eclipselink/Moxy:基于类型的继承和属性名称开发

Question

我正在使用MOXy的JAXB实现和外部元数据绑定文件来面对涉及继承和多态的编组/解组问题.

我无法控制XML文件或模型类.

模型中有多个继承其他DTO类的类.以下是我正在使用的环境示例.此示例仅用于某些语法目的,真实环境涉及嵌套继承,集合等.

这是将继承的类

  class A {

        private String name;

        public String getName(){
              return name;
        }

        public void setName(String value){
              name = value;
        }

  } 

这是一个继承的类

  class B extends A {

        private String attrFromB;

        public String getAttrFromB(){
              return attrFromB;
        }

        public void setAttrFromB(String value){
              attrFromB = value;
        }
  } 

而另一个

  class C extends A {

        private String attrFromC;

        public String getAttrFromC(){
              return attrFromC;
        }

        public void setAttrFromC(String value){
              attrFromC= value;
        }
  } 

这是一个容器类

  class MyContainerClass{

        private A myObject;

        public A getMyObject(){
           return myObject;
        }

        public void setMyObject(A value){
           myObject = value;
        }
  }

这是在包含A的MyContainer的情况下应该生成的XML

  <MyContainer>
        <MyObject nameA="foo" />
  </MyContainer>

MyContainer包含B

  <MyContainer>
        <MyObject nameB="foo" attrFromB="bar" />
  </MyContainer>

而MyContainer包含C.

  <MyContainer>
        <MyObject nameC="foo" attrFromC="bar" />
  </MyContainer>

所以你已经可以看到地平线上的问题......

这是我要写的映射文件:

  <?xml version="1.0"?>
     <xml-bindings 
        xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
        package-name="com.test.example"
        version="2.1">  

        <java-type name="A" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-element java-attribute="name" xml-path="@nameA" />
           </java-attributes>
        </java-type>  

        <java-type name="B" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <xml-see-also>
              com.test.example.A
           </xml.see.also>
           <java-attributes>
              <xml-element java-attribute="name" xml-path="@nameB" />
              <xml-element java-attribute="attrFromB" xml-path="@attrFromB" />
           </java-attributes>
        </java-type>

        <java-type name="C" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <xml-see-also>
              com.test.example.A
           </xml.see.also>
           <java-attributes>
              <xml-element java-attribute="name" xml-path="@nameC" />
              <xml-element java-attribute="attrFromC" xml-path="@attrFromC" />
           </java-attributes>
        </java-type>

        <java-type name="MyContainer" xml-accessor-type="NONE">
           <xml-root-element name="MyContainer" />
           <java-attributes>
              <xml-element java-attribute="myObject" type="com.test.example.A" xml-path="MyObject" />
           </java-attributes>
        </java-type>

     </xml-bindings>

---

第一个问题是,如果我绑定这样的类,我会得到以下异常:

  [Exception [EclipseLink-44] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.DescriptorException
  Exception Description: Missing class indicator field from database row [UnmarshalRecord()].

第一个问题:我理解这是正常的,Jaxb需要一些方法来确定MyContaioner.myObject属性的类型.问题是我无法访问传入的XML文件,所以我不能向它们添加xsi:type字段.有没有办法根据其中特定属性的存在来确定一个类？不管它的价值如何.如果源xml包含@attrFromC属性,我知道对象应该是C类型.如果它包含attrFromB,那么它是B.

---

第二个问题是B和C中不存在"name"属性,因此jaxb忽略了em.

  --Ignoring attribute [name] on class [com.test.example.B] as no Property was generated for it.
  --Ignoring attribute [name] on class [com.test.example.C] as no Property was generated for it.

第二个问题:另一个问题是我不知道Jaxb是否能够覆盖XML文件中的xml属性名称(@ nameA,@ nameB和nameC都指向A.name),有没有办法做它？

在此先感谢您的时间.

Answer 1

以下是您的问题的答案。问题2的答案，也是问题1的答案。

第一个问题：我知道这是正常的，Jaxb 需要某种方法来确定 MyContainer.myObject 属性的类型。问题是我无权访问传入的 XML 文件，因此无法向其中添加 xsi:type 字段。有没有办法根据类中是否存在特定属性来确定类？不管它的价值。如果源 xml 包含 @attrFromC 属性，我知道该对象应该是 C 类型。如果它包含 attrFromB，则它是 B 类型。

对于此用例，您可以利用EclipseLink JAXB (MOXy)ClassExtractor中的扩展：

我的类提取器

AClassExtractor是一些代码，您可以实现这些代码来帮助 MOXy 确定应该实例化哪个类。系统会向您传递 a Record，您可以通过 XPath 询问当前元素中是否存在属性，以确定应实例化哪个类。

package com.test.example;

import org.eclipse.persistence.descriptors.ClassExtractor;
import org.eclipse.persistence.sessions.*;

public class MyClassExtractor extends ClassExtractor{

    @Override
    public Class<?> extractClassFromRow(Record record, Session session) {
        if(null != record.get("@attrFromB")) {
            return B.class;
        } else if(null != record.get("@attrFromC")) {
            return C.class;
        } else {
            return A.class;
        }
    }

}

元数据 (oxm.xml)

您可以ClassExtractor使用@XmlClassExtractor注释进行配置。您还可以通过外部元数据文件来执行此操作。我已对您的问题中包含的内容进行了调整，以包括以下内容：

