Throw MavenParsingException when a POM is missing groupId or version

RawPom.toPom previously called Objects.requireNonNull on the project's
groupId and version, producing a context-free NullPointerException when
either coordinate was absent. The NPE bubbled all the way out of
mod build / parse without naming the offending artifact, making
diagnosis painful.

Treat blank (null or whitespace-only) values as missing in
getGroupId / getVersion, and throw MavenParsingException naming the
artifact and source repository when either coordinate cannot be
resolved from the POM or its <parent> element.

Author: steve-aom-elliott

rewrite-maven/src/main/java/org/openrewrite/maven/internal/RawPom.java

@@ -36,7 +36,6 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 
 import static java.util.Collections.emptyList;
 import static java.util.Collections.emptyMap;
@@ -404,22 +403,26 @@ public static class Profile {
     }
 
     public @Nullable String getGroupId() {
-        return groupId == null && parent != null ? parent.getGroupId() : groupId;
+        if (!StringUtils.isBlank(groupId)) {
+            return groupId;
+        }
+        if (parent != null && !StringUtils.isBlank(parent.getGroupId())) {
+            return parent.getGroupId();
+        }
+        return null;
     }
 
     public @Nullable String getVersion() {
-        if (version == null) {
-            if (currentVersion == null) {
-                if (parent == null) {
-                    return null;
-                } else {
-                    return parent.getVersion();
-                }
-            } else {
-                return currentVersion;
-            }
+        if (!StringUtils.isBlank(version)) {
+            return version;
         }
-        return version;
+        if (!StringUtils.isBlank(currentVersion)) {
+            return currentVersion;
+        }
+        if (parent != null && !StringUtils.isBlank(parent.getVersion())) {
+            return parent.getVersion();
+        }
+        return null;
     }
 
 
@@ -428,15 +431,26 @@ public Pom toPom(@Nullable Path inputPath, @Nullable MavenRepository repo) {
                 getParent().getGroupId(), getParent().getArtifactId(),
                 getParent().getVersion()), getParent().getRelativePath());
 
+        String resolvedGroupId = getGroupId();
+        String resolvedVersion = getVersion();
+        if (resolvedGroupId == null || resolvedVersion == null) {
+            throw new MavenParsingException(
+                    "POM is missing a required coordinate:" +
+                            (resolvedGroupId == null ? " groupId" : "") +
+                            (resolvedVersion == null ? " version" : "") +
+                            " for " + new GroupArtifactVersion(resolvedGroupId, artifactId, resolvedVersion) +
+                            (repo == null ? "" : " (from " + repo.getUri() + ")"));
+        }
+
         Pom.PomBuilder builder = Pom.builder()
                 .sourcePath(inputPath)
                 .repository(repo)
                 .parent(parent)
                 .gav(new ResolvedGroupArtifactVersion(
                         repo == null ? null : repo.getUri(),
-                        Objects.requireNonNull(getGroupId()),
+                        resolvedGroupId,
                         artifactId,
-                        Objects.requireNonNull(getVersion()),
+                        resolvedVersion,
                         null))
                 .name(name)
                 .obsoletePomVersion(pomVersion)

rewrite-maven/src/test/java/org/openrewrite/maven/internal/RawPomTest.java

@@ -610,4 +610,108 @@ void deserializePluginConfiguration() throws Exception {
         assertThat(plugin.getConfigurationList("grandparent.parent.child.stringList", String.class)).hasSize(4)
           .contains("f", "r", "e", "d");
     }
+
+    @Test
+    void missingGroupIdThrowsParsingException() {
+        RawPom pom = RawPom.parse(
+          //language=xml
+          new ByteArrayInputStream("""
+                <project>
+                    <modelVersion>4.0.0</modelVersion>
+                    <artifactId>my-app</artifactId>
+                    <version>1</version>
+                </project>
+            """.getBytes()),
+          null
+        );
+
+        assertThatThrownBy(() -> pom.toPom(null, null))
+          .isInstanceOf(MavenParsingException.class)
+          .hasMessageContaining("missing a required coordinate")
+          .hasMessageContaining("groupId")
+          .hasMessageContaining("my-app");
+    }
+
+    @Test
+    void missingVersionThrowsParsingException() {
+        RawPom pom = RawPom.parse(
+          //language=xml
+          new ByteArrayInputStream("""
+                <project>
+                    <modelVersion>4.0.0</modelVersion>
+                    <groupId>com.mycompany.app</groupId>
+                    <artifactId>my-app</artifactId>
+                </project>
+            """.getBytes()),
+          null
+        );
+
+        assertThatThrownBy(() -> pom.toPom(null, null))
+          .isInstanceOf(MavenParsingException.class)
+          .hasMessageContaining("missing a required coordinate")
+          .hasMessageContaining("version");
+    }
+
+    @Test
+    void emptyGroupIdElementThrowsParsingException() {
+        RawPom pom = RawPom.parse(
+          //language=xml
+          new ByteArrayInputStream("""
+                <project>
+                    <modelVersion>4.0.0</modelVersion>
+                    <groupId/>
+                    <artifactId>my-app</artifactId>
+                    <version>1</version>
+                </project>
+            """.getBytes()),
+          null
+        );
+
+        assertThatThrownBy(() -> pom.toPom(null, null))
+          .isInstanceOf(MavenParsingException.class)
+          .hasMessageContaining("groupId");
+    }
+
+    @Test
+    void whitespaceVersionElementThrowsParsingException() {
+        RawPom pom = RawPom.parse(
+          //language=xml
+          new ByteArrayInputStream("""
+                <project>
+                    <modelVersion>4.0.0</modelVersion>
+                    <groupId>com.mycompany.app</groupId>
+                    <artifactId>my-app</artifactId>
+                    <version>   </version>
+                </project>
+            """.getBytes()),
+          null
+        );
+
+        assertThatThrownBy(() -> pom.toPom(null, null))
+          .isInstanceOf(MavenParsingException.class)
+          .hasMessageContaining("version");
+    }
+
+    @Test
+    void groupIdInheritedFromParentStillResolves() {
+        RawPom pom = RawPom.parse(
+          //language=xml
+          new ByteArrayInputStream("""
+                <project>
+                    <modelVersion>4.0.0</modelVersion>
+                    <parent>
+                        <groupId>com.mycompany.app</groupId>
+                        <artifactId>my-parent</artifactId>
+                        <version>1</version>
+                    </parent>
+                    <artifactId>my-app</artifactId>
+                </project>
+            """.getBytes()),
+          null
+        );
+
+        Pom resolved = pom.toPom(null, null);
+        assertThat(resolved.getGroupId()).isEqualTo("com.mycompany.app");
+        assertThat(resolved.getVersion()).isEqualTo("1");
+    }
 }