To do a proper null check of a JsonNode field:

JsonNode jsonNode = response.get("item");

if(jsonNode == null || jsonNode.isNull()) { }

The item is either not present in response, or explicitly set to null .

There is difference in between these 2 methods

1. JsonNode.get() method returns null
2. Use JsonNode.path(String).asText() which checks if node is present or not, if not then it returns empty string.

The issues lies in the meaning of null in this context. In the problematic case, clearly op has been instantiated and is therefore not null. This can be shown by adding another output line in the constructor performing the validation:

System.out.println("    class:" + (null == op ? "Real Null" : op.getClass().toString()));

This, when the program is run again, produces the folloiwng output:

Initial value: {"type":"CREATE","op":null}
Document value: {"@type":"CrudOperation","type":"CREATE"}
 isUpdate: false
isNotNull: false
  isValid: false
    class:class com.fasterxml.jackson.databind.node.NullNode
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.cyberfront.test.json.nll.demonstration.CrudOperation, problem: Operation Failed Validation: {"type":"CREATE","op":null}
 at [Source: N/A; line: -1, column: -1]
Reconstructed: null

That is, op is instantiated as a NullNode object type, which when the toString() method is performed upon it, returns null as a string not as the type.

A solution to resolving this is use of the JsonNode.isNull() method to detect when this case arises.

@JsonCreator
public CrudOperation(@JsonProperty("type") Type type, @JsonProperty("op") JsonNode op) {
    this.type = type;
    this.op = null == op || op.isNull() ? null : op;

    boolean isUpdate = Type.UPDATE.equals(this.getType());
    boolean isNotNull = null == this.getOp();
    boolean isValid = isUpdate ^ isNotNull;

    if (!isValid) {
        System.out.println(" isUpdate: " + String.valueOf(isUpdate));
        System.out.println("isNotNull: " + String.valueOf(isNotNull));
        System.out.println("  isValid: " + String.valueOf(isValid));
        System.out.println("    class:" + (null == op ? "Real Null" : op.getClass().toString()));

        throw new IllegalArgumentException("Operation Failed Validation: " + this.toString());
    }
}

This additional check for isNull() will filter out the case where a JsonNode is an instantiated null node, and will provide

Initial value: {"type":"CREATE","op":null}
Document value: {"@type":"CrudOperation","type":"CREATE"}
Reconstructed: {"type":"CREATE","op":null}

This achieves the desired result.