Spring Jackson ObjectMapper Fails to JSON Deserialize While the Unit Test Passes - spring

Question

I've got a class in my Spring web application:
#Value // or #Data Lombok
public class Bar {
private final BigDecimal value;
#JsonCreator
public Bar(double value) {
this.value = BigDecimal.valueOf(value);
}
}
I wrote a unit test which passes:
#Test
void test() throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
Bar bar = new Bar(12.34);
assertEquals(mapper.readValue("12.34", Bar.class), bar);
}
But when I send a POST to the controller, it fails to deserialize the request body (which is just 12.34 to be deserialized to a Bar instance) with the following error:
JSON parse error: Cannot construct instance of com.example.demo.Bar (although at least one Creator exists): no double/Double-argument constructor/factory method to deserialize from Number value (12.34); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.example.demo.Bar (although at least one Creator exists): no double/Double-argument constructor/factory method to deserialize from Number value (12.34)
If I remove the #Value, it can deserialize it. To make it even more confusing, if I add the constructor (created by #Value) manually, still it works. But if I remove #JsonCreator it again can deserialize it.
What am I missing?

Answer 1

#Jsoncreator should be used in conjunction with #JsonProperty, to specify how to deserialize a JSON object. So if for example you have a rest controller that some JSON like:
{
"value": 123
}
Your constructor should be annotated like such:
#JsonCreator
public Bar(#JsonProperty("value") double value) {
this.value = BigDecimal.valueOf(value);
}
Although this might see redundant, the idea is that this allows for more flexibily in cases were the JSON you intend to deserialize doesn't match the name of your properties.
So for example if the object you are receiving still has a key value, but your class has a property myValue, the following will work:
public class Example {
private final BigDecimal myValue;
#JsonCreator
public Bar(#JsonProperty("value") double myValue) {
this.myValue= BigDecimal.valueOf(myValue);
}
}