java - 事件类随 Axon CQRS 变化
问题描述
在 axon 中测试事件迁移时,我发现具有默认值的新字段实际上设置为 null。将字段(具有默认值)实际添加到以前的事件中的好方法是什么?我不想有一个 MyEventV2 值是强制性的(没有默认值)并且不想指定一个“upcaster”。
首先,我们有这个事件,它被发布并存储在 axon 服务器中:
data class MyEvent(
val someId: String,
val name: String
)
后来我们改变了 MyEvent,我们给了一个默认值来保持与以前事件的兼容性,我们希望旧事件有这个默认值。真正的结果:'newField' 在接收旧事件时为空。
data class MyEvent(
val someId: String,
val name: String,
val newField: String = "defaultValueButWillBeNullInPreviousEvents... -> would need it to have the actual default value"
)
我使用 axon server、axon 4.2、spring boot 2.2.0 进行了测试,所有默认值都来自 axon boot starter,Jackson 2.10.0
我们应该使用类似的东西(但一点也不好......):
data class MyEvent(
val someId: String,
val name: String,
private val newFieldV2: String? = null
) {
val newField: String
get() = newFieldV2 ?: "my default value"
}
我还没有检查轴突如何从事件存储中重建事件,但我猜它是通过字段访问(反射)而不是构造函数来完成的。但很奇怪,因为数据类没有提供默认的空构造函数......
编辑:Axon 默认使用 XStreamSerializer,我已将配置切换为使用 Jackson,但问题仍然存在,并且 Jackson kotlin 模块已注册。
EDIT2:更新到 Jackson 2.10.1(请参阅问题),它应该修复但由于某种原因我收到另一个错误“com.fasterxml.jackson.databind.exc.InvalidDefinitionException:无法构造实例org.x.x.MyEvent
(没有创建者,如默认构造,存在) :不能从对象值反序列化(没有基于委托或属性的创建者)”。
如果添加“ParameterNamesModule”,则会出现错误,如果未添加,则事件的默认值仍为 null。
EDIT3:我写了测试,但轴突仍然有事件 org.axonframework.serialization.json.JacksonSerializer 错误
@ExperimentalStdlibApi
class JsonDefaultValueTest {
private lateinit var mapper1: ObjectMapper
private lateinit var mapper2: ObjectMapper
private lateinit var mapper3: ObjectMapper
private lateinit var mapper4: ObjectMapper
private lateinit var mapper5: ObjectMapper
private lateinit var mapper6: ObjectMapper
@BeforeEach
fun setup() {
mapper1 = Jackson2ObjectMapperBuilder.json()
.build()
mapper2 = ObjectMapper()
mapper3 = ObjectMapper().registerModule(KotlinModule())
mapper4 = ObjectMapper().registerModule(KotlinModule(nullisSameAsDefault = true))
mapper5 = ObjectMapper().registerModule(ParameterNamesModule())
mapper6 = ObjectMapper()
.registerModule(ParameterNamesModule())
.registerModule(KotlinModule())
.registerModule(Jdk8Module())
.registerModule(JavaTimeModule())
}
@Test
fun `only s1 passed with mapper 1`() {
val json = """{"s1":"only s1"}"""
val event = mapper1.readValue<MyEvent>(json)
val event2 = mapper1.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray())
val expected = MyEvent(
"only s1",
aInt = 0
)
Assertions.assertEquals(expected, event)
}
@Test
fun `only s1 passed with mapper 2`() {
val json = """{"s1":"only s1"}"""
assertThrows<InvalidDefinitionException> { mapper2.readValue<MyEvent>(json) }
assertThrows<InvalidDefinitionException> { mapper2.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray()) }
}
@Test
fun `only s1 passed with mapper 3`() {
val json = """{"s1":"only s1"}"""
val event = mapper3.readValue<MyEvent>(json)
val event2 = mapper3.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray())
val expected = MyEvent(
"only s1",
aInt = 0
)
Assertions.assertEquals(expected, event)
Assertions.assertEquals(expected, event2)
}
@Test
fun `only s1 passed with mapper 4`() {
val json = """{"s1":"only s1"}"""
val event = mapper4.readValue<MyEvent>(json)
val event2 = mapper4.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray())
val expected = MyEvent(
"only s1",
aInt = 0
)
Assertions.assertEquals(expected, event)
Assertions.assertEquals(expected, event2)
}
@Test
fun `only s1 passed with mapper 5`() {
val json = """{"s1":"only s1","s2":null}"""
assertThrows<ValueInstantiationException> { mapper5.readValue<MyEvent>(json) }
assertThrows<ValueInstantiationException> { mapper5.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray()) }
}
@Test
fun `only s1 passed with mapper 6`() {
val json = """{"s1":"only s1"}"""
val event = mapper6.readValue<MyEvent>(json)
val event2 = mapper6.readerFor(MyEvent::class.java).readValue<MyEvent>(json.encodeToByteArray())
val expected = MyEvent(
"only s1",
aInt = 0
)
Assertions.assertEquals(expected, event)
Assertions.assertEquals(expected, event2)
}
data class MyEvent(
val s1: String,
val aInt: Int,
val s2: String = "my default"
)
}
解决方案
这似乎是杰克逊-科特林的问题。您需要明确配置 Jackson 以将“null”(或缺少字段)视为“使用默认值”。
正如https://github.com/FasterXML/jackson-module-kotlin/issues/130#issuecomment-546625625中的建议,您应该使用:
ObjectMapper().registerModule(KotlinModule(nullisSameAsDefault = true)
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