Fix NPE when deserializing TestIdentifier

Issue: #3819

Author: dmlloyd

documentation/src/docs/asciidoc/release-notes/release-notes-5.11.0-M2.adoc

@@ -26,6 +26,7 @@ repository on GitHub.
 [[release-notes-5.11.0-M2-junit-platform-bug-fixes]]
 ==== Bug Fixes
 
+* Fixed a bug where `TestIdentifier` could cause a `NullPointerException` on deserialize when there is no parent identifier. See link:https://github.com/junit-team/junit5/issues/3819[issue 3819].
 * ❓
 
 [[release-notes-5.11.0-M2-junit-platform-deprecations-and-breaking-changes]]

junit-platform-launcher/src/main/java/org/junit/platform/launcher/TestIdentifier.java

@@ -284,7 +284,8 @@ private void readObject(ObjectInputStream s) throws ClassNotFoundException, IOEx
 		source = serializedForm.source;
 		tags = serializedForm.tags;
 		type = serializedForm.type;
-		parentId = UniqueId.parse(serializedForm.parentId);
+		String parentId = serializedForm.parentId;
+		this.parentId = parentId == null ? null : UniqueId.parse(parentId);
 		legacyReportingName = serializedForm.legacyReportingName;
 	}
 
@@ -307,7 +308,8 @@ private static class SerializedForm implements Serializable {
 
 		SerializedForm(TestIdentifier testIdentifier) {
 			this.uniqueId = testIdentifier.uniqueId.toString();
-			this.parentId = testIdentifier.parentId.toString();
+			UniqueId parentId = testIdentifier.parentId;
+			this.parentId = parentId == null ? null : parentId.toString();
 			this.displayName = testIdentifier.displayName;
 			this.legacyReportingName = testIdentifier.legacyReportingName;
 			this.source = testIdentifier.source;

platform-tests/src/test/java/org/junit/platform/launcher/TestIdentifierTests.java

@@ -16,10 +16,12 @@
 import static org.junit.platform.commons.util.SerializationUtils.deserialize;
 import static org.junit.platform.commons.util.SerializationUtils.serialize;
 
+import java.util.Optional;
 import java.util.Set;
 
 import org.junit.jupiter.api.Test;
 import org.junit.platform.engine.TestDescriptor;
+import org.junit.platform.engine.TestSource;
 import org.junit.platform.engine.TestTag;
 import org.junit.platform.engine.UniqueId;
 import org.junit.platform.engine.support.descriptor.AbstractTestDescriptor;
@@ -72,6 +74,74 @@ void initialVersionCanBeDeserialized() throws Exception {
 		}
 	}
 
+	@Test
+	void identifierWithNoParentCanBeSerializedAndDeserialized() throws Exception {
+		TestIdentifier ti = TestIdentifier.from(new TestDescriptor() {
+			@Override
+			public UniqueId getUniqueId() {
+				return UniqueId.root("example", "id");
+			}
+
+			@Override
+			public String getDisplayName() {
+				return "displayName";
+			}
+
+			@Override
+			public Set<TestTag> getTags() {
+				return Set.of();
+			}
+
+			@Override
+			public Optional<TestSource> getSource() {
+				return Optional.empty();
+			}
+
+			@Override
+			public Optional<TestDescriptor> getParent() {
+				return Optional.empty();
+			}
+
+			@Override
+			public void setParent(TestDescriptor parent) {
+				// ignore
+			}
+
+			@Override
+			public Set<? extends TestDescriptor> getChildren() {
+				return Set.of();
+			}
+
+			@Override
+			public void addChild(TestDescriptor descriptor) {
+				// ignore
+			}
+
+			@Override
+			public void removeChild(TestDescriptor descriptor) {
+				// ignore
+			}
+
+			@Override
+			public void removeFromHierarchy() {
+				// ignore
+			}
+
+			@Override
+			public Type getType() {
+				return Type.TEST;
+			}
+
+			@Override
+			public Optional<? extends TestDescriptor> findByUniqueId(UniqueId uniqueId) {
+				return Optional.empty();
+			}
+		});
+		byte[] bytes = serialize(ti);
+		TestIdentifier dti = (TestIdentifier) deserialize(bytes);
+		assertEquals(ti, dti);
+	}
+
 	private static void assertDeepEquals(TestIdentifier first, TestIdentifier second) {
 		assertEquals(first, second);
 		assertEquals(first.getUniqueId(), second.getUniqueId());