Handling of null-values in JSONObject and XML.toString()

[tobihein]

We have a json-string and want to convert it to a corresponding xml-string. In case, that there are null-values in the json-string, we get an unexpected result

JSon-String

{
   "myKey": null
}

What we do

String jsonString="{\"myKey\": null}";
final JSONObject jsonObject = new JSONObject( jsonString );
finalString xmlString = XML.toString( jsonObject));

What we get
<myKey>null</myKey>

What we expect
<myKey/>

Is our expectation correct? If yes, I would do a pull-request and change line 594 in class XML.java

[johnjaylward]

No, in this case it's working as designed. In Javascript and JSON, null is a value (as opposed to undefined). In order to maintain the JSON value of null when converting to XML we output that as the string "null".

[tobihein]

Thanks for your quick reply.

So in our case there is no difference in the xml for the input

{
   "myKey": null,
   "myKey2": "null"
}

Do you have a suggestion how we can get our expected output?

[johnjaylward]

hmm, now that I'm looking at the code and the history of it, this may be a bug. At https://github.com/stleary/JSON-java/blob/master/XML.java#L594 you can see a null check for the value. This is the java null which this library uses to represent javascript undefined. However, there is no further check for the JSON null (https://github.com/stleary/JSON-java/blob/master/JSONObject.java#L165) which has a toString method that returns"null". (This is why null is output for your sample JSON. The library internally converts it to that JSONObject.NULL object).

In reality, the null check in the XML class should never happen since the JSONObject.put methods all remove keys with null values. Hence, the keySet call should never contain values that are JAVA null. I feel like the intent of that check was to actually check for JSON null and not JAVA null.

@stleary Let me know your opinion on this. It may be a small tweak we want to make.

[stleary]

@johnjaylward thanks for investigating. Are you suggesting adding a check for JSON null or replacing the existing check?

[johnjaylward]

Replacing the existing check with JSONObject.NULL.equals(value).

[stleary]

Sounds reasonable, no objection on this end.

[tobihein]

Thanks, you two. My pull-request would have looked exactly like yours, johnjaylward.

I'm looking forward to the upcomming release.

[johnjaylward]

I'm still updating it a little due to test failures since we were validating that functionality

[javadev]

Hi,

It may be a new attribute <myKey null="true"/>.

[tobihein]

@javadev
For me this isn't an option because I can't change the schema of the output-xml.

We added a workaround in our code. We replace all JSONObject.NULL with "". But we would still be glad to have this changed in the library.

[tobihein]

I saw the pull-request #412 for another feature request. Can you imagine to also make the output configurable by an additional parameter to the XML.toString-method? This would avoid a breaking change.

[javadev]

Thanks for your quick reply.

So in our case there is no difference in the xml for the input

{
   "myKey": null,
   "myKey2": "null"
}

Do you have a suggestion how we can get our expected output?

It may be converter to xml like this:

<?xml version="1.0" encoding="UTF-8"?>
<root>
  <myKey null="true"/>
  <myKey2>null</myKey2>
</root>

[stleary]

#412 is a good idea. I will proceed with integrating with the unit tests and getting it committed. A section has been added to the Wiki FAQ that addresses how XML requests will be handled going forward.
https://github.com/stleary/JSON-java/wiki/FAQ#status-of-the-xml-code