Jackson with JSON: Unrecognized field, not marked as ignorable

Drill down into unfamiliar fields

When you're up against JSON deserialization, the hurdles you might stumble upon are the fields in your JSON data that have no representation in your Java classes. Let's conquer that:

Jackson's ObjectMapper : Default behaviour

Out-of-the-box, Jackson's ObjectMapper lives in an ideal world where each JSON field has a corresponding Java field. An unknown field is a party-crasher, resulting in a UnrecognizedPropertyException.

Smile and wave at the unknown: @JsonIgnoreProperties

To politely disregard any unwelcome JSON data, wave the @JsonIgnoreProperties(ignoreUnknown = true) flag at your class level:

@JsonIgnoreProperties(ignoreUnknown = true)
public class ClassThatPlaysItCool {
    // Sees no evil (unknown JSON fields)
    // Fields partying here do match with the JSON keys!
}
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This tactic guarantees your Java classes are gracious hosts to evolving JSON formats.

Setting ObjectMapper to chill mode

If you want your ObjectMapper to universally ignore unexpected guests (unknown properties), ask it to do so:

ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
// Now ObjectMapper declares, "Chill, I've got this!" for unknown JSON fields across all classes.


Sweet talking ObjectMapper with @JsonProperty

When your Java field names are too shy to introduce themselves to the corresponding JSON keys, @JsonProperty comes to the rescue:

public class ClassWithAliases {
    @JsonProperty("jsonKeyName")
    private String javaFieldName;
    // "Hi, I'm 'javaFieldName', but you can call me 'jsonKeyName'."
}
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This tells Jackson to quietly link the 'jsonKeyName' in the JSON data to javaFieldName in your class, ensuring no UnrecognizedPropertyException pops up.

Dealing with complex JSON structures

Sometimes JSON data is a layered cake rather than a simple flat cookie. We need to tackle nested JSON objects and align Java getters to sail this boat.

@JsonIgnoreProperties - The Wrapper's shield

Sometimes, you may have a wrapper class holding the fort for your nested POJOs. Use @JsonIgnoreProperties to protect it from unrecognized properties at any nesting level.

@JsonIgnoreProperties(ignoreUnknown = true)
public class ClassThatHugsInnerClasses {
    // Nested children classes having a nice party with their JSON friends here.
}


Getters that know the way: Aligning with JSON properties

Jackson requires your getter methods to dress like your JSON keys for a successful matchmaking.

public class ClassThatTransformsIntoJSON {
    private String aFieldNameThatGets;

    @JsonProperty("aFieldNameThatGets")
    public String getAFieldNameThatGets() {
        return aFieldNameThatGets;
    }
    // Transform to "aFieldNameThatGets" with matching getter for JSON.
}


Using well-configured getters, Jackson can serialize and deserialize between Java and nested JSON properties like a pro.

Making room for evolving JSON structures

Lastly, preparing your Java objects to overlook unknown fields paves the way for future JSON structure changes. Your deserialization process is now more resilient to unexpected field arrival or departure parties in your JSON data.