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I am learning to use Jackson's custom serialization and deserialization capabilities to handle the DatePair class. I hope to concatenate the fields of DatePair into a single string.
Here is a MCVE code:
DatePair is a unmodifiable class within a third-party library:

 import lombok.Data; import java.util.Date; @Data public class DatePair { Date date1; Date date2; }

My example:

 import com.fasterxml.jackson.annotation.JsonFormat; import com.fasterxml.jackson.core.JsonGenerator; import com.fasterxml.jackson.databind.JsonSerializer; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.SerializerProvider; import com.fasterxml.jackson.databind.module.SimpleModule; import lombok.Data; import java.io.IOException; import java.text.SimpleDateFormat; import java.util.Date; @Data public class Example { @Data public static class Model1 { @JsonFormat(pattern = "yyyy/MM/dd HH:mm:ss.SSS", timezone = "GMT+2") DatePair datePair; } public static class DatePairJsonSerializer extends JsonSerializer<DatePair> { @Override public void serialize(DatePair value, JsonGenerator gen, SerializerProvider serializers) throws IOException { gen.writeRaw("("); serializers.defaultSerializeValue(value.date1, gen); gen.writeRaw(", "); /* When serialize Model1 will throw JsonGenerationException: Can not write a string, expecting field name (context: Object) */ serializers.defaultSerializeValue(value.date2, gen); gen.writeRaw(")"); } } public static void main(String[] args) { ObjectMapper mapper = new ObjectMapper(); mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")); SimpleModule simpleModule = new SimpleModule(); simpleModule.addSerializer(DatePair.class, new DatePairJsonSerializer()); mapper.registerModule(simpleModule); DatePair datePair = new DatePair(); datePair.setDate1(new Date()); datePair.setDate2(new Date()); Model1 model1 = new Model1(); model1.setDatePair(datePair); try { /* serialize DatePair */ String json = mapper.writeValueAsString(datePair); System.out.println(json); // output: ("2025-06-22 10:02:48", "2025-06-22 10:02:48") /* serialize Model1 will throw JsonGenerationException: Can not write a string, expecting field name (context: Object) */ json = mapper.writeValueAsString(model1); // expected output: {"datePair": "(2025/06/22 10:02:48.000, 2025/06/22 10:02:48.000)"} System.out.println(json); } catch (Exception e) { e.printStackTrace(); } } }

dependencies:

 <dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.18.2</version> </dependency>

I encountered some issues:

  1. When serializing DatePair alone, I want the output Date to ​​not have quotes​​, like: (2025-06-22 10:02:48, 2025-06-22 10:02:48), instead of ("2025-06-22 10:02:48", "2025-06-22 10:02:48")
  2. When serializing Model1, I want @JsonFormat applied to the date fields in DatePair
  3. Current serialization of Model1 throws errors. How to resolve?
  4. Could you provide a deserialization example matching the serialization logic?
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  • Try writing the " that should surround your ( enclosed dates as well, looks like currently you are missing those. Commented Jun 22 at 3:59

2 Answers 2

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Jackson's serialization might be tricky. You can achieve your goal by StdConverter.

Here's an example:

@Data public static class Model1 { @JsonSerialize(converter = DatePairSerializerConverter.class) @JsonDeserialize(converter = DatePairDeserializerConverter.class) DatePair datePair; } public static class DatePairSerializerConverter extends StdConverter<DatePair, String> { private static final DateFormat FIRST_DATE_FORMAT = new SimpleDateFormat("(yyyy/MM/dd HH:mm:ss.SSS, "); private static final DateFormat SECOND_DATE_FORMAT = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS)"); @Override public String convert(DatePair datePair) { return FIRST_DATE_FORMAT.format(datePair.date1) + SECOND_DATE_FORMAT.format(datePair.date2); } } public static class DatePairDeserializerConverter extends StdConverter<String, DatePair> { private static final String DELIMITER = ", "; private static final DateFormat FIRST_DATE_FORMAT = new SimpleDateFormat("(yyyy/MM/dd HH:mm:ss.SSS"); private static final DateFormat SECOND_DATE_FORMAT = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS)"); @Override public DatePair convert(String value) { String[] dates = value.split(DELIMITER); DatePair datePair = new DatePair(); try { datePair.setDate1(FIRST_DATE_FORMAT.parse(dates[0])); datePair.setDate2(SECOND_DATE_FORMAT.parse(dates[1])); return datePair; } catch (ParseException e) { throw new RuntimeException(e); } } }

A few side notes:

  1. If you want to register DatePairSerializerConverter & DatePairDeserializerConverter globally (so you don't need to specify @JsonSerialize & @JsonDeserialize in each model explicitly), you might need to look into StdDelegatingSerializer & StdDelegatingDeserializer, because there's no such method as objectMapper.addConverter(new DatePairSerializerConverter()), unfortunatelly.
  2. It's recommended to store each class in a separate file.
  3. It's recommended to add private modifier to the fields your model classes.
  4. If you're on Java 17+, try migrating your model classes to record, so you don't have to bother with Lombok annotations.
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First, the output ("2025-06-22 10:02:48", "2025-06-22 10:02:48") of serialized DatePair is not a standard JSON, that mean's you can not using SerializerProvider.defaultSerializeValue . You need create a format with JsonFormator define a default, and the Serializer (recommended extends StdSerializer) should implements ContextualSerializer. You can refer to the implementation of DateSerializer and DateTimeSerializerBase.

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