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Jackson Custom Deserializer for polymorphic objects and String literals as defaults

I'd like to deserialize an object from YAML with the following properties, using Jackson in a Spring Boot application:
Abstract class Vehicle, implemented by Boat and Car
For simplicity, imagine both have a name, but only Boat has also a seaworthy property, while Car has a top-speed.
mode-of-transport:
type: boat
name: 'SS Boatface'
seaworthy: true
----
mode-of-transport:
type: car`
name: 'KITT'
top-speed: 123
This all works fine in my annotated subclasses using #JsonTypeInfo and #JsonSubTypes!
Now, I'd like to create a shorthand using only a String value, which should create a Car by default with that name:
mode-of-transport: 'KITT'
I tried creating my own custom serializer, but got stuck on most of the relevant details. Please help me fill this in, if this is the right approach:
public class VehicleDeserializer extends StdDeserializer<Merger> {
/* Constructors here */
#Override
public Vehicle deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
if (/* it is an OBJECT */){
// Use the default polymorphic deserializer
} else if (/* it is a STRING */) {
Car car = new Car();
car.setName( /* the String value */ );
return car;
}
return ???; /* what to return here? */
}
}
I found these 2 answers for inspiration, but it looks like combining it with polymorphic types makes it more difficult: How do I call the default deserializer from a custom deserializer in Jackson and Deserialize to String or Object using Jackson
A few things are different than the solutions offered in those questions:
I am processing YAML, not JSON. Not sure about the subtle differences there.
I have no problem hardcoding the 'default' type for Strings inside my Deserializer, hopefully making it simpler.

This was actually easier than I thought to solve it. I got it working using the following:
Custom deserializer implementation:
public class VehicleDeserializer extends StdDeserializer<Vehicle> {
public VehicleDeserializer() {
super(Vehicle.class);
}
#Override
public Vehicle deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
if (jp.currentToken() == JsonToken.VALUE_STRING) {
Car car = new Car();
car.setName(jp.readValueAs(String.class));
return car;
}
return jp.readValueAs(Vehicle.class);
}
}
To avoid circular dependencies and to make the custom deserializer work with the polymorphic #JsonTypeInfo and #JsonSubTypes annotations I kept those annotations on the class level of Vehicle, but put the following annotations on the container object I am deserializing:
public class Transport {
#JsonDeserialize(using = VehicleDeserializer.class)
#JsonTypeInfo(use = JsonTypeInfo.Id.NONE)
private Vehicle modeOfTransport;
// Getter, setters
}
This means that by default a Vehicle is deserialized as a polymorphic object, unless explicitly specified to deserialize it using my custom deserializer. This deserializer will then in turn defer to the polymorphism if the input is not a String.
Hopefully this will help someone running into this issue :)

So there is a solution that requires you to handle the jackson errors using a DeserializationProblemHandler (since you want to parse the same type using different inputs, this is not achieved easily using regular means):
public class MyTest {
#Test
public void doTest() throws JsonParseException, JsonMappingException, IOException {
final ObjectMapper om = new ObjectMapper();
om.addHandler(new DeserializationProblemHandler() {
#Override
public Object handleMissingInstantiator(final DeserializationContext ctxt, final Class<?> instClass, final JsonParser p, final String msg) throws IOException {
if (instClass.equals(Car.class)) {
final JsonParser parser = ctxt.getParser();
final String text = parser.getText();
switch (text) {
case "KITT":
return new Car();
}
}
return NOT_HANDLED;
}
#Override
public JavaType handleMissingTypeId(final DeserializationContext ctxt, final JavaType baseType, final TypeIdResolver idResolver, final String failureMsg) throws IOException {
// if (baseType.isTypeOrSubTypeOf(Vehicle.class)) {
final JsonParser parser = ctxt.getParser();
final String text = parser.getText();
switch (text) {
case "KITT":
return TypeFactory.defaultInstance().constructType(Car.class);
}
return super.handleMissingTypeId(ctxt, baseType, idResolver, failureMsg);
}
});
final Container objectValue = om.readValue(getObjectJson(), Container.class);
assertTrue(objectValue.getModeOfTransport() instanceof Car);
final Container stringValue = om.readValue(getStringJson(), Container.class);
assertTrue(stringValue.getModeOfTransport() instanceof Car);
}
private String getObjectJson() {
return "{ \"modeOfTransport\": { \"type\": \"car\", \"name\": \"KITT\", \"speed\": 1}}";
}
private String getStringJson() {
return "{ \"modeOfTransport\": \"KITT\"}";
}
}
class Container {
private Vehicle modeOfTransport;
public Vehicle getModeOfTransport() {
return modeOfTransport;
}
public void setModeOfTransport(final Vehicle modeOfTransport) {
this.modeOfTransport = modeOfTransport;
}
}
#JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type", visible = true)
#JsonSubTypes({
#Type(name = "car", value = Car.class)
})
abstract class Vehicle {
protected String type;
protected String name;
public String getType() {
return type;
}
public void setType(final String type) {
this.type = type;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
}
#JsonTypeName("car")
class Car extends Vehicle {
private int speed;
public int getSpeed() {
return speed;
}
public void setSpeed(final int speed) {
this.speed = speed;
}
}
Note that I used JSON, not YAML, and you need to add your other subtypes as well.

