如何在Jackson中使用自定义Serializer？ [英] How do I use a custom Serializer with Jackson?

问题描述

我想要使用Jackson将两个Java类序列化为JSON：

I have two Java classes that I want to serialize to JSON using Jackson:

public class User {
    public final int id;
    public final String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

public class Item {
    public final int id;
    public final String itemNr;
    public final User createdBy;

    public Item(int id, String itemNr, User createdBy) {
        this.id = id;
        this.itemNr = itemNr;
        this.createdBy = createdBy;
    }
}

我想将一个Item序列化为这个JSON：

I want to serialize an Item to this JSON:

{"id":7, "itemNr":"TEST", "createdBy":3}

用户序列化为仅包含 id 。我还可以将所有用户对象邮件化为JSON，如：

with User serialized to only include the id. I will also be able to serilize all user objects to JSON like:

{"id":3, "name": "Jonas", "email": "[e-mail]"}

所以我想我需要为项目编写自定义序列化程序并尝试使用：

So I guess that I need to write a custom serializer for Item and tried with this:

public class ItemSerializer extends JsonSerializer<Item> {

@Override
public void serialize(Item value, JsonGenerator jgen,
        SerializerProvider provider) throws IOException,
        JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeNumberField("id", value.id);
    jgen.writeNumberField("itemNr", value.itemNr);
    jgen.writeNumberField("createdBy", value.user.id);
    jgen.writeEndObject();
}

}

我使用此代码序列化JSON来自 Jackson How-to：Custom Serializers ：

I serialize the JSON with this code from Jackson How-to: Custom Serializers:

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                              new Version(1,0,0,null));
simpleModule.addSerializer(new ItemSerializer());
mapper.registerModule(simpleModule);
StringWriter writer = new StringWriter();
try {
    mapper.writeValue(writer, myItem);
} catch (JsonGenerationException e) {
    e.printStackTrace();
} catch (JsonMappingException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

但我收到此错误：

Exception in thread "main" java.lang.IllegalArgumentException: JsonSerializer of type com.example.ItemSerializer does not define valid handledType() (use alternative registration method?)
    at org.codehaus.jackson.map.module.SimpleSerializers.addSerializer(SimpleSerializers.java:62)
    at org.codehaus.jackson.map.module.SimpleModule.addSerializer(SimpleModule.java:54)
    at com.example.JsonTest.main(JsonTest.java:54)

如何我可以在杰克逊上使用自定义串行器吗？

这就是我用Gson做的方法：

This is how I would do it with Gson:

public class UserAdapter implements JsonSerializer<User> {

    @Override 
    public JsonElement serialize(User src, java.lang.reflect.Type typeOfSrc,
            JsonSerializationContext context) {
        return new JsonPrimitive(src.id);
    }
}

    GsonBuilder builder = new GsonBuilder();
    builder.registerTypeAdapter(User.class, new UserAdapter());
    Gson gson = builder.create();
    String json = gson.toJson(myItem);
    System.out.println("JSON: "+json);

但我现在需要和杰克逊一起做，因为Gson不支持接口。 / p>

But I need to do it with Jackson now, since Gson doesn't have support for interfaces.

推荐答案

如上所述，@ JsonValue是一个好方法。但是如果你不介意自定义序列化程序，则不需要为Item编写一个，而是为User写一个 - 如果是这样的话，它就像这样简单：

As mentioned, @JsonValue is a good way. But if you don't mind a custom serializer, there's no need to write one for Item but rather one for User -- if so, it'd be as simple as:

public void serialize(Item value, JsonGenerator jgen,
    SerializerProvider provider) throws IOException,
    JsonProcessingException {
  jgen.writeNumber(id);
}

另一种可能性是实现 JsonSerializable ，在这种情况下不需要注册。

Yet another possibility is to implement JsonSerializable, in which case no registration is needed.

关于错误;这很奇怪 - 你可能想要升级到更高版本。但扩展 org.codehaus.jackson.map.ser.SerializerBase 也更安全，因为它将具有非必要方法的标准实现（即除了实际的序列化调用之外的所有内容） 。

As to error; that is weird -- you probably want to upgrade to a later version. But it is also safer to extend org.codehaus.jackson.map.ser.SerializerBase as it will have standard implementations of non-essential methods (i.e. everything but actual serialization call).

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