play.libs.Jsonを継承してJacksonでDateFormatを指定する。

JsonEx.java

import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.map.ObjectMapper;
import play.libs.Json;

import java.text.SimpleDateFormat;

public class JsonEx extends Json {

    /**
     * Convert an object to JsonNode.
     *
     * @param data Value to convert in Json.
     */
    public static JsonNode toJson(final Object data) {
        try {

            ObjectMapper mapper = new ObjectMapper();
            mapper.setDateFormat(new SimpleDateFormat("yyyy/MM/dd HH:mm:ss"));

            return mapper.valueToTree(data);
        } catch(Exception e) {
            throw new RuntimeException(e);
        }
    }

}