MOXy反序列化异常：在项目中找不到带有默认根元素的描述符

Question

这是我的课程：

@XmlRootElement(name="Zoo")
class Zoo {
    //@XmlElementRef
    public Collection<? extends Animal> animals;
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso({Bird.class, Cat.class, Dog.class})
@XmlDiscriminatorNode("@type")
abstract class Animal {
    @XmlElement
    public String name; 
}

@XmlDiscriminatorValue("Bird")
@XmlRootElement(name="Bird")
class Bird extends Animal {
    @XmlElement
    public String wingSpan;
    @XmlElement
    public String preferredFood;
}

@XmlDiscriminatorValue("Cat")
@XmlRootElement(name="Cat")
class Cat extends Animal {
    @XmlElement
    public String favoriteToy;
}

@XmlDiscriminatorValue("Dog")
@XmlRootElement(name="Dog")
class Dog extends Animal {
    @XmlElement
    public String breed;
    @XmlElement
    public String leashColor;
}

这是序列化的JSON：

   {
        "animals": [
            {
                "type": "Bird",
                "name": "bird-1",
                "wingSpan": "6 feets",
                "preferredFood": "food-1"
            },
            {
                "type": "Cat",
                "name": "cat-1",
                "favoriteToy": "toy-1"
            },
            {
                "type": "Dog",
                "name": "dog-1",
                "breed": "bread-1",
                "leashColor": "black"
            }
        ]
    }

这是反序列化器代码：

public static <T> T Deserialize_Moxy(String jsonStr, Class<?>[] cl) throws JAXBException {
    InputStream is = new ByteArrayInputStream(jsonStr.getBytes());
    JAXBContext jc = JAXBContext.newInstance(cl);         
    Unmarshaller unmarshaller = jc.createUnmarshaller();

    // Marshal to JSON
    unmarshaller.setProperty(MarshallerProperties.MEDIA_TYPE, "application/json");
    unmarshaller.setProperty(MarshallerProperties.JSON_INCLUDE_ROOT, false);
    @SuppressWarnings("unchecked")
    T obj = (T)unmarshaller.unmarshal(is);
    return obj;
}

这是例外：

Exception in thread "main" javax.xml.bind.UnmarshalException
 - with linked exception:
[Exception [EclipseLink-25008] (Eclipse Persistence Services - 2.4.1.v20121003-ad44345): org.eclipse.persistence.exceptions.XMLMarshalException
Exception Description: A descriptor with default root element  was not found in the project]
    at org.eclipse.persistence.jaxb.JAXBUnmarshaller.handleXMLMarshalException(JAXBUnmarshaller.java:1014)
    at org.eclipse.persistence.jaxb.JAXBUnmarshaller.unmarshal(JAXBUnmarshaller.java:147)
    at com.bp.samples.json.generics.Foo.Deserialize_Moxy(Foo.java:271)
    at com.bp.samples.json.generics.Foo.main(Foo.java:111)
Caused by: Exception [EclipseLink-25008] (Eclipse Persistence Services - 2.4.1.v20121003-ad44345): org.eclipse.persistence.exceptions.XMLMarshalException
Exception Description: A descriptor with default root element  was not found in the project
    at org.eclipse.persistence.exceptions.XMLMarshalException.noDescriptorWithMatchingRootElement(XMLMarshalException.java:143)
    at org.eclipse.persistence.internal.oxm.record.SAXUnmarshallerHandler.startElement(SAXUnmarshallerHandler.java:222)
    at org.eclipse.persistence.internal.oxm.record.json.JSONReader.parseRoot(JSONReader.java:161)
    at org.eclipse.persistence.internal.oxm.record.json.JSONReader.parse(JSONReader.java:118)
    at org.eclipse.persistence.internal.oxm.record.SAXUnmarshaller.unmarshal(SAXUnmarshaller.java:827)
    at org.eclipse.persistence.internal.oxm.record.SAXUnmarshaller.unmarshal(SAXUnmarshaller.java:350)
    at org.eclipse.persistence.internal.oxm.record.SAXUnmarshaller.unmarshal(SAXUnmarshaller.java:334)
    at org.eclipse.persistence.oxm.XMLUnmarshaller.unmarshal(XMLUnmarshaller.java:407)
    at org.eclipse.persistence.jaxb.JAXBUnmarshaller.unmarshal(JAXBUnmarshaller.java:133)
    ... 2 more

还有一个关于序列化JSON的问题：是否有一种方法可以使JSON序列化程序发布“ @type”而不是“
type”。当前，它看起来像具有属性“类型”的对象。如果我们可以用“ @”修饰它，那么显然这更多的是类型信息而不是属性。

谢谢，Behzad

Answer 1

以下是我对您的两个问题的回答：

问题＃1-例外

当使用该MarshallerProperties.JSON_INCLUDE_ROOT属性关闭根元素时，则需要使用一种unmarshal采用Class参数的方法来告诉MOXy您想要解组的对象的类型。

StreamSource json = new StreamSource("src/forum14246033/input.json");
Zoo zoo = unmarshaller.unmarshal(json, Zoo.class).getValue();

问题2

还有一个关于序列化JSON的问题：是否有一种方法可以使JSON序列化程序发布“ @type”而不是“
type”。当前，它看起来像具有属性“类型”的对象。如果我们可以用“ @”修饰它，那么显然这更多的是类型信息而不是属性。

该@前缀表示字段/属性映射为XML属性。您可以使用该JAXBContextProperties.JSON_ATTRIBUTE_PREFIX属性指定前缀以限定映射到XML属性的数据。

properties.put(JAXBContextProperties.JSON_ATTRIBUTE_PREFIX, "@");

完整的例子

演示版

package forum14246033;

import java.util.*;
import javax.xml.bind.*;
import javax.xml.transform.stream.StreamSource;
import org.eclipse.persistence.jaxb.JAXBContextProperties;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>(2);
        properties.put(JAXBContextProperties.MEDIA_TYPE, "application/json");
        properties.put(JAXBContextProperties.JSON_INCLUDE_ROOT, false);
        properties.put(JAXBContextProperties.JSON_ATTRIBUTE_PREFIX, "@");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {Zoo.class}, properties);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        StreamSource json = new StreamSource("src/forum14246033/input.json");
        Zoo zoo = unmarshaller.unmarshal(json, Zoo.class).getValue();

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(zoo, System.out);
    }

}

input.json /输出

{
   "animals" : [ {
      "@type" : "Bird",
      "name" : "bird-1",
      "wingSpan" : "6 feets",
      "preferredFood" : "food-1"
   }, {
      "@type" : "Cat",
      "name" : "cat-1",
      "favoriteToy" : "toy-1"
   }, {
      "@type" : "Dog",
      "name" : "dog-1",
      "breed" : "bread-1",
      "leashColor" : "black"
   } ]
}

领域模型

我不建议在域模型中使用公共字段，但是如果您这样做，则可以将元数据缩减为以下内容：

动物园

import java.util.Collection;

class Zoo {
    public Collection<? extends Animal> animals;
}

动物

import javax.xml.bind.annotation.XmlSeeAlso;
import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorNode;

@XmlSeeAlso({Bird.class, Cat.class, Dog.class})
@XmlDiscriminatorNode("@type")
abstract class Animal {

    public String name;

}

鸟

import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorValue;

@XmlDiscriminatorValue("Bird")
class Bird extends Animal {
    public String wingSpan;
    public String preferredFood;
}

jaxb.properties

要将MOXy指定为JAXB（JSR-222）提供程序，您需要jaxb.properties在与域模型相同的程序包中包含一个名为的文件，并包含以下条目：

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory