getting "NULL" instead of null in Json response in java

Question

I have my own object when there is no value it is coming as "NULL" in a string from downstream and it is failing while converting to java object in ObjectMapper

class Test 
private String value1; 
private String value2; 
.... getters and setters.

Json Response is

{"Test": "NULL"}

How I can handle this in java Error message is :

** error : com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.test.Test (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('NULL') **

This is how I am converting json to java

ResponseEntity<String> responseEntity = getDownstreamResponse(id);
ObjectMapper mapper = new ObjectMapper();
Test test =  mapper.readValue(responseEntity.getBody(), Test.class);

ObjectMapper are related to your get/set and constructor methods since thats how they scan the fields and possibilities. In this case the defined json-string (response) does not match the object at the top so it can not be mapped correct. You need to make sure that it can be built with all information being present and also for ignoring null values, etc. — LenglBoy
– LenglBoy, Commented Feb 14, 2024 at 7:32
Can you show the code that you use to parse the response (and not only the property Test)? — Matteo NNZ
– Matteo NNZ, Commented Feb 14, 2024 at 9:14
@MatteoNNZ I have updated the post could you please check that. — Shuaib Shaikh
– Shuaib Shaikh, Commented Feb 14, 2024 at 10:04
Where is that JSON coming from? It's clearly a string field that has the value "NULL", not a _null_/empty value. Thus, trying to parse it as a class with multiple fields cannot work. That is, there's a mismatch between the JSON and what you're expecting. — Doug Breaux
– Doug Breaux, Commented Feb 14, 2024 at 13:49

Matteo NNZ · Accepted Answer · 2024-02-14 12:22:10Z

Inside the body of:

ResponseEntity<String> responseEntity = getDownstreamResponse(id);

... you will have:

{"Test": "NULL"}

... but that's not a Test.class. The Test.class, eventually, it's only the value of the key "Test", so the following line of code would fail anyway even if the response was correct:

Test test =  mapper.readValue(responseEntity.getBody(), Test.class);

To make it simpler, I'd do this:

ObjectMapper mapper = new ObjectMapper();
JsonNode fullResponse = mapper.readTree(responseEntity.getBody());
JsonNode testNode = fullResponse.get("Test");
if ("NULL".equals(testNode.asText())) {
    //you won't be able to parse the response into class Test.class. 
    //do what you wish to do here (throw an exception, return null etc.)
}
Test test =  mapper.readValue(testNode, Test.class);
//here you should have parsed test, so do what you want with it

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