Issues with Struct Read/Write in Milo SDK using JsonBsdParser – Null Values for LocalizedText and Arrays

[ravi-kothapally]

Hi, I'm using the UaAnsiCServer from Unified Automation for testing, and working with Milo SDK version 0.6.15, restricted to Java 11.

I'm trying to read a structured value and write it back directly using the following code:

client.addSessionInitializer(new DataTypeDictionarySessionInitializer(new JsonBsdParser()));
client.connect().get();

final String nodeIdStr = "ns=4;s=Demo.Static.Scalar.Structure";
DataValue dataValue = client.readValue(
    0.0,
    TimestampsToReturn.Neither,
    NodeId.parse(nodeIdStr)
).get();

ExtensionObject xo = (ExtensionObject) dataValue.getValue().getValue();
Object value = xo.decode(client.getDynamicSerializationContext());

logger.info("value: {}", value);

final ExtensionObject extensionObject = ExtensionObject.encodeDefaultBinary(
    client.getDynamicSerializationContext(),
    value,
    xo.getEncodingId()
);

StatusCode statusCode = client.writeValue(
    NodeId.parse(nodeIdStr),
    DataValue.valueOnly(new Variant(extensionObject))
).get();

When reading a value like this:

value: {"EngineeringUnits":{"NamespaceUri":null,"UnitId":0,"DisplayName":{},"Description":{}},"EURange":{"Low":0.0,"High":0.0},"Title":{},"AxisScaleType":0,"AxisSteps":[]}

I get a NullPointerException:

java.lang.NullPointerException
	at org.eclipse.milo.opcua.binaryschema.gson.JsonStructureCodec.memberTypeToOpcUaScalar(JsonStructureCodec.java:181)

It seems to fail when DisplayName, Description, or similar fields are empty or null. These fields are of type LocalizedText, and Milo tries to access .get("locale") and .get("text") without null checks:

case "LocalizedText": {
    return new LocalizedText(
        jsonObject.get("locale").getAsString(),
        jsonObject.get("text").getAsString()
    );
}

how to handle this?

---

I’m also facing a similar issue with another structure:

Node: "ns=4;s=Demo.Static.Scalar.WorkOrder"

value: {"ID":"00000000-0000-0000-0000-000000000000","AssetID":null,"StartTime":0,"StatusComments":null}

When trying to write this back, I get the following error:

Not a JSON Array: null
java.lang.IllegalStateException: Not a JSON Array: null
	at com.google.gson.JsonElement.getAsJsonArray(JsonElement.java:107)
	at org.eclipse.milo.opcua.binaryschema.gson.JsonStructureCodec.memberTypeToOpcUaArray(JsonStructureCodec.java:259)

However, if I provide StatusComments: [], it works fine. So the question is:

Shouldn’t Milo treat null as an empty array when the expected type is an array?

---

1. How can we avoid or fix these issues with null or {} values when dealing with LocalizedText and arrays?
2. Is there a recommended way to pre-process or sanitize the decoded struct before encoding and writing it back?

Thank you in advance for your help!

[kevinherron]

Decoders are supposed to preserve the difference between null and empty array, but even so it the result shouldn't be an NPE or uncaught exception. It seems that this is just buggy behavior in BsdParser.

I'd recommend you fork and fix this in BsdParser as required. BsdParser and the JSON encoding for structs is deprecated / unmaintained in 1.0.

I would consider merging a PR that fixed this against 0.6.x, though I'm not really planning to continue supporting and releasing 0.6.x versions except perhaps one more to wrap things up.