Rest Json Jackson Mapper Custom Object Mapper - spring

Question

I am having an issue with the Jackson Json mapper which I can't figure out how to solve.
I am having a Spring MVC Rest application and the endpoints are converted to Json using Jackson.
Some of the result objects contain a type that I want to tamper with before it gets converted.
More specifically, a result object could look like this.
ResultObject
- getDoubleMap() : DoubleMap
- getDoubleEntries() : List<DoubleEntry>
- toMap() : Map<String, Double>
What I want to do is to not have Jackson convert the DoubleMap instance but much rather override it like this
Object someJacksonMapInterceptor(Object object) {
if(object instanceof DoubleMap) {
return ((DoubleMap) object).toMap();
}
return object;
}
I have tortured google quite a while now and not a simple solution. Hope someone can advise.
Many thanks in advance.

Answer 1

In one application, we are custom-deserealizing date, probably you can use it for your custom deserealization.
public class VitalSign {
public static final String DATE_FORMAT1 = "yyyy-MM-dd'T'HH:mm:ssZ";
public static final String DATE_FORMAT2 = "yyyy-MM-dd'T'HH:mm:ss";
//public static final String DATE_FORMAT3 = "yyyy-MM-dd'T'HH:mm:ssTDZ";
public static final String DATE_FORMAT4 = "MMM dd, yyyy h:mm:ss aa";
#NotNull
#Column(name = "observed")
#Temporal(TemporalType.TIMESTAMP)
#DateTimeFormat(style = "M-")
#JsonDeserialize(using = CustomJsonDateDeserializer.class)
private Date timestamp;
public static class CustomJsonDateDeserializer extends JsonDeserializer<Date> {
public CustomJsonDateDeserializer() {
super();
}
#Override
public Date deserialize(JsonParser jsonparser, DeserializationContext deserializationcontext) throws IOException, JsonProcessingException {
SimpleDateFormat[] formats = { new SimpleDateFormat(DATE_FORMAT1), new SimpleDateFormat(DATE_FORMAT2), new SimpleDateFormat(DATE_FORMAT4, Locale.US) };
String date = jsonparser.getText();
for (SimpleDateFormat format : formats) {
try {
return format.parse(date);
} catch (ParseException e) {
}
}
throw new RuntimeException("Unparseable date " + date);
}
}
}

Answer 2

For serializing, you can just annotate your toMap() method with #JsonValue. For deserializing, if you have a static factory to create a DoubleMap from a Map<String, Double>, you can just annotate that with #JsonCreator.
private final ObjectMapper mapper = new ObjectMapper();
#Test
public void serialize_doublemap() throws Exception {
DoubleMap map = new DoubleMap();
map.put("red", 0.5);
map.put("orange", 0.7);
assertThat(mapper.writeValueAsString(map), equivalentTo("{ red: 0.5, orange: 0.7 }"));
}
#Test
public void deserialize_doublemap() throws Exception {
assertThat(mapper.readValue("{ \"red\": 0.5, \"orange\": 0.7 }", DoubleMap.class).toMap(),
equalTo(ImmutableMap.of("red", 0.5, "orange", 0.7)));
}
public static class DoubleMap {
public List<DoubleEntry> entries = new ArrayList<>();
public void put(String label, double value) {
entries.add(new DoubleEntry(label, value));
}
#JsonCreator
public static DoubleMap fromJson(Map<String, Double> input) {
DoubleMap map = new DoubleMap();
input.forEach(map::put);
return map;
}
public List<DoubleEntry> getDoubleEntries() {
return entries;
}
#JsonValue
public Map<String, Double> toMap() {
return entries.stream().collect(Collectors.toMap(e -> e.label, e -> e.value));
}
}
public static final class DoubleEntry {
public final String label;
public final double value;
public DoubleEntry(String label, double value) {
this.label = label;
this.value = value;
}
}