Jackson JSON notes

Russell Bateman
March 2019
last update: 03/11/2019 22:27:15

Jackson is a very good JSON utility library for Java.

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1.<dependency>
2.  <groupId>com.fasterxml.jackson.core</groupId>
3.  <artifactId>jackson-databind</artifactId>
4.  <version>2.9.8</version>                       <-- 18 December 2018 -->
5.</dependency>

The dependency above will cause the following to load:

jackson-annotations-2.9.8.jar
jackson-core-2.9.8.jar
jackson-databind-2.9.8.jar

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1.public class Car
2.{
3.  private String color;
4.  private String type;
5. 
6.  // standard accessors...
7.}

Serialization of a POJO (bean) into JSON:

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1.ObjectMapper mapper = new ObjectMapper();
2.Car          car    = new Car( "Yellow", "Renault" );
3.final String JSON   = mapper.writeValueAsString( car );

Deserialization of JSON input (string) to POJO (bean):

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1.final String JSON = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
2.Car          car  = mapper.readValue( JSON, Car.class );

For (lots) more, see https://www.baeldung.com/jackson-object-mapper-tutorial.

Annotations

@JsonInclude —used to define which properties (i.e.: fields) of a POJO (bean) are to be included in serialization (i.e.: POJO-to-JSON). For example, ...

@JsonInclude( JsonInclude.Include.ALWAYS )      // always include this field
private UUID  id;

@JsonInclude( JsonInclude.Include.NON_NULL )    // include this field when not nil
private Ojbect  metadata;

@JsonInclude( JsonInclude.Include.NON_EMPTY )   // include this field when not null or empty
private String version;

@JsonInclude( JsonInclude.Include.NON_DEFAULT ) // (see documentation)

@JsonAnyGetter —denotes a Map field as containing standard properties:

private Map< String, String > properties;

@JsonAnyGetter
public Map< String, String > getProperties() { return properties; }

@JsonGetter —marks method as accessor (when not obvious):

private String name;

@JsonGetter( "name" )
public String getTheName() { return name; }

@JsonPropertyOrder —designates the order of properties (fields) upon serialization:

@JsonPropertyOrder( "name", "id", "date" )
public class Thing
{
  private int    id;
  private String name;
  private Date   date;
}

See many more at https://www.baeldung.com/jackson-annotations:
- @JsonValue
- @JsonRootName
- @JsonSerialize
- @JsonDeserialize
- @JsonCreator
- @JacksonInject
- @JsonAnySetter
- @JsonSetter
- @JsonIgnoreProperties
- @JsonIgnore
- @JsonIgnoreType
- @JsonAutoDetect
- @JsonProperty
- @JsonFormat
- @JsonUnwrapped
- @JsonView
- @JsonManagedReference
- @JsonBackReference
- @JsonIdentityInfo
- @JsonFilter

Tailoring Jackson behavior

Here are some examples. These can be used to extend new ObjectMapper() with .disable( ... ) or .enable( ... ), etc.

MapperFeature.ALLOW_COERCION_OF_SCALARS —basically, stuff like "json_boolean" : true instead of "json_boolean" : 1.
SerializationFeature.WRITE_DATES_AS_TIMESTAMPS —deals with whether Date-based things are serialized as numeric timestamps or as a textual representation.
DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES —unknown properties are ones that do not map to a property in the sense that there is no setter annotated to handle it, results in a failure.