new DefaultRepositoryKeyFunctionFactory()),
                 new DefaultRepositorySystemLifecycle(),
                 Collections.emptyMap(),
-                new DefaultRepositorySystemValidator(Collections.singletonList(countingValidator)));
+                new DefaultRepositorySystemValidator(Collections.singletonMap("counting", countingValidator)));
         session = TestUtils.newSession();
     }
 
@@ -162,7 +162,7 @@ void reentrantCallAllowsUninterpolatedExpressions() throws Exception {
                 new DefaultRepositorySystemLifecycle(),
                 Collections.emptyMap(),
                 new DefaultRepositorySystemValidator(
-                        Collections.singletonList(EXPRESSION_REJECTING_VALIDATOR_FACTORY)));
+                        Collections.singletonMap("expressionRejecting", EXPRESSION_REJECTING_VALIDATOR_FACTORY)));
 
         // An outermost call with an uninterpolated expression MUST fail validation
         VersionRangeRequest outerBadRequest =
@@ -221,7 +221,7 @@ public float getPriority() {
                         new DefaultRepositoryKeyFunctionFactory()),
                 new DefaultRepositorySystemLifecycle(),
                 Collections.singletonMap("test", decoratorFactory),
-                new DefaultRepositorySystemValidator(Collections.emptyList()));
+                new DefaultRepositorySystemValidator(Collections.emptyMap()));
 
         // Outermost call: decorator should run
         ArtifactDescriptorRequest outerRequest =

maven-resolver-impl/src/test/java/org/eclipse/aether/internal/impl/DefaultRepositorySystemTest.java

@@ -69,7 +69,7 @@ void init() {
                         new DefaultRepositoryKeyFunctionFactory()),
                 new DefaultRepositorySystemLifecycle(),
                 Collections.emptyMap(),
-                new DefaultRepositorySystemValidator(Collections.emptyList()));
+                new DefaultRepositorySystemValidator(Collections.emptyMap()));
         session = TestUtils.newSession();
     }
 

maven-resolver-impl/src/test/java/org/eclipse/aether/internal/impl/DefaultRepositorySystemValidatorTest.java

