序列化 Map<Date, String>与杰克逊 [英] Serializing Map&lt;Date, String&gt; with Jackson

问题描述

我想用 Jackson 序列化一个 Map.日期应该序列化为时间戳，就像我所有的其他日期一样.

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

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

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)

我认为这是因为类型擦除而 jackson 在运行时不知道键是 Date.但是我没有找到将 TypeReference 传递给任何 writeValue 方法的方法.

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.

是否有一种简单的方法可以实现我想要的行为，还是所有的键总是被 jackson 渲染为字符串?

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

感谢您的提示.

推荐答案

默认的映射键序列化器是 StdKeySerializer，它只是这样做.

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);

你可以使用 SimpleModule 功能，并使用 addKeySerializer 方法指定自定义密钥序列化程序.

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

这是如何做到的.

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()));
  }
}

<小时>

最新 Jackson (2.0.4) 的更新:

---

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()));
  }
}

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