我的服务器 api 使用不同类型的多态对象。比如我有一个json:
{
"eventType": "FirstType",
"eventData": "some-useful-object"
}
和它的 kotlin 类:
sealed class NotificationEvent {
data class SomeUsefulEvent(val eventData: String): NotificationEvent()
}
和混合类:
@JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.PROPERTY,
property = "eventType"
)
@JsonSubTypes(
JsonSubTypes.Type(
value = NotificationEvent.SomeUsefulEvent::class,
name = "FirstType"
)
)
abstract class NotificationEventMixIn
当我从客户端调用时
jsonMapper.readValue<NotificationEvent>(event.data)
应用程序因 NPE 而崩溃:
java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.get(java.lang.Object)' on a null object reference at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._addFieldMixIns(AnnotatedFieldCollector.java:110) at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._findFields(AnnotatedFieldCollector.java:87) at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collect(AnnotatedFieldCollector.java:41) at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collectFields(AnnotatedFieldCollector.java:36) at com.fasterxml.jackson.databind.introspect.AnnotatedClass._fields(AnnotatedClass.java:349) at com.fasterxml.jackson.databind.introspect.AnnotatedClass.fields(AnnotatedClass.java:321) at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._addFields(POJOPropertiesCollector.java:379) at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.collectAll(POJOPropertiesCollector.java:308) at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getPropertyMap(POJOPropertiesCollector.java:287) at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getProperties(POJOPropertiesCollector.java:170) at com.fasterxml.jackson.databind.introspect.BasicBeanDescription._properties(BasicBeanDescription.java:164) at com.fasterxml.jackson.databind.introspect.BasicBeanDescription.findProperties(BasicBeanDescription.java:239) at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._findCreatorsFromProperties(BasicDeserializerFactory.java:346) at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._constructDefaultValueInstantiator(BasicDeserializerFactory.java:330) at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory.findValueInstantiator(BasicDeserializerFactory.java:255) at com.fasterxml.jackson.databind.deser.BeanDeserializerFactory.buildBeanDeserializer(BeanDeserializerFactory.java:214) at com.fasterxml.jackson.databind.deser.BeanDeserializerFactory.createBeanDeserializer(BeanDeserializerFactory.java:137) at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer2(DeserializerCache.java:411) at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer(DeserializerCache.java:349) at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:264) at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244) at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:142) at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:477) at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:4178) at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3997) at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3011)
我怎样才能避免这种崩溃?
最佳答案
在研究上浪费了一些时间后,我发现在 AnnotatedFieldCollector#_findFields
中有 for
block 调用 JavaType.getRawClass().getDeclaredFields()
如果它返回一个空数组,则跳过所有 for
block , map _fields
未初始化并保持 null
当 _addFieldMixIns
被调用。
发生这种情况是因为属性不是 kotlin 中的字段并且父类没有。
可以通过向 NotificationEvent
类添加 @JvmField
注释属性来解决此问题,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!
我希望这个问题对某人有所帮助,我鼓励 Jackson 团队修复这个错误
关于java - 使用 MixIn 时 Jackson android NPE,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/55238235/