I am attempting to work through a tutorial from Programmer Bruce that is supposed to allow the deserialization of polymorphic JSON.
The complete list can be found here Programmer Bruce tutorials (Great stuff btw)
I have worked through the first five with no problems but I have hit a snag on the last one (Example 6), which of course is the one I really need to get working.
I am getting the following error at compile time
The method readValue(JsonParser, Class) in the type ObjectMapper is not applicable for the arguments (ObjectNode, Class)
and it's being caused by the chunk of code
public Animal deserialize(
JsonParser jp, DeserializationContext ctxt)
throws IOException, JsonProcessingException
{
ObjectMapper mapper = (ObjectMapper) jp.getCodec();
ObjectNode root = (ObjectNode) mapper.readTree(jp);
Class<? extends Animal> animalClass = null;
Iterator<Entry<String, JsonNode>> elementsIterator =
root.getFields();
while (elementsIterator.hasNext())
{
Entry<String, JsonNode> element=elementsIterator.next();
String name = element.getKey();
if (registry.containsKey(name))
{
animalClass = registry.get(name);
break;
}
}
if (animalClass == null) return null;
return mapper.readValue(root, animalClass);
}
}
Specifically by the line
return mapper.readValue(root, animalClass);
Has anyone run into this before and if so, was there a solution?
I'd appreciate any help anyone can give Thanks in advance Jon D.
This question is related to
java
json
compiler-errors
polymorphism
jackson
Whereas @jbarrueta answer is perfect, in the 2.12 version of Jackson was introduced a new long-awaited type for the @JsonTypeInfo annotation, DEDUCTION.
It is useful for the cases when you have no way to change the incoming json or must not do so. I'd still recommend to use use = JsonTypeInfo.Id.NAME, as the new way may throw an exception in complex cases when it has no way to determine which subtype to use.
Now you can simply write
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
@JsonSubTypes({
@JsonSubTypes.Type(Dog.class),
@JsonSubTypes.Type(Cat.class) }
)
public abstract class Animal {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
And it will produce {"name":"ruffus", "breed":"english shepherd"} and {"name":"goya", "favoriteToy":"mice"}
Once again, it's safer to use NAME if some of the fields may be not present, like breed or favoriteToy.
You need only one line before the declaration of the class Animal for correct polymorphic serialization/deserialization:
@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "@class")
public abstract class Animal {
...
}
This line means: add a meta-property on serialization or read a meta-property on deserialization (include = JsonTypeInfo.As.PROPERTY) called "@class" (property = "@class") that holds the fully-qualified Java class name (use = JsonTypeInfo.Id.CLASS).
So, if you create a JSON directly (without serialization) remember to add the meta-property "@class" with the desired class name for correct deserialization.
More information here
If using the fasterxml then,
these changes might be needed
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.Version;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.node.ObjectNode;
in main method--
use
SimpleModule module =
new SimpleModule("PolymorphicAnimalDeserializerModule");
instead of
new SimpleModule("PolymorphicAnimalDeserializerModule",
new Version(1, 0, 0, null));
and in Animal deserialize() function, make below changes
//Iterator<Entry<String, JsonNode>> elementsIterator = root.getFields();
Iterator<Entry<String, JsonNode>> elementsIterator = root.fields();
//return mapper.readValue(root, animalClass);
return mapper.convertValue(root, animalClass);
This works for fasterxml.jackson. If it still complains of the class fields. Use the same format as in the json for the field names (with "_" -underscore). as this
//mapper.setPropertyNamingStrategy(new CamelCaseNamingStrategy());
might not be supported.
abstract class Animal
{
public String name;
}
class Dog extends Animal
{
public String breed;
public String leash_color;
}
class Cat extends Animal
{
public String favorite_toy;
}
class Bird extends Animal
{
public String wing_span;
public String preferred_food;
}
Handling polymorphism is either model-bound or requires lots of code with various custom deserializers. I'm a co-author of a JSON Dynamic Deserialization Library that allows for model-independent json deserialization library. The solution to OP's problem can be found below. Note that the rules are declared in a very brief manner.
public class SOAnswer {
@ToString @Getter @Setter
@AllArgsConstructor @NoArgsConstructor
public static abstract class Animal {
private String name;
}
@ToString(callSuper = true) @Getter @Setter
@AllArgsConstructor @NoArgsConstructor
public static class Dog extends Animal {
private String breed;
}
@ToString(callSuper = true) @Getter @Setter
@AllArgsConstructor @NoArgsConstructor
public static class Cat extends Animal {
private String favoriteToy;
}
public static void main(String[] args) {
String json = "[{"
+ " \"name\": \"pluto\","
+ " \"breed\": \"dalmatian\""
+ "},{"
+ " \"name\": \"whiskers\","
+ " \"favoriteToy\": \"mouse\""
+ "}]";
// create a deserializer instance
DynamicObjectDeserializer deserializer = new DynamicObjectDeserializer();
// runtime-configure deserialization rules;
// condition is bound to the existence of a field, but it could be any Predicate
deserializer.addRule(DeserializationRuleFactory.newRule(1,
(e) -> e.getJsonNode().has("breed"),
DeserializationActionFactory.objectToType(Dog.class)));
deserializer.addRule(DeserializationRuleFactory.newRule(1,
(e) -> e.getJsonNode().has("favoriteToy"),
DeserializationActionFactory.objectToType(Cat.class)));
List<Animal> deserializedAnimals = deserializer.deserializeArray(json, Animal.class);
for (Animal animal : deserializedAnimals) {
System.out.println("Deserialized Animal Class: " + animal.getClass().getSimpleName()+";\t value: "+animal.toString());
}
}
}
Maven depenendency for pretius-jddl (check newest version at maven.org/jddl:
<dependency>
<groupId>com.pretius</groupId>
<artifactId>jddl</artifactId>
<version>1.0.0</version>
</dependency>
A simple way to enable polymorphic serialization / deserialization via Jackson library is to globally configure the Jackson object mapper (jackson.databind.ObjectMapper) to add information, such as the concrete class type, for certain kinds of classes, such as abstract classes.
To do that, just make sure your mapper is configured correctly. For example:
Option 1: Support polymorphic serialization / deserialization for abstract classes (and Object typed classes)
jacksonObjectMapper.enableDefaultTyping(
ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
Option 2: Support polymorphic serialization / deserialization for abstract classes (and Object typed classes), and arrays of those types.
jacksonObjectMapper.enableDefaultTyping(
ObjectMapper.DefaultTyping.NON_CONCRETE_AND_ARRAYS);
Reference: https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization
Source: Stackoverflow.com