Jackson XML反序列化问题

Question

我能够编写XML文档，但无法反序列化创建的XML。原始代码是Kotlin，但我已经发布了Java的等效代码。

@JacksonXmlRootElement(localName = "assets")
public class Assets {
    @JacksonXmlElementWrapper(useWrapping = false)
    private Asset[] asset;

    public Assets(Asset[] asset) {
        this.asset = asset;
    }

    public Asset[] getAsset() {
        return asset;
    }

    public void setAsset(Asset[] asset) {
        this.asset = asset;
    }
}

public class Asset {
    @JacksonXmlProperty(isAttribute = true)
    private String type;
    @JacksonXmlProperty(isAttribute = true)
    private String name;
    @JacksonXmlProperty(isAttribute = true)
    private String displayName;
    @JacksonXmlElementWrapper(localName = "permissions")
    private Permission[] permission;

    public Asset(String type, String name, String displayName, Permission[] permission) {
        this.type = type;
        this.name = name;
        this.displayName = displayName;
        this.permission = permission;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDisplayName() {
        return displayName;
    }

    public void setDisplayName(String displayName) {
        this.displayName = displayName;
    }

    public Permission[] getPermission() {
        return permission;
    }

    public void setPermission(Permission[] permission) {
        this.permission = permission;
    }
}

public class Permission {
    @JacksonXmlProperty(isAttribute = true)
    private String name;
    private Group[] group;

    public Permission(String name, Group[] group) {
        this.name = name;
        this.group = group;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Group[] getGroup() {
        return group;
    }

    public void setGroup(Group[] group) {
        this.group = group;
    }
}

用Kotlin实例化：

fun writeToXml(obj: Any) : String {
    val xmlMapper = XmlMapper()
    xmlMapper.enable(SerializationFeature.INDENT_OUTPUT)
    return xmlMapper.writeValueAsString(obj)
}

val assets = Assets(arrayOf(
            Asset("bdo", "approval-matrix", "Approval Matrix", arrayOf(Permission("Create", arrayOf(Group(arrayOf("Admin", "Approver")))))),
            Asset("bdo", "approval-matrix-2", "Approval Matrix 2", arrayOf(Permission("Delete", arrayOf(Group(arrayOf("Admin", "Approver")), Group(arrayOf("Admin-2", "Approver-2"))))))))

val xmlModule = JacksonXmlModule()
val objectMapper = XmlMapper(xmlModule)
println(writeToXml(assets))

生成的XML符合预期：

<assets>
  <asset type="bdo" name="approval-matrix" displayName="Approval Matrix">
    <permissions>
      <permission name="Create">
        <groups>
          <group>Admin</group>
          <group>Approver</group>
        </groups>
      </permission>
    </permissions>
  </asset>
  <asset type="bdo" name="approval-matrix-2" displayName="Approval Matrix 2">
    <permissions>
      <permission name="Delete">
        <groups>
          <group>Admin</group>
          <group>Approver</group>
        </groups>
        <groups>
          <group>Admin-2</group>
          <group>Approver-2</group>
        </groups>
      </permission>
    </permissions>
  </asset>
</assets>

然而，在尝试反序列化时：

val generated = objectMapper.readValue<Assets>(xmlAssets)

我收到了以下错误提示：

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `uk.co.processflows.configuration.entities.Assets` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (StringReader); line: 2, column: 3]
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
    at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1451)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1027)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1290)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
    at com.fasterxml.jackson.dataformat.xml.deser.WrapperHandlingDeserializer.deserialize(WrapperHandlingDeserializer.java:113)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3011)
    at uk.co.processflows.module.EntryPoint.main(EntryPoint.kt:58)

任何帮助都将不胜感激。

- Wayne Hunter

---

你尝试过像错误提示中建议的那样添加一个默认构造函数吗？public Asset() {} - OldCurmudgeon

是的，我做了，问题还是一样。 - Wayne Hunter

Assets也是一样吗？抱歉 - 我刚注意到错误是关于Assets类的。 - OldCurmudgeon

是的，我已经为每个类型添加了默认构造函数。 - Wayne Hunter

谢谢，每种类型的默认值已解决了问题。 - Wayne Hunter

Answer 1

为了澄清答案：

我包括了

dependencies {
    ...
    compile(kotlin("reflect"))
    compile("com.fasterxml.jackson.module:jackson-module-kotlin:2.9.+")
    compile("com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.9.+")
    ...
}

在build.gradle.kts文件中，我遇到了同样的问题。通过向构造函数添加默认值来更改数据类似乎可以解决此问题。例如：

data class HelloWorld(val hello: Boolean, val world: Boolean) <- 出错

变成

data class HelloWorld(val hello: Boolean = false, val world: Boolean = false) <- 正常工作

然后：

const val xml = """
<HelloWorld>
    <hello>true</hello>
    <world>true</world>
</HelloWorld>
"""

使用Kotlin数据类，可以正常进行序列化和反序列化。谢谢！

- Brooks DuBois

Answer 2

现在有一个Jackson模块，可以将其反序列化为Kotlin数据类，而无需添加默认构造函数。

请参见https://github.com/FasterXML/jackson-module-kotlin。

compile "com.fasterxml.jackson.module:jackson-module-kotlin:2.9.+"

尝试将此代码添加到您的 Kotlin 项目中，以便您可以保持您的领域类“纯净”。

- hexkid