How to fix the Sonar issues (returning null Maps) in JsonHelper?

[sleberknight]

The original JsonHelper implementation returns null for the following methods:

• public Map<String, Object> toMap(@Nullable String json)
• public <K, V> Map<K, V> toMap(@Nullable String json, TypeReference<Map<K, V>> targetMapType)
• public <T> Map<String, T> toFlatMap(@Nullable Object object, Class<T> valueClass)
• public <K, V> Map<K, V> convertToMap(Object fromObject, TypeReference<Map<K, V>> targetMapType)

Sonar is unhappy that we're returning null for a collection return type. I agree. I did some research into null in JSON and, as far as I can determine, the specification permits null values inside a JSON object, but does not appear to permit having a null JSON object at the "top level". i.e. this is valid:

{
  "name": "Bob",
  "nickname": null
}

but this doesn't seem to be allowed:

null

as the entire definition of a JSON object.

Per RFC-7159:

JSON can represent four primitive types (strings, numbers, booleans,
and null) and two structured types (objects and arrays).

and:

An object is an unordered collection of zero or more name/value
pairs, where a name is a string and a value is a string, number,
boolean, null, object, or array.

An array is an ordered sequence of zero or more values.

and

An object structure is represented as a pair of curly brackets
surrounding zero or more name/value pairs (or members)

The way I read this is that you can assign null to the things inside a JSON object, but that a JSON object with nothing in it would be represented as:

{}

I am definitely not pretty sure about this. But regardless I actually tried passing null to Jackson's ObjectMapper:

var objectMapper = new ObjectMapper();
String json = null;
var person = objectMapper.readValue(json, Person.class);
System.out.println("person = " + person);

and the above code throws an IllegalArgumentException:

java.lang.IllegalArgumentException: argument "content" is null

So, it appears Jackson (which is the de-facto standard JSON library in Java) doesn't permit this, so maybe we shouldn't either. In addition, if you pass an empty (or blank) string instead of null in the above sample code, you get a Jackson MismatchedInputException:

com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input
 at [Source: (String)""; line: 1, column: 0]

And last, passing {} as the JSON works just fine, and the output is:

person = KiwiUrlsTest.Person(firstName=null, lastName=null)

I'd propose throwing IllegalArgumentException if someone passes null or an empty string to any of the above four methods.

This would necessitate a major version bump in kiwi, though, since while it doesn't change a public API in terms of method signature, it is a major change in behavior.

---

Refs:

• Sonar problems in kiwi - https://sonarcloud.io/project/issues?id=kiwiproject_kiwi&resolved=false&rules=java%3AS1168
• RFC-7159 https://datatracker.ietf.org/doc/html/rfc7159

[sleberknight]

This will be addressed in 5.0.0