事件类随 Axon CQRS 变化

Question

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）实际添加到先前事件的好方法是什么？ 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，它被发布并存储在轴突服务器中：

data class MyEvent(
    val someId: String,
    val name: String
)

稍后我们更改 MyEvent，我们给了一个默认值以保持与以前事件的兼容性持久化，我们希望旧事件具有此默认值。 真实结果：'newField'在接收旧事件时为null。

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

我们是否应该使用类似的东西（但根本不是很好......）：

data class MyEvent(
    val someId: String,
    val name: String,
    private val newFieldV2: String? = null
) {
    val newField: String
        get() = newFieldV2 ?: "my default value"
}

我还没有检查过 axon 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。 但是很奇怪，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，并且注册了 Jackson kotlin 模块。

编辑2： 更新到 Jackson 2.10.1（请参阅 issue）应该可以修复，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 org.x.x.MyEvent 的实例（没有创作者，如默认构造, 存在）：无法从 Object 值反序列化（没有基于委托或属性的 Creator）”。

如果添加“ParameterNamesModule”会出现错误，如果没有添加，则事件的默认值仍然为空。

编辑3： 我写了测试，但是 axon 仍然有事件 org.axonframework.serialization.json.JacksonSerializer 的错误

@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"
    )
}

Answer 1

这似乎是 Jackson-Kotlin 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”。

按照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)

Answer 2

这听起来像是用于反序列化事件的 Jackson ObjectMapper 配置不正确。为了让它与 Kotlin 特定的功能（例如默认值）一起使用，您需要注册 Kotlin 模块：https://github.com/FasterXML/jackson-module-kotlin

val mapper = ObjectMapper().registerModule(KotlinModule())