如何将 JSON null 反序列化为 NullNode 而不是 Java null？
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How to deserialize JSON null to a NullNode instead of Java null?
提问by fge
Note: Hymanson 2.1.x.
注意：Hyman逊 2.1.x。
The problem is quite simple but I could not find a solution so far. I have browsed through existing documentation etc and could not find an answer.
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。
The base class is this:
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, Hymanson 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 + '"';
}
}
And the problematic class is this:
有问题的课程是这样的：
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;
}
}
When I try to deserialize:
当我尝试反序列化时：
{ "op": "add", "path": "/x", "value": null }
I'd like the null value to be deserialized as a NullNode, not Java null. And I couldn't find a way to do it so far.
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做到这一点。
How do you achieve that?
你如何做到这一点？
(note: all constructors of concrete classes are @JsonCreators with the appropriate @JsonPropertyannotations -- they work without problem, my only problem is JSON null handling)
（注意：具体类的所有构造函数都@JsonCreator带有适当的@JsonProperty注释——它们可以正常工作，我唯一的问题是 JSON 空处理）
回答by fge
OK, well, I haven't read the documentation enough, and it is in fact very simple.
好吧，我还没有阅读足够的文档，实际上很简单。
There is a JsonNodeDeserializerimplementation of JsonDeserializer, which you can extend. And JsonDeserializerhas a.getNullValue() method.
有一个JsonNodeDeserializer实现JsonDeserializer，您可以对其进行扩展。并且JsonDeserializer有一个.getNullValue() 方法。
So, a custom deserializer was in order:
所以，一个自定义的解串器是有序的：
public final class JsonNullAwareDeserializer
extends JsonNodeDeserializer
{
@Override
public JsonNode getNullValue()
{
return NullNode.getInstance();
}
}
And, in the problematic class:
而且，在有问题的课程中：
@JsonDeserialize(using = JsonNullAwareDeserializer.class)
protected final JsonNode value;
