Serializing Map<Date, String> with Jackson

Question

I want to serialize a Map with Jackson. The Date should be serialized as a timestamp, like all my other dates.

The following code renders the keys in the form "Tue Mar 11 00:00:00 CET 1952" (which is Date.toString()) instead of the timestamp.

Map<Date, String> myMap = new HashMap<Date, String>(); ... ObjectMapper.writeValue(myMap) 

I assume this is because of type erasure and jackson doesn't know at runtime that the key is a Date. But I didn't find a way to pass a TypeReference to any writeValue method.

Is there a simple way to achieve my desired behaviour or are all keys always rendered as Strings by jackson?

Thanks for any hint.

Answer 1

The default map key serializer is StdKeySerializer, and it simply does this.

String keyStr = (value.getClass() == String.class) ? ((String) value) : value.toString(); jgen.writeFieldName(keyStr); 

You could make use of the SimpleModule feature, and specify a custom key serializer, using the addKeySerializer method.

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And here's how that could be done.

import java.io.IOException; import java.util.Date; import java.util.HashMap; import java.util.Map;  import org.codehaus.jackson.JsonGenerator; import org.codehaus.jackson.JsonProcessingException; import org.codehaus.jackson.Version; import org.codehaus.jackson.map.JsonSerializer; import org.codehaus.jackson.map.ObjectMapper; import org.codehaus.jackson.map.ObjectWriter; import org.codehaus.jackson.map.SerializerProvider; import org.codehaus.jackson.map.module.SimpleModule; import org.codehaus.jackson.map.type.MapType; import org.codehaus.jackson.map.type.TypeFactory;  public class CustomKeySerializerDemo {   public static void main(String[] args) throws Exception   {     Map<Date, String> myMap = new HashMap<Date, String>();     myMap.put(new Date(), "now");     Thread.sleep(100);     myMap.put(new Date(), "later");      ObjectMapper mapper = new ObjectMapper();     System.out.println(mapper.writeValueAsString(myMap));     // {"Mon Jul 04 11:38:36 MST 2011":"now","Mon Jul 04 11:38:36 MST 2011":"later"}      SimpleModule module =         new SimpleModule("MyMapKeySerializerModule",             new Version(1, 0, 0, null));     module.addKeySerializer(Date.class, new DateAsTimestampSerializer());      MapType myMapType = TypeFactory.defaultInstance().constructMapType(HashMap.class, Date.class, String.class);      ObjectWriter writer = new ObjectMapper().withModule(module).typedWriter(myMapType);     System.out.println(writer.writeValueAsString(myMap));     // {"1309806289240":"later","1309806289140":"now"}   } }  class DateAsTimestampSerializer extends JsonSerializer<Date> {   @Override   public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)        throws IOException, JsonProcessingException   {     jgen.writeFieldName(String.valueOf(value.getTime()));   } } 

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Update for the latest Jackson (2.0.4):

import java.io.IOException; import java.util.Date; import java.util.HashMap; import java.util.Map;  import com.fasterxml.jackson.core.JsonGenerator; import com.fasterxml.jackson.core.JsonProcessingException; import com.fasterxml.jackson.databind.JsonSerializer; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.ObjectWriter; import com.fasterxml.jackson.databind.SerializerProvider; import com.fasterxml.jackson.databind.module.SimpleModule; import com.fasterxml.jackson.databind.type.MapType; import com.fasterxml.jackson.databind.type.TypeFactory;  public class CustomKeySerializerDemo {   public static void main(String[] args) throws Exception   {     Map<Date, String> myMap = new HashMap<Date, String>();     myMap.put(new Date(), "now");     Thread.sleep(100);     myMap.put(new Date(), "later");      ObjectMapper mapper = new ObjectMapper();     System.out.println(mapper.writeValueAsString(myMap));     // {"2012-07-13T21:14:09.499+0000":"now","2012-07-13T21:14:09.599+0000":"later"}      SimpleModule module = new SimpleModule();     module.addKeySerializer(Date.class, new DateAsTimestampSerializer());      MapType myMapType = TypeFactory.defaultInstance().constructMapType(HashMap.class, Date.class, String.class);      ObjectWriter writer = new ObjectMapper().registerModule(module).writerWithType(myMapType);     System.out.println(writer.writeValueAsString(myMap));     // {"1342214049499":"now","1342214049599":"later"}   } }  class DateAsTimestampSerializer extends JsonSerializer<Date> {   @Override   public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)        throws IOException, JsonProcessingException   {     jgen.writeFieldName(String.valueOf(value.getTime()));   } }