如何使用Jackson的objectMapper反序列化接口字段？ [英] How to deserialize interface fields using Jackson's objectMapper?

问题描述

ObjectMapper 的 readValue（InputStream in，Class< T> valueType） function需要Class。但是如果我在内部传递的类有一些接口作为数据成员，我该如何使用它。

ObjectMapper's readValue(InputStream in, Class<T> valueType) function requires the Class. But how do I use it if the class I am passing internally, is having some Interface as data member.

虽然我能理解这个异常背后的原因，因为Jackson没有得到传递类的内部接口的具体类，但我的问题是如何解决它？
我如何反序列化呢？我试图反序列化的类是：

although I can understand the reason behind this exception, as Jackson is not getting the concrete class of the internal Interface of the passed class, but my question is how to resolve it? how do I deserialize it then? The class I am trying to deserialize is:

class BaseMetricImpl<N> implements Metric<N> {
    protected MetricValueDescriptor descriptor;
}

此处 MetricValueDescriptor 是一个接口，所以这给了我以下错误： -

Here MetricValueDescriptor is an interface, so this gives me following error : -

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of MetricValueDescriptor, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
 at [Source: java.io.ByteArrayInputStream@2ede2c9f; line: 1, column: 2] (through reference chain: SingleValueMetricImpl["descriptor"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:624)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:115)
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:375)
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:98)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:308)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2793)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1989)

推荐答案

杰克逊显然无法构造 MetricValueDescriptor 对象，因为它是一个接口。您需要在json和ObjectMapper中获取其他信息，以告诉jackson如何构造一个对象。这是一种方法，假设 MVDImpl 是一个实现 MetricValueDescriptor 的具体类：

Jackson obviously cannot construct the MetricValueDescriptor object since it is an interface. You will need to have additional information in your json and in your ObjectMapper to tell jackson how to construct an object out of it. Here is one way to do it, assuming MVDImpl is a concrete class which implements MetricValueDescriptor:

您可以通过json本身的字段告诉Jackson所需的类型信息，例如type。为此，您需要使用 JsonTypeInfo 和 界面中的JsonSubTypes 注释。例如，

You can tell Jackson the required type information through a field in the json itself, say "type". To do this, you need to use JsonTypeInfo and JsonSubTypes annotations in your interface. For example,

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "type")
@JsonSubTypes({
    @Type(value = MVDImpl.class, name = "mvdimpl") })
interface MetricValueDescriptor
{
   ...
}

您需要添加你的json中的type：mvdimpl字段。

You will need to add a "type":"mvdimpl" field in your json as well.

我打算指出你官方文档了解更多信息，但后来我发现了一个很好的博客，涵盖了这个主题 - 用杰克逊反序列化JSON 。它非常全面地并通过示例介绍了这个主题。所以如果你需要更多的自定义，你一定要阅读它。

I was going to point you to the official doc for more info, but then I found an excellent blog covering this topic - Deserialize JSON with Jackson. It covers this topic pretty comprehensively and with examples. So you should definitely read it if you need more customisation.

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