Question

我的服务器api使用不同类型的多态对象。例如，我有一个json：

{
  "eventType": "FirstType",
  "eventData": "some-useful-object"
}

和kotlin类：

sealed class NotificationEvent {

  data class SomeUsefulEvent(val eventData: String): NotificationEvent()

}

和mixin类：

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "eventType"
)
@JsonSubTypes(
    JsonSubTypes.Type(
        value = NotificationEvent.SomeUsefulEvent::class,
        name = "FirstType"
    )
)
abstract class NotificationEventMixIn

当我从客户端致电时

jsonMapper.readValue<NotificationEvent>(event.data)

应用程序因NPE而崩溃：

java.lang.NullPointerException：尝试调用接口方法 'java.lang.Object java.util.Map.get（java.lang.Object）'在null上对象参考在com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._addFieldMixIns（AnnotatedFieldCollector.java:110）处在com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._findFields（AnnotatedFieldCollector.java:87）在com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collect（AnnotatedFieldCollector.java:41）在com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collectFields（AnnotatedFieldCollector.java:36）在com.fasterxml.jackson.databind.introspect.AnnotatedClass._fields（AnnotatedClass.java:349）在com.fasterxml.jackson.databind.introspect.AnnotatedClass.fields（AnnotatedClass.java:321）在com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._addFields（POJOPropertiesCollector.java:379）在com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.collectAll（POJOPropertiesCollector.java:308）在com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getPropertyMap（POJOPropertiesCollector.java:287）在com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getProperties（POJOPropertiesCollector.java:170）在com.fasterxml.jackson.databind.introspect.BasicBeanDescription._properties（BasicBeanDescription.java:164）在com.fasterxml.jackson.databind.introspect.BasicBeanDescription.findProperties（BasicBeanDescription.java:239）在com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._findCreatorsFromProperties（BasicDeserializerFactory.java:346）在com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._constructDefaultValueInstantiator（BasicDeserializerFactory.java:330）在com.fasterxml.jackson.databind.deser.BasicDeserializerFactory.findValueInstantiator（BasicDeserializerFactory.java:255）在com.fasterxml.jackson.databind.deser.BeanDeserializerFactory.buildBeanDeserializer（BeanDeserializerFactory.java:214）在com.fasterxml.jackson.databind.deser.BeanDeserializerFactory.createBeanDeserializer（BeanDeserializerFactory.java:137）在com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer2（DeserializerCache.java:411）在com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer（DeserializerCache.java:349）在com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2（DeserializerCache.java:264）在com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer（DeserializerCache.java:244）在com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer（DeserializerCache.java:142）在com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer（DeserializationContext.java:477）在com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer（ObjectMapper.java:4178）在com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose（ObjectMapper.java:3997）在com.fasterxml.jackson.databind.ObjectMapper.readValue（ObjectMapper.java:3011）

如何避免此崩溃？

Answer 1

经过一段时间的研究，我发现在AnnotatedFieldCollector#_findFields中有一个for块调用JavaType.getRawClass().getDeclaredFields()，如果它返回一个空数组，则所有for块被跳过，映射_fields未初始化，并在调用null时保持_addFieldMixIns。

发生这种情况是因为属性不是kotlin中的字段，并且父类没有属性。

可以通过向@JvmField类中添加NotificationEvent带注释的属性来解决此问题，如下所示：

sealed class NotificationEvent {

  @Suppress("unused")
  @JvmField val stub: Int = 0

  data class SomeUsefulEvent(val eventData: String): NotificationEvent()

}

一切正常！

我希望这个问题能对某人有所帮助，并鼓励杰克逊团队修复此错误

使用MixIn时，杰克逊android NPE

1 个答案: