如何在解组期间使用 JAXB XmlAdapter 处理 XML IDREF 的前向引用？

Question

是否可以在解组过程中处理 JAXB XmlAdapter 中 XML IDREF 元素的前向引用？例如，我有以下 XML complexType：

<xs:complexType name="person">
    <xs:complexContent>
        <xs:sequence>
            <xs:element name="dateOfBirth" type="xs:dateTime" minOccurs="0"/>
            <xs:element name="firstName" type="xs:string" minOccurs="0"/>
            <xs:element name="gender" type="xs:string" minOccurs="0"/>
            <xs:element name="guardian" type="xs:IDREF" minOccurs="0"/>
            <xs:element name="homePhone" type="xs:string" minOccurs="0"/>
            <xs:element name="lastName" type="xs:string" minOccurs="0"/>
        </xs:sequence>
    </xs:complexContent>
</xs:complexType>

guardian 字段可以引用文档中其他位置的另一个Person-type 元素。我目前在编组时使用 XmlAdapter，以便第一次编组对象时，它由包含编组，并且该对象的任何后续出现都通过引用进行编组。请参阅mine 的上一个问题。但是，由于我的 XML 实例文档的创建方式，Person 元素的第一次出现可能在 IDREF 出现之后发生。

这是可能的吗？还是我需要以不同的方式处理这个问题？谢谢！

Answer 1

我对您的related question 有一个回答我概述了如何使用XmlAdapter 来实现对象的第一次出现通过包含/嵌套编组并且所有其他事件通过引用编组的用例：

• Can JAXB marshal by containment at first then marshal by @XmlIDREF for subsequent references?

选项 #1 - @XmlID/@XmlIDREF

如果您的所有Person 对象都通过嵌套表示，并且您想引入一些基于键的关系，那么您最好使用@XmlID 将字段/属性标记为键，并使用@XmlID 进行映射字段/属性作为外键。你的 Person 类看起来像：

@XmlAccessorType(XmlAccessType.FIELD)
public class Person {

    @XmlID
    private String id;

    @XmlIDREF
    private Person guardian;
}

更多信息

• http://blog.bdoughan.com/2010/10/jaxb-and-shared-references-xmlid-and.html

选项 #2 - 使用 XmlAdapter

如果您将我的previous answer 中的XmlAdapter 更新为：

package forum7587095;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

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

public class PhoneNumberAdapter extends XmlAdapter<PhoneNumberAdapter.AdaptedPhoneNumber, PhoneNumber>{

    private List<PhoneNumber> phoneNumberList = new ArrayList<PhoneNumber>();
    private Map<String, PhoneNumber> phoneNumberMap = new HashMap<String, PhoneNumber>();

    @XmlSeeAlso(AdaptedWorkPhoneNumber.class)
    @XmlType(name="phone-number")
    public static class AdaptedPhoneNumber {
        @XmlAttribute public String id;
        public String number;

        public AdaptedPhoneNumber() {
        }

        public AdaptedPhoneNumber(PhoneNumber phoneNumber) {
            id = phoneNumber.getId();
            number = phoneNumber.getNumber();
        }

        public PhoneNumber getPhoneNumber() {
            PhoneNumber phoneNumber = new PhoneNumber();
            phoneNumber.setId(id);
            phoneNumber.setNumber(number);
            return phoneNumber;
        }

    }

    @XmlType(name="work-phone-number")
    public static class AdaptedWorkPhoneNumber extends AdaptedPhoneNumber {

        public String extension;

        public AdaptedWorkPhoneNumber() {
        }

        public AdaptedWorkPhoneNumber(WorkPhoneNumber workPhoneNumber) {
            super(workPhoneNumber);
            extension = workPhoneNumber.getExtension();
        }

        @Override
        public WorkPhoneNumber getPhoneNumber() {
            WorkPhoneNumber phoneNumber = new WorkPhoneNumber();
            phoneNumber.setId(id);
            phoneNumber.setNumber(number);
            phoneNumber.setExtension(extension);
            return phoneNumber;
        }
}

    @Override
    public AdaptedPhoneNumber marshal(PhoneNumber phoneNumber) throws Exception {
        AdaptedPhoneNumber adaptedPhoneNumber;
        if(phoneNumberList.contains(phoneNumber)) {
            if(phoneNumber instanceof WorkPhoneNumber) {
                adaptedPhoneNumber = new AdaptedWorkPhoneNumber();
            } else {
                adaptedPhoneNumber = new AdaptedPhoneNumber();
            }
            adaptedPhoneNumber.id = phoneNumber.getId();
        } else {
            if(phoneNumber instanceof WorkPhoneNumber) {
                adaptedPhoneNumber = new AdaptedWorkPhoneNumber((WorkPhoneNumber)phoneNumber);
            } else {
                adaptedPhoneNumber = new AdaptedPhoneNumber(phoneNumber);
            }
            phoneNumberList.add(phoneNumber);
        }
        return adaptedPhoneNumber;
    }

    @Override
    public PhoneNumber unmarshal(AdaptedPhoneNumber adaptedPhoneNumber) throws Exception {
        PhoneNumber phoneNumber = phoneNumberMap.get(adaptedPhoneNumber.id);
        if(null != phoneNumber) {
            if(adaptedPhoneNumber.number != null) {
                phoneNumber.setNumber(adaptedPhoneNumber.number);
            }
            return phoneNumber;
        }
        phoneNumber = adaptedPhoneNumber.getPhoneNumber();
        phoneNumberMap.put(phoneNumber.getId(), phoneNumber);
        return phoneNumber;
    }

}

然后，您将能够解组如下所示的 XML 文档，其中首先出现引用：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <phone-number id="A"/>
    <phone-number id="B">
        <number>555-BBBB</number>
    </phone-number>
    <phone-number id="A">
        <number>555-AAAA</number>
    </phone-number>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W">
        <number>555-WORK</number>
        <extension>1234</extension>
    </phone-number>
    <phone-number xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="work-phone-number" id="W"/>
</customer>