Skip dependencies with no resolved artifact file instead of throwing NPE

src/main/java/org/apache/maven/plugins/antrun/AntRunMojo.java

@@ -429,11 +429,7 @@ public void copyProperties(MavenProject mavenProject, Project antProject) {
 
         // Add properties for dependency artifacts
         Set<Artifact> depArtifacts = mavenProject.getArtifacts();
-        for (Artifact artifact : depArtifacts) {
-            String propName = artifact.getDependencyConflictId();
-
-            antProject.setProperty(propertyPrefix + propName, artifact.getFile().getPath());
-        }
+        setDependencyFileProperties(depArtifacts, antProject);
 
         // Add a property containing the list of versions for the mapper
         StringBuilder versionsBuffer = new StringBuilder();
@@ -443,6 +439,37 @@ public void copyProperties(MavenProject mavenProject, Project antProject) {
         antProject.setProperty(versionsPropertyName, versionsBuffer.toString());
     }
 
+    /**
+     * Registers one <code>${propertyPrefix}${dependencyConflictId}</code> property per dependency,
+     * holding the path to that dependency's artifact file.
+     * <p>
+     * An artifact's file may be <code>null</code> when it was never resolved, for instance under
+     * partial or offline resolution. Such dependencies are skipped with a warning naming them,
+     * rather than aborting the build: the property is simply absent, so a build that never
+     * references it still runs.
+     *
+     * @param depArtifacts the project's dependency artifacts, may be null
+     * @param antProject the Ant project to set properties on, not null
+     */
+    void setDependencyFileProperties(Set<Artifact> depArtifacts, Project antProject) {
+        if (depArtifacts == null) {
+            return;
+        }
+
+        for (Artifact artifact : depArtifacts) {
+            String propName = artifact.getDependencyConflictId();
+
+            File artifactFile = artifact.getFile();
+            if (artifactFile == null) {
+                getLog().warn("Not setting property \"" + propertyPrefix + propName + "\": dependency "
+                        + artifact.getId() + " has no resolved artifact file.");
+                continue;
+            }
+
+            antProject.setProperty(propertyPrefix + propName, artifactFile.getPath());
+        }
+    }
+
     /**
      * Copy properties from the Ant project to the Maven project.
      *

src/test/java/org/apache/maven/plugins/antrun/AntRunMojoDependencyPropertiesTest.java

@@ -0,0 +1,110 @@
+/*
+ * 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.plugins.antrun;
+
+import java.io.File;
+import java.lang.reflect.Field;
+import java.util.LinkedHashSet;
+import java.util.Set;
+
+import org.apache.maven.artifact.Artifact;
+import org.apache.maven.artifact.DefaultArtifact;
+import org.apache.maven.artifact.handler.DefaultArtifactHandler;
+import org.apache.maven.artifact.versioning.VersionRange;
+import org.apache.tools.ant.Project;
+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.assertNull;
+
+/**
+ * A dependency whose artifact was never resolved has a null file. Registering the
+ * per-dependency path properties must not fall over on it.
+ */
+class AntRunMojoDependencyPropertiesTest {
+
+    /**
+     * execute() defaults propertyPrefix to "" before any of this runs; a directly
+     * constructed mojo has not been through that, so mirror it here.
+     */
+    private static AntRunMojo mojoWithEmptyPrefix() throws Exception {
+        AntRunMojo mojo = new AntRunMojo(null);
+        Field prefix = AntRunMojo.class.getDeclaredField("propertyPrefix");
+        prefix.setAccessible(true);
+        prefix.set(mojo, "");
+        return mojo;
+    }
+
+    private static Artifact artifact(String artifactId, File file) {
+        Artifact artifact = new DefaultArtifact(
+                "org.example",
+                artifactId,
+                VersionRange.createFromVersion("1.0"),
+                "compile",
+                "jar",
+                null,
+                new DefaultArtifactHandler("jar"));
+        artifact.setFile(file);
+        return artifact;
+    }
+
+    @Test
+    void unresolvedDependencyIsSkippedInsteadOfThrowing() throws Exception {
+        Set<Artifact> artifacts = new LinkedHashSet<>();
+        artifacts.add(artifact("unresolved", null));
+
+        Project antProject = new Project();
+        AntRunMojo mojo = mojoWithEmptyPrefix();
+
+        assertDoesNotThrow(() -> mojo.setDependencyFileProperties(artifacts, antProject));
+        assertNull(
+                antProject.getProperty("org.example:unresolved:jar"),
+                "no path property should be set for a dependency with no artifact file");
+    }
+
+    @Test
+    void resolvedDependenciesStillGetTheirPathProperty() throws Exception {
+        File resolved = new File("target", "resolved-1.0.jar");
+
+        Set<Artifact> artifacts = new LinkedHashSet<>();
+        artifacts.add(artifact("unresolved", null));
+        artifacts.add(artifact("resolved", resolved));
+
+        Project antProject = new Project();
+        AntRunMojo mojo = mojoWithEmptyPrefix();
+
+        mojo.setDependencyFileProperties(artifacts, antProject);
+
+        // The unresolved artifact is listed first, so this also proves one bad
+        // dependency no longer prevents the rest from being registered.
+        assertEquals(
+                resolved.getPath(),
+                antProject.getProperty("org.example:resolved:jar"),
+                "resolved dependencies must still get their path property");
+    }
+
+    @Test
+    void nullArtifactSetIsTolerated() throws Exception {
+        Project antProject = new Project();
+        AntRunMojo mojo = mojoWithEmptyPrefix();
+
+        assertDoesNotThrow(() -> mojo.setDependencyFileProperties(null, antProject));
+    }
+}