Fix !vthreads command error with uninitialized Continuations

Continuation objects are added to GC list on create, which may not
yet be assoicated with vthread object yet at time when coredump is
generated. DDR needs to correctly handle null vthread reference.

Signed-off-by: Jack Lu <[email protected]>

Author: fengxue-IS

debugtools/DDR_VM/src/com/ibm/j9ddr/vm29/tools/ddrinteractive/VirtualThreadsCommand.java

@@ -69,11 +69,16 @@ private static long getVmRef(J9ObjectPointer continuation) throws CorruptDataExc
 		return J9ObjectHelper.getLongField(continuation, vmRefOffset);
 	}
 
-	private static J9ObjectPointer getName(J9ObjectPointer vthread) throws CorruptDataException {
-		if (nameOffset == null) {
-			nameOffset = J9ObjectHelper.getFieldOffset(vthread, "name", "Ljava/lang/String;");
+	private static String getName(J9ObjectPointer vthread) throws CorruptDataException {
+		String name = null;
+		if (vthread.notNull()) {
+			if (nameOffset == null) {
+				nameOffset = J9ObjectHelper.getFieldOffset(vthread, "name", "Ljava/lang/String;");
+			}
+			name = J9ObjectHelper.getStringField(vthread, nameOffset);
 		}
-		return J9ObjectHelper.getObjectField(vthread, nameOffset);
+
+		return (name != null) ? name : "<N/A>";
 	}
 
 	public VirtualThreadsCommand() {
@@ -112,15 +117,15 @@ private static void displayVirtualThreads(J9JavaVMPointer vm, PrintStream out) t
 			while (continuation.notNull()) {
 				long vmRef = getVmRef(continuation);
 				J9ObjectPointer vthread = getVirtualThread(continuation);
-				J9ObjectPointer name = getName(vthread);
+				String name = getName(vthread);
 
 				out.format(
 						outputFormat,
 						vmRef,
 						vmRef,
 						continuation.getHexAddress(),
 						vthread.getHexAddress(),
-						J9ObjectHelper.stringValue(name));
+						name);
 				continuation = ObjectReferencePointer.cast(continuation.addOffset(linkOffset)).at(0);
 			}
 			continuationObjectList = continuationObjectList._nextList();