Jackson Deserialization not calling deserialize on Custom Deserializer

I want to deserialize classes of the form:
public class TestFieldEncryptedMessage implements ITextMessage {
#JsonProperty("text")
#Encrypted(cipherAlias = "testAlias")
private String text;
public TestFieldEncryptedMessage() {
}
#JsonCreator
public TestFieldEncryptedMessage(#JsonProperty("text") String text) {
this.text = text;
}
public String getText() {
return text;
}
public void setText(String text) {
this.text = text;
}
}
Where the text is encrypted and deserialization should unencrypt the value before rebuilding the TestFieldEncryptedMessage instance.
I am following an approach very similar to: https://github.com/codesqueak/jackson-json-crypto
That is, I am building a module extending SimpleModule:
public class CryptoModule extends SimpleModule {
public final static String GROUP_ID = "au.com.auspost.messaging";
public final static String ARTIFACT_ID = "jackson-json-crypto";
private EncryptedSerializerModifier serializerModifier;
private EncryptedDeserializerModifier deserializerModifier;
public CryptoModule() {
}
public CryptoModule addEncryptionService(final EncryptionService encryptionService) {
serializerModifier = new EncryptedSerializerModifier(encryptionService);
deserializerModifier = new EncryptedDeserializerModifier(encryptionService);
return this;
}
#Override
public String getModuleName() {
return ARTIFACT_ID;
}
#Override
public Version version() {
return new Version(major, minor, patch, null, GROUP_ID, ARTIFACT_ID);
}
#Override
public void setupModule(final SetupContext context) {
if ((null == serializerModifier) || (null == deserializerModifier))
throw new EncryptionException("Crypto module not initialised with an encryption service");
context.addBeanSerializerModifier(serializerModifier);
context.addBeanDeserializerModifier(deserializerModifier);
}
}
As you can see, two modifiers are set up: the EncryptedSerializerModifier works perfectly and is called by the ObjectMapper, but the deserializer behind the EncryptedDeserializerModifier is ignored.
As is seen in many other examples on SO such as here: How can I include raw JSON in an object using Jackson?, I set up the EncryptedDeserializerModifier with:
public class EncryptedDeserializerModifier extends BeanDeserializerModifier {
private final EncryptionService encryptionService;
private Map<String, SettableBeanProperty> properties = new HashMap<>();
public EncryptedDeserializerModifier(final EncryptionService encryptionService) {
this.encryptionService = encryptionService;
}
#Override
public BeanDeserializerBuilder updateBuilder(final DeserializationConfig config, final BeanDescription beanDescription, final BeanDeserializerBuilder builder) {
Encrypted annotation = beanDescription.getType().getRawClass().getAnnotation(Encrypted.class);
Iterator it = builder.getProperties();
while (it.hasNext()) {
SettableBeanProperty p = (SettableBeanProperty) it.next();
if (null != p.getAnnotation(Encrypted.class)) {
JsonDeserializer<Object> current = p.getValueDeserializer();
properties.put(p.getName(), p);
builder.addOrReplaceProperty(p.withValueDeserializer(new EncryptedJsonDeserializer(encryptionService, current, p)), true);
}
}
return builder;
}
}
Finally, the EncryptedJsonDeserializer itself overrides the following:
#Override
public Object deserialize(final JsonParser parser, final DeserializationContext context) throws JsonMappingException {
JsonDeserializer<?> deserializer = baseDeserializer;
if (deserializer instanceof ContextualDeserializer) {
deserializer = ((ContextualDeserializer) deserializer).createContextual(context, property);
}
return service.decrypt(parser, deserializer, context, property != null ? property.getType() : type);
}
#Override
public JsonDeserializer<?> createContextual(final DeserializationContext context, final BeanProperty property) throws JsonMappingException {
JsonDeserializer<?> wrapped = context.findRootValueDeserializer(property.getType());
return new EncryptedJsonDeserializer(service, wrapped, property);
}
The createContextual() method is called, but the deserialize method is not called. The property throughout the execution is always the "text" property, so I seem to have the right context.
anyone know why the ObjectMapper doesn't find the right Deserializer?
EDIT added implements ITextMessage to decrypted class, which I thought was an unimportant detail, but turned out to be the cause of the issue.