@@ -0,0 +1,158 @@
+/*
+ * 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.eclipse.aether.internal.impl;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.eclipse.aether.DefaultRepositoryCache;
+import org.eclipse.aether.DefaultRepositorySystemSession;
+import org.eclipse.aether.artifact.Artifact;
+import org.eclipse.aether.artifact.DefaultArtifact;
+import org.eclipse.aether.collection.CollectRequest;
+import org.eclipse.aether.graph.Dependency;
+import org.eclipse.aether.internal.test.util.TestUtils;
+import org.eclipse.aether.resolution.ArtifactDescriptorRequest;
+import org.eclipse.aether.spi.validator.Validator;
+import org.eclipse.aether.spi.validator.ValidatorFactory;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class DefaultRepositorySystemValidatorTest {
+    @Test
+    void abstain() {
+        DefaultRepositorySystemValidator validator = new DefaultRepositorySystemValidator(
+                Collections.singletonMap("abstain", session -> ValidatorFactory.NOOP));
+        ArtifactDescriptorRequest request =
+                new ArtifactDescriptorRequest(new DefaultArtifact("foo:bar:1.0"), Collections.emptyList(), "test");
+        assertDoesNotThrow(() -> validator.validateArtifactDescriptorRequest(TestUtils.newSession(), request));
+    }
+
+    @Test
+    void fail() {
+        DefaultRepositorySystemValidator validator =
+                new DefaultRepositorySystemValidator(Collections.singletonMap("abstain", session -> new Validator() {
+                    @Override
+                    public void validateArtifact(Artifact artifact) throws IllegalArgumentException {
+                        throw new IllegalArgumentException("Artifact validation failed");
+                    }
+                }));
+        ArtifactDescriptorRequest request =
+                new ArtifactDescriptorRequest(new DefaultArtifact("foo:bar:1.0"), Collections.emptyList(), "test");
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> validator.validateArtifactDescriptorRequest(TestUtils.newSession(), request));
+    }
+
+    @Test
+    void withCaching() {
+        final ConcurrentHashMap<Integer, AtomicInteger> instanceCounter = new ConcurrentHashMap<>();
+        DefaultRepositorySystemValidator validator =
+                new DefaultRepositorySystemValidator(Collections.singletonMap("counter", session -> new Validator() {
+                    @Override
+                    public void validateArtifact(Artifact artifact) throws IllegalArgumentException {
+                        instanceCounter
+                                .computeIfAbsent(System.identityHashCode(this), k -> new AtomicInteger(0))
+                                .incrementAndGet();
+                    }
+                }));
+        DefaultRepositorySystemSession sameSession = TestUtils.newSession();
+        sameSession.setCache(new DefaultRepositoryCache()); // with cache
+        assertDoesNotThrow(() -> validator.validateArtifactDescriptorRequest(
+                sameSession,
+                new ArtifactDescriptorRequest(new DefaultArtifact("foo:bar:1.0"), Collections.emptyList(), "test")));
+        assertDoesNotThrow(() -> validator.validateArtifactDescriptorRequest(
+                sameSession,
+                new ArtifactDescriptorRequest(new DefaultArtifact("baz:foo:1.0"), Collections.emptyList(), "test")));
+        // one instance is
+        assertEquals(1, instanceCounter.size());
+        // used twice
+        assertEquals(
+                2,
+                instanceCounter
+                        .get(instanceCounter.keySet().stream().iterator().next())
+                        .get());
+    }
+
+    @Test
+    void withoutCaching() {
+        final ConcurrentHashMap<Integer, AtomicInteger> instanceCounter = new ConcurrentHashMap<>();
+        DefaultRepositorySystemValidator validator =
+                new DefaultRepositorySystemValidator(Collections.singletonMap("counter", session -> new Validator() {
+                    @Override
+                    public void validateArtifact(Artifact artifact) throws IllegalArgumentException {
+                        instanceCounter
+                                .computeIfAbsent(System.identityHashCode(this), k -> new AtomicInteger(0))
+                                .incrementAndGet();
+                    }
+                }));
+        DefaultRepositorySystemSession sameSession = TestUtils.newSession();
+        sameSession.setCache(null); // without cache
+        assertDoesNotThrow(() -> validator.validateArtifactDescriptorRequest(
+                sameSession,
+                new ArtifactDescriptorRequest(new DefaultArtifact("foo:bar:1.0"), Collections.emptyList(), "test")));
+        assertDoesNotThrow(() -> validator.validateArtifactDescriptorRequest(
+                sameSession,
+                new ArtifactDescriptorRequest(new DefaultArtifact("baz:foo:1.0"), Collections.emptyList(), "test")));
+        // two instances
+        assertEquals(2, instanceCounter.size());
+        for (AtomicInteger counter : instanceCounter.values()) {
+            // each used once
+            assertEquals(1, counter.get());
+        }
+    }
+
+    @Test
+    void dependencies() {
+        final List<Dependency> dependencies =
+                Collections.singletonList(new Dependency(new DefaultArtifact("foo:bar:1.0"), "compile"));
+        final List<Dependency> managedDependencies =
+                Collections.singletonList(new Dependency(new DefaultArtifact("baz:foo:1.0"), "compile"));
+
+        final AtomicInteger validateDependencies = new AtomicInteger(0);
+        final AtomicInteger validateManagedDependencies = new AtomicInteger(0);
+        DefaultRepositorySystemValidator validator = new DefaultRepositorySystemValidator(Collections.singletonMap(
+                "dependencies", session -> new Validator() {
+                    @Override
+                    public void validateDependency(Dependency dependency) throws IllegalArgumentException {
+                        if (dependency.getArtifact().getArtifactId().equals("bar")) {
+                            validateDependencies.incrementAndGet();
+                        }
+                    }
+
+                    @Override
+                    public void validateManagedDependency(Dependency managedDependency)
+                            throws IllegalArgumentException {
+                        if (managedDependency.getArtifact().getArtifactId().equals("foo")) {
+                            validateManagedDependencies.incrementAndGet();
+                        }
+                    }
+                }));
+        assertDoesNotThrow(() -> validator.validateCollectRequest(
+                TestUtils.newSession(),
+                new CollectRequest(dependencies, managedDependencies, Collections.emptyList())));
+        assertEquals(1, validateDependencies.get());
+        assertEquals(1, validateManagedDependencies.get());
+    }
+}

