How can I deserialize a JSON to a Java class which has known mandatory fields, but can have several unknown fields?

You can use JsonAnySetter annotation:

class PersonDTO {    @JsonProperty("first_name")    private String firstName;    @JsonProperty("last_name")    private String lastName;    private Map<String, String> extras = new HashMap<>();    @JsonAnySetter    public void setExtras(String name, String value) {        this.extras.put(name, value);    }    // No args constructor    // Getters    // Setters}

See also:

• JSON Jackson deserialization multiple keys into same field
• Adding a dynamic json property as java pojo for jackson
• How to use dynamic property names for a Json object

There are couple of things that you can do if you are using Jackson:-

1. You can make use of @JsonAnyGetter and @JsonAnySetter annotation and create your class as:-

import com.fasterxml.jackson.annotation.JsonAnyGetter;import com.fasterxml.jackson.annotation.JsonAnySetter;import java.util.HashMap;import java.util.Map;public class PersonDTO {    // mandatory field    private String firstName;    // mandatory field    private String lastName;    // Unknown optional fields?    // Capture all other fields that Jackson do not match other members    @JsonIgnore    private Map<String, Object> additionalProperties = new HashMap<String, Object>();    @JsonAnyGetter    public Map<String, Object> getAdditionalProperties() {        return this.additionalProperties;    }    @JsonAnySetter    public void setAdditionalProperty(String name, Object value) {        this.additionalProperties.put(name, value);    }    // No args constructor    // Getters    // Setters}

2. If you don't want the unknown fields in your PersonDTO then you can simply ignore unknown properties using annotation @JsonIgnoreProperties at class level eg

@JsonIgnoreProperties(ignoreUnknown = true)public class PersonDTO { //TODO:- }

3. The ObjectMapper can also be configured to ignore unknowns e.g.:-

new ObjectMapper()  .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)