Hello! I have another example that feels like it _should_ work but actually doesn't. Again, this is a drastically simplified example that comes from a real existing schema, and uses sealed interfaces that are currently a preview feature (at least until JDK 17 appears tomorrow).
~~~
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import
com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;
import
com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlRootElement;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.util.Objects;
import static java.nio.charset.StandardCharsets.UTF_8;
public final class SerialDemo3
{
private SerialDemo3()
{
}
@JsonSubTypes({
@JsonSubTypes.Type(value = Ex1Bike.class, name = "Bike"),
@JsonSubTypes.Type(value = Ex1Car.class, name = "Car"),
@JsonSubTypes.Type(value = Ex1RollerSkates.class, name =
"RollerSkates") })
public sealed interface Vehicle
{
@JacksonXmlRootElement(localName = "Bike")
record Ex1Bike(
@JacksonXmlProperty(isAttribute = true)
boolean bell,
@JacksonXmlProperty(isAttribute = true)
boolean diskBrakes)
implements Vehicle
{
}
@JacksonXmlRootElement(localName = "Car")
record Ex1Car(
@JacksonXmlProperty(isAttribute = true)
String manufacturer)
implements Vehicle
{
public Ex1Car
{
Objects.requireNonNull(manufacturer, "manufacturer");
}
}
@JacksonXmlRootElement(localName = "RollerSkates")
record Ex1RollerSkates()
implements Vehicle
{
public Ex1RollerSkates
{
}
}
}
public record VehicleHolder(Vehicle vehicle)
{
public VehicleHolder
{
Objects.requireNonNull(vehicle, "vehicle");
}
}
public static void main(
final String[] args)
throws Exception
{
final var mapper =
XmlMapper.builder()
.enable(SerializationFeature.INDENT_OUTPUT)
.build();
System.out.println("""
Expected:
<RollerSkates/>
""");
serializeAndParse(mapper, new Ex1RollerSkates());
System.out.println("""
Expected:
<Car manufacturer="Mitsubishi"/>
""");
serializeAndParse(mapper, new Ex1Car("Mitsubishi"));
System.out.println("""
Expected:
<Bike bell="true" diskBrakes="true"/> """);
serializeAndParse(mapper, new Ex1Bike(true, false));
System.out.println("""
Expected:
<VehicleHolder>
<Bike bell="true" diskBrakes="true"/>
</VehicleHolder>
""");
serializeAndParse(mapper, new VehicleHolder(new Ex1Bike(true,
false))); }
private static void serializeAndParse(
final XmlMapper mapper,
final Object input)
{
System.out.println("Serializing:");
System.out.println(input);
System.out.println();
try (var output = new ByteArrayOutputStream()) {
mapper.writeValue(output, input);
output.flush();
System.out.println("Serialized:");
System.out.println(output.toString(UTF_8));
System.out.println("Parsing...");
final Object result;
try (var stream = new ByteArrayInputStream(output.toByteArray()))
{ result = mapper.readValue(stream, input.getClass());
}
System.out.println("Received: " + result);
} catch (final Exception e) {
System.out.println(e);
} finally {
System.out.println("--");
System.out.println();
}
}
}
~~~
The output of the program is:
~~~
Expected:
<RollerSkates/>
Serializing:
Ex1RollerSkates[]
Serialized:
<RollerSkates/>
Parsing...
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
construct instance of
`SerialDemo3$Vehicle$Ex1RollerSkates`
(although at least one Creator exists): no default no-arguments
constructor found at [Source: (ByteArrayInputStream); line: 1, column:
1] --
Expected:
<Car manufacturer="Mitsubishi"/>
Serializing:
Ex1Car[manufacturer=Mitsubishi]
Serialized:
<Car manufacturer="Mitsubishi"/>
Parsing...
com.fasterxml.jackson.databind.exc.ValueInstantiationException: Cannot
construct instance of `SerialDemo3$Vehicle$Ex1Car`,
problem: manufacturer at [Source: (ByteArrayInputStream); line: 1,
column: 1] --
Expected:
<Bike bell="true" diskBrakes="true"/>
Serializing:
Ex1Bike[bell=true, diskBrakes=false]
Serialized:
<Bike bell="true" diskBrakes="false"/>
Parsing...
com.fasterxml.jackson.databind.exc.InvalidDefinitionException:
Duplicate creator property "" (index 0 vs 1) for type
`SerialDemo3$Vehicle$Ex1Bike` at [Source: (ByteArrayInputStream); line:
1, column: 1] --
Expected:
<VehicleHolder>
<Bike bell="true" diskBrakes="true"/>
</VehicleHolder>
Serializing:
VehicleHolder[vehicle=Ex1Bike[bell=true, diskBrakes=false]]
Serialized:
<VehicleHolder>
<vehicle bell="true" diskBrakes="false"/>
</VehicleHolder>
Parsing...
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot
construct instance of
`SerialDemo3$Vehicle`
(no Creators, like default constructor, exist): abstract types either
need to be mapped to concrete types, have custom deserializer, or
contain additional type information at [Source: (ByteArrayInputStream);
line: 2, column: 3] (through reference chain:
SerialDemo3$VehicleHolder["vehicle"])
-- ~~~
There are several issues here:
1. Deserialization fails for Ex1RollerSkates with the error "(although
at least one Creator exists): no default no-arguments constructor
found". This is obviously incorrect as it's a record class and the
only and only constructor available has no arguments.
2. Deserialization fails for Ex1Car for reasons I don't fully
understand. I've tried adding a JsonCreator annotation onto the
constructor, but it doesn't change anything.
3. Deserialization fails for Ex1Bike, claiming there's a duplicate
creator property.
4. Serialization of the VehicleHolder type essentially loses type
information and creates a lowercase "vehicle" element containing
the attributes of whatever record type was used.
5. Deserialization of the VehicleHolder type obviously fails due to
point 4.
Interestingly, trying the following:
~~~
mapper.readValue("""
<VehicleHolder>
<Bile bell="true" diskBrakes="false"/>
</VehicleHolder>
""".getBytes(UTF_8),
VehicleHolder.class
);
~~~
... Also fails, but for the same reason as point 2. Jackson appears
to either handle constructor injection for POJOs, or field injection
for mutable values, but seems to trip over when it comes to immutable
records.
--
Mark Raynsford | https://www.io
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