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/validator/Validator.java

@@ -54,6 +54,20 @@ default void validateMetadata(Metadata metadata) throws IllegalArgumentException
      */
     default void validateDependency(Dependency dependency) throws IllegalArgumentException {}
 
+    /**
+     * Validates managed dependency.
+     * <em>Important:</em> They are declarative constraints (version/scope/exclusion overrides) that only take effect
+     * when a matching dependency is encountered during collection. Validating them eagerly may reject valid builds
+     * where a BOM imports managed dependencies with uninterpolated property expressions (e.g. {@code ${osgi.version}})
+     * that are never actually used. If a managed dependency IS matched and its coordinates are invalid, the error
+     * will surface naturally during version resolution or artifact resolution.
+     *
+     * @param managedDependency The managed dependency to validate, never {@code null}.
+     * @throws IllegalArgumentException if dependency is invalid.
+     * @since 2.0.22
+     */
+    default void validateManagedDependency(Dependency managedDependency) throws IllegalArgumentException {}
+
     /**
      * Validates local repository.
      *

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/validator/ValidatorFactory.java

@@ -26,11 +26,21 @@
  * @since 2.0.8
  */
 public interface ValidatorFactory {
+
+    /**
+     * The no-op (does not validate anything) instance of validator.
+     *
+     * @since 2.0.22
+     */
+    Validator NOOP = new Validator() {};
+
     /**
-     * Creates a new validator for the session.
+     * Creates a new validator for the session, or {@link #NOOP} to abstain from validation. Factory is consulted
+     * once per session (if cache present in session) and returned instances (even {@code null}s) are cached and reused
+     * across single session.
      *
      * @param session The repository system session from which to configure the validator, must not be {@code null}.
-     * @return The validator for the session, never {@code null}.
+     * @return The validator to be used for the session, or {@link #NOOP} to abstain from validation, never {@code null}.
      */
     Validator newInstance(RepositorySystemSession session);
 }

maven-resolver-supplier-mvn3/src/main/java/org/eclipse/aether/supplier/RepositorySystemSupplier.java

@@ -18,9 +18,7 @@
  */
 package org.eclipse.aether.supplier;
 
-import java.util.ArrayList;
 import java.util.HashMap;
-import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Supplier;
@@ -1131,18 +1129,18 @@ protected ModelCacheFactory createModelCacheFactory() {
         return new DefaultModelCacheFactory();
     }
 
-    private List<ValidatorFactory> validatorFactories;
+    private Map<String, ValidatorFactory> validatorFactories;
 
-    public final List<ValidatorFactory> getValidatorFactories() {
+    public final Map<String, ValidatorFactory> getValidatorFactories() {
         checkClosed();
         if (validatorFactories == null) {
             validatorFactories = createValidatorFactories();
         }
         return validatorFactories;
     }
 
-    protected List<ValidatorFactory> createValidatorFactories() {
-        return new ArrayList<>();
+    protected Map<String, ValidatorFactory> createValidatorFactories() {
+        return new HashMap<>();
     }
 
     private RepositorySystemValidator repositorySystemValidator;

maven-resolver-supplier-mvn4/src/main/java/org/eclipse/aether/supplier/RepositorySystemSupplier.java

@@ -18,10 +18,8 @@
  */
 package org.eclipse.aether.supplier;
 
-import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
-import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Supplier;
@@ -1156,18 +1154,18 @@ protected ModelCacheFactory createModelCacheFactory() {
         return new DefaultModelCacheFactory();
     }
 
-    private List<ValidatorFactory> validatorFactories;
+    private Map<String, ValidatorFactory> validatorFactories;
 
-    public final List<ValidatorFactory> getValidatorFactories() {
+    public final Map<String, ValidatorFactory> getValidatorFactories() {
         checkClosed();
         if (validatorFactories == null) {
             validatorFactories = createValidatorFactories();
         }
         return validatorFactories;
     }
 
-    protected List<ValidatorFactory> createValidatorFactories() {
-        return new ArrayList<>();
+    protected Map<String, ValidatorFactory> createValidatorFactories() {
+        return new HashMap<>();
     }
 
     private RepositorySystemValidator repositorySystemValidator;