Custom Serializer / Deserializer not working with map keys

[tanayhub]

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Describe the bug

Context

Wanted to use Jackson Databind to convert a Map<Duration, UUID> into a JSON string. But object mapper should use custom serde methods instead of the default Duration::toString. Therefore, created few serde implementations, added it to a module and registered that module with a object mapper ( GithubSnippet::main on L67 ). Further created 2 objects:

UUID uuid = UUID.fromString("00000000-0000-0000-0000-000000000000");
Duration duration = Duration.ofMillis(12345);

Input Cases

Map<UUID, Duration> input1 = Map.of(uuid, duration);
Map<Duration, UUID> input2 = Map.of(duration, uuid);

Current / Expected Behavior

Converting input1 into a JSON string gives output {"00000000-0000-0000-0000-000000000000":"12345ms"} which is expected. Whereas converting input2 into a JSON string gives incorrect output {"PT12.345S":"00000000-0000-0000-0000-000000000000"} ( expected {"12345ms":"00000000-0000-0000-0000-000000000000"} )

Snippet

import com.fasterxml.jackson.core.JacksonException;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;

import java.io.IOException;
import java.time.Duration;
import java.util.Map;
import java.util.Objects;
import java.util.UUID;

class DurationSerializer extends StdSerializer<Duration> {

    public DurationSerializer() {
        super(Duration.class);
    }

    @Override
    public void serialize(Duration duration, JsonGenerator generator, SerializerProvider provider) throws IOException {
        generator.writeString(duration.toMillis() + "ms");
    }

    public static DurationSerializer getInstance() {
        return new DurationSerializer();
    }
}

class DurationDeserializer extends StdDeserializer<Duration> {

    public DurationDeserializer() {
        super(Duration.class);
    }

    @Override
    public Duration deserialize(JsonParser parser, DeserializationContext context) throws IOException, JacksonException {
        String duration = Objects.requireNonNullElse(parser.getValueAsString(), "0ms");
        return Duration.ofMillis(Long.parseLong(duration.substring(0, duration.length() - 2)));
    }

    public static DurationDeserializer getInstance() {
        return new DurationDeserializer();
    }
}

class DurationModule extends SimpleModule {
    public DurationModule() {
        addSerializer(Duration.class, DurationSerializer.getInstance());
        addDeserializer(Duration.class, DurationDeserializer.getInstance());
    }

    public static DurationModule getInstance() {
        return new DurationModule();
    }
}

public class GithubSnippet {
    public static void main(String[] args) throws JsonProcessingException {
        UUID uuid = UUID.fromString("00000000-0000-0000-0000-000000000000");
        Duration duration = Duration.ofMillis(12345);

        ObjectMapper mapper = new ObjectMapper().registerModule(DurationModule.getInstance());

        /* correct */
        /* case input: Map<UUID, Duration> */
        /* actual & expected output: {"00000000-0000-0000-0000-000000000000":"12345ms"} */
        System.out.println(mapper.writeValueAsString(Map.of(uuid, duration)));
        
        /* incorrect */
        /* case input: Map<Duration, UUID> */
        /* actual output: {"PT12.345S":"00000000-0000-0000-0000-000000000000"} */
        /* expected output: {"12345ms":"00000000-0000-0000-0000-000000000000"} */
        System.out.println(mapper.writeValueAsString(Map.of(duration, uuid)));
    }
}

NOTE: you can directly copy paste and run this snippet as long as required libraries as available in the classpath

Version Information

2.17.2

[pjfanning]

Jackson uses separate Key Serializers and Deserializers to handle the keys of Maps.

Please have a read of https://www.baeldung.com/jackson-map

See https://www.javadoc.io/static/com.fasterxml.jackson.core/jackson-databind/2.17.2/com/fasterxml/jackson/databind/module/SimpleModule.html#addKeySerializer-java.lang.Class-com.fasterxml.jackson.databind.JsonSerializer-

[tanayhub]

@pjfanning thanks for quick reply, tried this and it worked. Thanks again