POJOToMapModule.java

package com.foo.bar;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.DeserializationConfig;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.deser.BeanDeserializerModifier;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
import java.io.IOException;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

public class POJOToMapModule extends SimpleModule {

  public static void main(String[] args) {
    class Foo {

      Instant createdAt;

      public Foo(Instant createdAt){
        this.createdAt = createdAt;
      }

      public Instant getCreatedAt(){ return createdAt; }

    }
    Foo foo = new Foo(Instant.now());
    ObjectMapper om = new ObjectMapper()
        .registerModule(new POJOToMapModule())
        .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
    Map<String,Object> map = om.convertValue(foo, om.getTypeFactory().constructMapType(HashMap.class, String.class, Object.class));
    System.out.println("createdAt type => " + map.get("createdAt").getClass());
  }

  public static class InstantToDateSerializer extends JsonSerializer<Instant> {
    @Override
    public void serialize(Instant value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
      if (value != null) {
        // Convert from Instant to Date...
        gen.writeObject(Date.from(value));
      }
    }
  }

  public static class DateStringToInstantDeserializer extends JsonDeserializer<Object> {

    private final JsonDeserializer<?> deserializer;

    public DateStringToInstantDeserializer(JsonDeserializer<?> deserializer) {
      this.deserializer = deserializer;
    }

    @Override
    public Object deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
      if (jsonParser.getCurrentToken() != JsonToken.VALUE_STRING) {
        // The current token is not a string... pass along to the default deserializer:
        return deserializer.deserialize(jsonParser, deserializationContext);
      }
      try {
        // It's a string, so let's attempt to parse and if successful, return an Instant.
        return Date
            .from(LocalDateTime
                .parse(jsonParser.getValueAsString(), DateTimeFormatter.ISO_ZONED_DATE_TIME)
                .toInstant(ZoneOffset.UTC));
      } catch (DateTimeParseException e) {
        // Fallback...
        return deserializer.deserialize(jsonParser, deserializationContext);
      }
    }
  }

  public POJOToMapModule() {
    this.setSerializerModifier(new BeanSerializerModifier() {
      @Override
      public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
        // Register the Instant to Date serializer if the current field type is an Instant:
        if (beanDesc.getBeanClass().isAssignableFrom(Instant.class)) {
          return new InstantToDateSerializer();
        }
        // Fallback to default behavior:
        return super.modifySerializer(config, beanDesc, serializer);
      }
    });
    this.setDeserializerModifier(new BeanDeserializerModifier() {
      @Override
      public JsonDeserializer<?> modifyDeserializer(
          DeserializationConfig config,
          BeanDescription beanDesc,
          JsonDeserializer<?> deserializer
      ) {
        return new DateStringToInstantDeserializer(deserializer);
      }
    });
  }
}