用Jackson序列化Map <Date,String>

Question

我想用Jackson序列化一张地图.应将日期序列化为时间戳,就像我所有其他日期一样.

以下代码以"Tue Mar 11 00:00:00 CET 1952"(即Date.toString())的形式呈现密钥,而不是时间戳.

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

我假设这是因为类型擦除而且jackson在运行时不知道密钥是Date.但我没有找到将TypeReference传递给任何writeValue方法的方法.

有没有一种简单的方法来实现我想要的行为,或者所有的键总是被杰克逊渲染为Strings？

谢谢你的任何提示.

Answer 1

默认的映射键序列化程序是StdKeySerializer,它只是这样做.

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

您可以使用SimpleModule功能,并使用该addKeySerializer方法指定自定义键序列化程序.

这就是如何做到的.

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()));
  }
}

更新最新杰克逊(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()));
  }
}

Answer 2

像往常一样，布鲁斯的答案就在现场。

还有一种想法是，由于存在用于将Date值序列化为时间戳的全局设置：

SerializationConfig.Feature.WRITE_DATES_AS_TIMESTAMPS

也许这也适用于此。和/或至少使用标准ISO-8601格式的文本。主要的实际问题是向后兼容 ; 但是，我怀疑当前使用plain toString()是否非常有用，因为它既无效又不方便（回读值）。

因此，如果需要，您可能要提出功能请求；这听起来像杰克逊（Jackson）对地图键的处理欠佳。