序列化 Map<Date, String>与杰克逊 [英] Serializing Map<Date, String> 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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