I found the issue! If you look closely at the TestFieldEncryptedMessage class, whose text field is encrypted, you can see that it implements an interface. The interface is used so that the messages give some extra tooling for asserts in tests, however for deserialization, there is an unintended consequence. When the ObjectMapper is working its way through the json string, it tries, I think, to match a deserializer to a field inside ITextMessage, not to a field inside TestFieldEncryptedMessage, which is why the custom deserializer was not called (there is no text field in ITextMessage).
Once I stopped implementing ITextMessage, the custom deserializer was called.

Jackson Json deserialization of an object to a list

I'm consuming a web service using Spring's RestTemplate and deserializing with Jackson.
In my JSON response from the server, one of the fields can be either an object or a list. meaning it can be either "result": [{}] or "result": {}.
Is there a way to handle this kind of things by annotations on the type I'm deserializing to ? define the member as an array[] or List<> and insert a single object in case of the second example ?
Can I write a new HttpMessageConverter that will handle it ?

Since you are using Jackson I think what you need is JsonDeserializer class (javadoc).
You can implement it like this:
public class ListOrObjectGenericJsonDeserializer<T> extends JsonDeserializer<List<T>> {
private final Class<T> cls;
public ListOrObjectGenericJsonDeserializer() {
final ParameterizedType type = (ParameterizedType) this.getClass().getGenericSuperclass();
this.cls = (Class<T>) type.getActualTypeArguments()[0];
}
#Override
public List<T> deserialize(final JsonParser p, final DeserializationContext ctxt) throws IOException, JsonProcessingException {
final ObjectCodec objectCodec = p.getCodec();
final JsonNode listOrObjectNode = objectCodec.readTree(p);
final List<T> result = new ArrayList<T>();
if (listOrObjectNode.isArray()) {
for (JsonNode node : listOrObjectNode) {
result.add(objectCodec.treeToValue(node, cls));
}
} else {
result.add(objectCodec.treeToValue(listOrObjectNode, cls));
}
return result;
}
}
...
public class ListOrObjectResultItemJsonDeserializer extends ListOrObjectGenericJsonDeserializer<ResultItem> {}
Next you need to annotate your POJO field. Let's say you have classes like Result and ResultItem:
public class Result {
// here you add your custom deserializer so jackson will be able to use it
#JsonDeserialize(using = ListOrObjectResultItemJsonDeserializer.class)
private List<ResultItem> result;
public void setResult(final List<ResultItem> result) {
this.result = result;
}
public List<ResultItem> getResult() {
return result;
}
}
...
public class ResultItem {
private String value;
public String getValue() {
return value;
}
public void setValue(final String value) {
this.value = value;
}
}
Now you can check your deserializer:
// list of values
final String json1 = "{\"result\": [{\"value\": \"test\"}]}";
final Result result1 = new ObjectMapper().readValue(json1, Result.class);
// one value
final String json2 = "{\"result\": {\"value\": \"test\"}}";
final Result result2 = new ObjectMapper().readValue(json2, Result.class);
result1 and result2 contain the same value.

You can achieve what you want with a configuration flag in Jackson's ObjectMapper:
ObjectMapper mapper = Jackson2ObjectMapperBuilder.json()
.featuresToEnable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
.build();
Just set this ObjectMapper instance to your RestTemplate as explained in this answer, and in the class you are deserializing to, always use a collection, i.e. a List:
public class Response {
private List<Result> result;
// getter and setter
}

Jackson custom deserialization for polymorphic objects

I looked in many questions related to this but i can't find this exact use case.
Assumption here is that i am using Java Jackson library.
I have the following class hierarchy:
public class Event {
#JsonProperty("si")
String sessionId;
#JsonProperty("eventType")
String eventType;
...
}
#JsonSerialize(include= JsonSerialize.Inclusion.NON_NULL)
public class InitEvent extends Event
{
#JsonProperty("pm")
Params params;
public Params getParams()
{
return params;
}
.....
}
#JsonSerialize(include= JsonSerialize.Inclusion.NON_NULL)
public class RecoEvent extends Event
{
#JsonProperty("ti")
String targetId;
#JsonProperty("tt")
int targetType;
public String getTargetId()
{
return targetId;
}
....
}
The rule of deserialization is:
If eventType == 0 then deserialize to a InitEvent
If eventType == 0 then deserialize to a RecoEvent
Out of the box the Jackson deserialization will not work because it does not know which class to deserialize with. One way to handle that is by doing as follows on the base class:
#JsonTypeInfo(use=JsonTypeInfo.Id.CLASS, include=JsonTypeInfo.As.PROPERTY, property="#class")
#JsonSerialize(include= JsonSerialize.Inclusion.NON_NULL)
public class Event
The problem of this solution is assumes the client will serialize with the same mapper since the #class element needs to be present now in the JSON.
My client will not send the extra #class element in the incoming JSON.
What is the required solution?
How I could write a custom deserializer that picks the right derived class based on the eventType value?
Thx in advance

At the end I had to write my own deserializer.
This works in 2 steps, first implement the custom deserializer and then register it.
The implementation works as follows:
public class EvtListDeserializer extends JsonDeserializer<EventList>
{
private static ObjectMapper mapper = new ObjectMapper();
#Override
public EventList deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException, JsonProcessingException
{
EventList eventList = new EventList();
List<Event> listOfEvents = new ArrayList<Event>();
ObjectCodec oc = jsonParser.getCodec();
JsonNode eventListNode = oc.readTree(jsonParser);
ArrayNode node = (ArrayNode) eventListNode.get("evt");
Iterator<JsonNode> events = node.elements();
for (; events.hasNext(); )
{
JsonNode eventNode = events.next();
int eventType = eventNode.get("type").asInt();
Event event;
if (eventType == EventTypes.MoveEvent.getValue())
{
event = mapper.readValue(eventNode.toString(), MoveEvent.class);
}
else if (eventType == EventTypes.CopyEvent.getValue())
{
event = mapper.readValue(eventNode.toString(), CopyEvent.class);
}
else
{
throw new InvalidEventTypeException("Invalid event type:" + eventType);
}
listOfEvents.add(event);
}
eventList.setEvents(listOfEvents);
return eventList;
}
}
Then you just register it as follows in the class that uses your mapper:
public void init()
{
SimpleModule usageModule = new SimpleModule().addDeserializer(EventList.class, new EvtListDeserializer());
mapper.registerModule(usageModule);
}
That works perfectly.

(De-)Serialize Bean in a custom way at runtime

Let's imagine I have the following POJO:
class Pojo {
String s;
Object o;
Map<String, String> m;
}
And at runtime, I want default serialization / deserialization for all properties except one. Typically, I want to replace a field by its ID in a database when serializing, similarly to this other question.
For example, I want to replace o by a string obtained from an external mapping (for example: object1 <=> "123" and object2 <=> "456"):
serialization: read o and replace (so if o is object1, serialize as string "123")
deserialization: read "123", query some table to get the original value of o back (i.e. object1), recreate a Pojo object with o = object1.
I understand that Modules would be one way to do that but I'm not sure how to use them while keeping the automatic BeanSerializer/Deserializer for the properties that don't need to be changed.
Can someone give an example (even contrived) or an alternative approach?
Notes:
I can't use annotations or Mixins as the changes are unknown at compile time (i.e. any properties might be changed in a way that is not determinable).
This other question points to using a CustomSerializerFactory, which seems to do the job. Unfortunately, the official site indicates that it is not the recommended approach any more and that modules should be used instead.
Edit
To be a little clearer, I can do the following with Mixins for example:
ObjectMapper mapper = new ObjectMapper(MongoBsonFactory.createFactory());
mapper.addMixInAnnotations(Pojo.class, PojoMixIn.class);
ObjectReader reader = mapper.reader(Pojo.class);
DBEncoder dbEncoder = DefaultDBEncoder.FACTORY.create();
OutputBuffer buffer = new BasicOutputBuffer();
dbEncoder.writeObject(buffer, o);
with the following Mixin:
abstract class PojoMixIn {
#JsonIgnore Object o;
}
And then add the required string to the JSON content. But I would need to know at compile time that it is the o field that needs to be replaced, which I don't.

I think #JsonSerialize and #JsonDeserialize is what you need. These annotations give you control on the serialization/deserialization of particular fields. This question shows elegant way to combine them into one annotation.
UPD. For this complex scenario you could take a look at BeanSerializerModifier/BeanDeserializerModifier classes. The idea is to modify general BeanSerializer/BeanDeserializer with your custom logic for particular fields and let basic implementation to do other stuff. Will post an example some time later.
UPD2. As I see, one of the way could be to use changeProperties method and assign your own serializer.
UPD3. Updated with working example of custom serializer. Deserialization could be done in similar way.
UPD4. Updated example with full custom serialization/deserialization. (I have used jakson-mapper-asl-1.9.8)
public class TestBeanSerializationModifiers {
static final String PropertyName = "customProperty";
static final String CustomValue = "customValue";
static final String BaseValue = "baseValue";
// Custom serialization
static class CustomSerializer extends JsonSerializer<Object> {
#Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
String customValue = CustomValue; // someService.getCustomValue(value);
jgen.writeString(customValue);
}
}
static class MyBeanSerializerModifier extends BeanSerializerModifier {
#Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BasicBeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
for (int i = 0; i < beanProperties.size(); i++) {
BeanPropertyWriter beanPropertyWriter = beanProperties.get(i);
if (PropertyName.equals(beanPropertyWriter.getName())) {
beanProperties.set(i, beanPropertyWriter.withSerializer(new CustomSerializer()));
}
}
return beanProperties;
}
}
// Custom deserialization
static class CustomDeserializer extends JsonDeserializer<Object> {
#Override
public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
// serialized value, 'customValue'
String serializedValue = jp.getText();
String baseValue = BaseValue; // someService.restoreOldValue(serializedValue);
return baseValue;
}
}
static class MyBeanDeserializerModifier extends BeanDeserializerModifier {
#Override
public BeanDeserializerBuilder updateBuilder(DeserializationConfig config, BasicBeanDescription beanDesc, BeanDeserializerBuilder builder) {
Iterator<SettableBeanProperty> beanPropertyIterator = builder.getProperties();
while (beanPropertyIterator.hasNext()) {
SettableBeanProperty settableBeanProperty = beanPropertyIterator.next();
if (PropertyName.equals(settableBeanProperty.getName())) {
SettableBeanProperty newSettableBeanProperty = settableBeanProperty.withValueDeserializer(new CustomDeserializer());
builder.addOrReplaceProperty(newSettableBeanProperty, true);
break;
}
}
return builder;
}
}
static class Model {
private String customProperty = BaseValue;
private String[] someArray = new String[]{"one", "two"};
public String getCustomProperty() {
return customProperty;
}
public void setCustomProperty(String customProperty) {
this.customProperty = customProperty;
}
public String[] getSomeArray() {
return someArray;
}
public void setSomeArray(String[] someArray) {
this.someArray = someArray;
}
}
public static void main(String[] args) {
SerializerFactory serializerFactory = BeanSerializerFactory
.instance
.withSerializerModifier(new MyBeanSerializerModifier());
DeserializerFactory deserializerFactory = BeanDeserializerFactory
.instance
.withDeserializerModifier(new MyBeanDeserializerModifier());
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setSerializerFactory(serializerFactory);
objectMapper.setDeserializerProvider(new StdDeserializerProvider(deserializerFactory));
try {
final String fileName = "test-serialization.json";
// Store, "customValue" -> json
objectMapper.writeValue(new File(fileName), new Model());
// Restore, "baseValue" -> model
Model model = objectMapper.readValue(new File(fileName), Model.class);
} catch (IOException e) {
e.printStackTrace();
}
}
}