Serialize Range with Jackson - json

Question

I have a Range that I need to serialize in a certain format, namely "[lower,upper)". To do so I have written a basic serializer:
public class RangeSerializer extends StdSerializer<Range<?>> {
#Override
public void serialize(final Range<?> value, final JsonGenerator gen, final SerializerProvider provider) throws IOException {
if (value != null) {
gen.writeRaw('"');
gen.writeRaw('[');
provider.defaultSerializeValue(value.lowerEndpoint(), gen);
gen.writeRaw(',');
provider.defaultSerializeValue(value.upperEndpoint(), gen);
gen.writeRaw(')');
gen.writeRaw('"');
}
}
(Note that in reality the serializer handles the various possibilities of Range such as closed/open ranges, the possibility of unbounded ranges at either end, etc. but for the purposes of my question that's not relevant so I've removed it to keep the code simple).
My problem is that by falling back to the default serializer for each class I end up with quotes in the wrong place. For example, if I had a Range<String> with an entry "[foo,bar)" then when serializing it I obtain "["foo","bar")". I need the result without quotes around the lower and upper endpoint values.
I understand that the additional quotes are because gen.writeString() in the underlying serializer doesn't realize that it's already in a string. Is there some way to let the generator know this, or an alternative way to achieve what I'm attempting to do?
Note that Range<?> really is generic, so I can't hard-code the serialization of the values. It needs to work for Range<Integer>, Range<String>, Range<DateTime> and anything else.

Answer 1

I was unable to get your approach of using a single generator to serialize everything working. You might be able to use a DelegatingJsonGenerator and hook certain calls, but I decided the way shown below was significantly simpler (at the price of minor performance loss).
Here's a Spock test class demonstrating how I was able to get this working.
#Grab('com.fasterxml.jackson.core:jackson-databind:2.8.1')
#Grab('org.spockframework:spock-core:1.0-groovy-2.4')
import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.databind.SerializationFeature
import com.fasterxml.jackson.databind.annotation.JsonSerialize
import com.fasterxml.jackson.databind.util.StdConverter
import spock.lang.Specification
import spock.lang.Unroll
class RangeSerializationTest extends Specification {
static class Range<T> {
T lower
T upper
}
#JsonSerialize(converter = RangeConverter)
static interface RangeMixin {
}
static class RangeConverter extends StdConverter<Range, String> {
private static final mapper = new ObjectMapper().disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
#Override
String convert(Range value) {
def lower = mapper.convertValue(value.lower, String)
def upper = mapper.convertValue(value.upper, String)
return "[$lower,$upper)"
}
}
#Unroll
def 'range of #lower.class'() {
given:
def mapper = new ObjectMapper()
mapper.addMixIn(Range, RangeMixin)
expect:
mapper.writeValueAsString(new Range(lower: lower, upper: upper)) == expectedJson
where:
lower | upper | expectedJson
'abc' | 'def' | '"[abc,def)"'
123 | 456 | '"[123,456)"'
new Date(123) | new Date(456) | '"[1970-01-01T00:00:00.123+0000,1970-01-01T00:00:00.456+0000)"'
}
}