Serializing Map 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

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.

---

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 myMap = new HashMap();
    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
{
  @Override
  public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider) 
      throws IOException, JsonProcessingException
  {
    jgen.writeFieldName(String.valueOf(value.getTime()));
  }
}

---

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 myMap = new HashMap();
    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
{
  @Override
  public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider) 
      throws IOException, JsonProcessingException
  {
    jgen.writeFieldName(String.valueOf(value.getTime()));
  }
}