启用将空对象的Jackson反序列化为空 [英] Enable Jackson Deserialization of Empty Objects to Null

问题描述

我被要求更改杰克逊映射配置，以便我们反序列化（从JSON）的每个空对象都将反序列化为null。

I was asked to change our jackson mapping configuration so that each empty object we deserialize (from JSON) is going to be deserialized as null.

问题是我在努力做到这一点，但没有任何运气。这是我们的 ObjectMapper 配置的示例（和示例）：

The problem is that I'm struggling to do it, but without any luck. Here is a sample of our ObjectMapper configuration (and example):

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, true);
mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
JavaTimeModule javaTimeModule = new JavaTimeModule();
javaTimeModule.addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer(DateTimeFormatter.ISO_DATE_TIME));
javaTimeModule.addDeserializer(Instant.class, InstantDeserializer.INSTANT);
mapper.registerModule(javaTimeModule);
mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
warmupMapper(mapper);

return mapper;

我考虑过添加以下内容：

I thought about something like adding:

mapper.configure(
    DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);

但它仅适用于字符串。

我恐怕使用a自定义反序列化器对我没有帮助，因为我正在编写一个通用的（对于所有对象）映射器。因此，我可能需要委托人或后期处理反序列化方法。

I'm afraid that using a custom deserializer will not help me, because I'm writing a generic (for all objects) mapper. So I probably need something like a delegator or a post process deserialization method.

因此对于 或 {} 我希望在Java中转换为 null （而不是空字符串或 Object 实例）。

So for json like "" or {} I expect to be converted to null in java (and not to empty string or Object instance).

推荐答案

什么是空对象？具有空值字段的对象？没有字段的对象？您可以创建一个自定义来检查节点并反序列化所需的方式。我认为以通用方式使用它没有问题。

What is a empty object for you? A object with null value fields? A object with no fields? You can create a custom to check the nodes and deserialize how you want. I see no problem to use it in a generic way.

我做了一个小例子：

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import java.io.IOException;
import java.util.Objects;

public class DeserializerExample<T> extends StdDeserializer<T> {

    private final ObjectMapper defaultMapper;

    public DeserializerExample(Class<T> clazz) {
        super(clazz);
        defaultMapper = new ObjectMapper();
    }

    @Override
    public T deserialize(JsonParser jp, DeserializationContext dc) throws IOException, JsonProcessingException {
        System.out.println("Deserializing...");

        JsonNode node = jp.getCodec().readTree(jp);

        for (JsonNode jsonNode : node) {
            if (!jsonNode.isNull()) {
                return defaultMapper.treeToValue(node, (Class<T>) getValueClass());
            }
        }

        return null;
    }

    public static void main(String[] args) throws IOException {

        ObjectMapper mapper = new ObjectMapper();
        SimpleModule module = new SimpleModule();
        module.addDeserializer(Person.class, new DeserializerExample(Person.class));
        mapper.registerModule(module);

        Person person = mapper.readValue("{\"id\":1, \"name\":\"Joseph\"}", Person.class);

        Person nullPerson = mapper.readValue("{\"id\":null, \"name\":null}", Person.class);

        System.out.println("Is null: " + Objects.isNull(person));
        System.out.println("Is null: " + Objects.isNull(nullPerson));
    }

}

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