如何对 Jackson 使用自定义 Serializer？
我有两个 Java 类，我想用 Jackson 把它们序列化为 JSON:
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;
}
}
我想序列化一个 Item 到这个 JSON:
{"id":7, "itemNr":"TEST", "createdBy":3}
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": "jonas@example.com"}
因此，我想我需要为 Item编写一个自定义序列化程序，并尝试这样做:
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();
}
}
我用 Jackson How-to: Custom Serializers中的代码序列化 JSON:
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();
}
But I get this error:
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)
如何对 Jackson 使用自定义 Serializer？
如果是我，我会这么对付格森:
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);
但是我现在需要用 Jackson 来做这件事，因为 Gson 不支持接口。
最佳答案