如何将JSON null反序列化为NullNode而不是Java null？

Question

注:杰克逊2.1.x.

问题很简单但到目前为止我找不到解决方案.我浏览了现有的文档等,但找不到答案.

基类是这样的:

@JsonTypeInfo(use = Id.NAME, include = As.PROPERTY, property = "op")

@JsonSubTypes({
    @Type(name = "add", value = AddOperation.class),
    @Type(name = "copy", value = CopyOperation.class),
    @Type(name = "move", value = MoveOperation.class),
    @Type(name = "remove", value = RemoveOperation.class),
    @Type(name = "replace", value = ReplaceOperation.class),
    @Type(name = "test", value = TestOperation.class)
})

public abstract class JsonPatchOperation
{
    /*
     * Note: no need for a custom deserializer, Jackson will try and find a
     * constructor with a single string argument and use it
     */
    protected final JsonPointer path;

    protected JsonPatchOperation(final JsonPointer path)
    {
        this.path = path;
    }

    public abstract JsonNode apply(final JsonNode node)
        throws JsonPatchException;

    @Override
    public String toString()
    {
        return "path = \"" + path + '"';
    }
}

有问题的课是这样的:

public abstract class PathValueOperation
    extends JsonPatchOperation
{
    protected final JsonNode value;

    protected PathValueOperation(final JsonPointer path, final JsonNode value)
    {
        super(path);
        this.value = value.deepCopy();
    }

    @Override
    public String toString()
    {
        return super.toString() + ", value = " + value;
    }
}

当我尝试反序列化时:

{ "op": "add", "path": "/x", "value": null }

我希望将null值反序列化为a NullNode,而不是Java null.到目前为止我找不到办法.

你是如何实现这一目标的？

(注意:具体类的所有构造函数都是@JsonCreator带有相应@JsonProperty注释的 - 它们没有问题,我唯一的问题是JSON null处理)

Answer 1

好吧,我没有阅读足够的文档,事实上它非常简单.

有一个JsonNodeDeserializer实现JsonDeserializer,您可以扩展它.并且JsonDeserializer有一个.getNullValue()方法.

因此,定制的反序列化程序是有序的:

public final class JsonNullAwareDeserializer
    extends JsonNodeDeserializer
{
    @Override
    public JsonNode getNullValue()
    {
        return NullNode.getInstance();
    }
}

并且,在有问题的课程中:

@JsonDeserialize(using = JsonNullAwareDeserializer.class)
protected final JsonNode value;