<?xml version="1.0"?>
<xml-bindings 
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="com.test.example"
    version="2.3">
    <java-types>
        <java-type name="A" xml-accessor-type="NONE">
           <xml-class-extractor class="com.test.example.MyClassExtractor"/>
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameA" />
           </java-attributes>
        </java-type>  
        <java-type name="B" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameB" />
              <xml-attribute java-attribute="attrFromB"/>
           </java-attributes>
        </java-type>
        <java-type name="C" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameC" />
              <xml-attribute java-attribute="attrFromC"/>
           </java-attributes>
        </java-type>
        <java-type name="MyContainerClass" xml-accessor-type="NONE">
           <xml-root-element name="MyContainer" />
           <java-attributes>
              <xml-element java-attribute="myObject" name="MyObject" />
           </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

演示

以下演示代码对您的问题中的每个 XML 文档进行解组，并输出该属性所持有的类型myObject：

package com.test.example;

import java.io.StringReader;
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextFactory;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "com/test/example/oxm.xml");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {MyContainerClass.class}, properties);
        Unmarshaller unmarshaller = jc.createUnmarshaller();

        StringReader aXml = new StringReader("<MyContainer><MyObject nameA='foo'/></MyContainer>");
        MyContainerClass myContainerA = (MyContainerClass) unmarshaller.unmarshal(aXml);
        System.out.println(myContainerA.getMyObject().getClass());

        StringReader bXml = new StringReader("<MyContainer><MyObject nameB='foo' attrFromB='bar'/></MyContainer>");
        MyContainerClass myContainerB = (MyContainerClass) unmarshaller.unmarshal(bXml);
        System.out.println(myContainerB.getMyObject().getClass());

        StringReader cXml = new StringReader("<MyContainer><MyObject nameC='foo' attrFromC='bar'/></MyContainer>");
        MyContainerClass myContainerC = (MyContainerClass) unmarshaller.unmarshal(cXml);
        System.out.println(myContainerC.getMyObject().getClass());
    }

}

输出

[EL Warning]: 2012-01-20 10:36:41.828--Ignoring attribute [name] on class [com.test.example.B] as no Property was generated for it.
[EL Warning]: 2012-01-20 10:36:41.828--Ignoring attribute [name] on class [com.test.example.C] as no Property was generated for it.
class com.test.example.A
class com.test.example.B
class com.test.example.C

第二个问题：另一个问题是我不知道 Jaxb 是否能够像 XML 文件中所期望的那样覆盖 xml 属性名称（@nameA、@nameB 和 nameC 均指 A.name），有没有办法做到它 ？

您可以利用 anXmlAdapter来回答这个问题。这种方法也可以用来回答你的第一个问题：

适配器

package com.test.example;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class AAdapter extends XmlAdapter<AAdapter.AdaptedA, A> {

    @Override
    public AdaptedA marshal(A a) throws Exception {
        if(null == a) {
            return null;
        }
        AdaptedA adaptedA = new AdaptedA();
        if(a instanceof C) {
            C c = (C) a;
            adaptedA.nameC = c.getName();
            adaptedA.attrFromC = c.getAttrFromC();
        } else if(a instanceof B) {
            B b = (B) a;
            adaptedA.nameB = b.getName();
            adaptedA.attrFromB = b.getAttrFromB();
        } else if(a instanceof A) {
            adaptedA.nameA = a.getName();
        }
        return adaptedA;
    }

    @Override
    public A unmarshal(AdaptedA adaptedA) throws Exception {
        if(null == adaptedA) {
            return null;
        }
        if(null != adaptedA.attrFromC) {
            C c = new C();
            c.setName(adaptedA.nameC);
            c.setAttrFromC(adaptedA.attrFromC);
            return c;
        } else if(null != adaptedA.attrFromB) {
            B b = new B();
            b.setName(adaptedA.nameB);
            b.setAttrFromB(adaptedA.attrFromB);
            return b;
        } 
        A a = new A();
        a.setName(adaptedA.nameA);
        return a;
    }

    public static class AdaptedA {
        @XmlAttribute public String nameA;
        @XmlAttribute public String nameB;
        @XmlAttribute public String nameC;
        @XmlAttribute public String attrFromB;
        @XmlAttribute public String attrFromC;
    }

}

元数据 (oxm-2.xml)

<?xml version="1.0"?>
<xml-bindings 
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="com.test.example"
    version="2.3">
    <java-types>
        <java-type name="MyContainerClass" xml-accessor-type="NONE">
           <xml-root-element name="MyContainer" />
           <java-attributes>
              <xml-element java-attribute="myObject" name="MyObject">
                <xml-java-type-adapter value="com.test.example.AAdapter"/>
              </xml-element>
           </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

演示2

package com.test.example;

import java.io.StringReader;
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextFactory;

public class Demo2 {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "com/test/example/oxm-2.xml");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {MyContainerClass.class}, properties);
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        StringReader aXml = new StringReader("<MyContainer><MyObject nameA='foo'/></MyContainer>");
        MyContainerClass myContainerA = (MyContainerClass) unmarshaller.unmarshal(aXml);
        System.out.println(myContainerA.getMyObject().getClass());
        marshaller.marshal(myContainerA, System.out);

        StringReader bXml = new StringReader("<MyContainer><MyObject nameB='foo' attrFromB='bar'/></MyContainer>");
        MyContainerClass myContainerB = (MyContainerClass) unmarshaller.unmarshal(bXml);
        System.out.println(myContainerB.getMyObject().getClass());
        marshaller.marshal(myContainerB, System.out);

        StringReader cXml = new StringReader("<MyContainer><MyObject nameC='foo' attrFromC='bar'/></MyContainer>");
        MyContainerClass myContainerC = (MyContainerClass) unmarshaller.unmarshal(cXml);
        System.out.println(myContainerC.getMyObject().getClass());
        marshaller.marshal(myContainerC, System.out);
    }

}

输出

class com.test.example.A
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameA="foo"/>
</MyContainer>
class com.test.example.B
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameB="foo" attrFromB="bar"/>
</MyContainer>
class com.test.example.C
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameC="foo" attrFromC="bar"/>
</MyContainer>