It is possible to deserialize to a class with private fields and a custom argument constructor without using annotations and without modifying the class, using Jackson?
I know it's possible in Jackson when using this combination: 1) Java 8, 2) compile with "-parameters" option, and 3) the parameters names match JSON. But it's also possible in GSON by default without all these restrictions.
For example:
public class Person {
private final String firstName;
private final String lastName;
private final int age;
public Person(String firstName, String lastName, int age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public static void main(String[] args) throws IOException {
String json = "{firstName: \"Foo\", lastName: \"Bar\", age: 30}";
System.out.println("GSON: " + deserializeGson(json)); // works fine
System.out.println("Jackson: " + deserializeJackson(json)); // error
}
public static Person deserializeJackson(String json) throws IOException {
ObjectMapper mapper = new ObjectMapper();
mapper.enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES);
mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
return mapper.readValue(json, Person.class);
}
public static Person deserializeGson(String json) {
Gson gson = new GsonBuilder().create();
return gson.fromJson(json, Person.class);
}
}
Which works fine for GSON, but Jackson throws:
Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `jacksonParametersTest.Person` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
at [Source: (String)"{firstName: "Foo", lastName: "Bar", age: 30}"; line: 1, column: 2]
at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
It's possible in GSON, so I would expect that there must be some way in Jackson without modifying the Person class, without Java 8, and without an explicit custom deserializer. Does anybody know a solution?
Gson seems to skip the argument constructor, so it must be creating a no-argument constructor behind the scenes using reflections.
Also, there exists a Kotlin Jackson module which is able to do this for Kotlin data classes, even without the "-parameters" compiler flag. So it is strange that such a solution doesn't seem to exist for Java Jackson.
This is the (nice and clean) solution available in Kotlin Jackson (which IMO should also become available in Java Jackson via a custom module):
val mapper = ObjectMapper()
.enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES)
.registerModule(KotlinModule())
val person: Person = mapper.readValue(json, Person::class.java)
You could use mix-in annotations. It's a great alternative when modifying the classes is not an option. You can think of it as kind of aspect-oriented way of adding more annotations during runtime, to augment the statically defined ones.
Assuming that your Person
class is defined as follows:
public class Person {
private final String firstName;
private final String lastName;
private final int age;
public Person(String firstName, String lastName, int age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
// Getters omitted
}
First define a mix-in annotation abstract class:
public abstract class PersonMixIn {
PersonMixIn(@JsonProperty("firstName") String firstName,
@JsonProperty("lastName") String lastName,
@JsonProperty("age") int age) {
}
}
Then configure ObjectMapper
to use the defined class as a mix-in for your POJO:
ObjectMapper mapper = new ObjectMapper();
mapper.enable(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES);
mapper.addMixIn(Person.class, PersonMixIn.class);
And deserialize the JSON:
String json = "{firstName: \"Foo\", lastName: \"Bar\", age: 30}";
Person person = mapper.readValue(json, Person.class);