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What happens if I annotate a constructor parameter using @JsonProperty but the Json doesn't specify that property. What value does the constructor get?
How do I differentiate between a property having a null value versus a property that is not present in the JSON?
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Jackson API provides two ways to ignore unknown fields, first at the class level using @JsonIgnoreProperties annotation and second at the ObjectMapper level using configure() method.
Ignore All Fields by Type Finally, we can ignore all fields of a specified type, using the @JsonIgnoreType annotation. If we control the type, then we can annotate the class directly: @JsonIgnoreType public class SomeType { ... } More often than not, however, we don't have control of the class itself.
In order to solve this issue, we disable the object mapper to deserialize on unknown properties. So the object mapper will unmarshal/deserialize only those properties from json file which are mapped to the java class. We have emp1. json which is similar to the previous example.
FAIL_ON_UNKNOWN_PROPERTIES. Feature that determines whether encountering of unknown properties (ones that do not map to a property, and there is no "any setter" or handler that can handle it) should result in a failure (by throwing a JsonMappingException ) or not.
Summarizing excellent answers by Programmer Bruce and StaxMan:
Missing properties referenced by the constructor are assigned a default value as defined by Java.
You can use setter methods to differentiate between properties that are implicitly or explicitly set. Setter methods are only invoked for properties with explicit values. Setter methods can keep track of whether a property was explicitly set using a boolean flag (e.g. isValueSet).
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