使用JAXB，如何将具有属性定义类型的XMLelement解组到基于该属性的对象？

Question

My application is receiving an object payload marshalled into XML, eg:

<targetedMessage>
   <sender>external application</sender>
   <payload class="class.path.from.external.application.Foo">
      <id>1</id>
   </payload>
</targetedMessage>

The payload class (Foo in example) can be one of a number of classes with a common inheritance structure.

Each has a corresponding class in my application:

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Foo extends Baz {
    private Long id;
    //other fields, getters, setters
}

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Bar extends Baz {
    private Long id;
    // other fields, getters, setters
}

and an enum can be used to find the local class of the object based on the foreign classpath:

public enum ForeignClass {

FOO("class.path.from.external.application.Foo", Foo.class),
BAR("class.path.from.external.application.Bar", Bar.class);

public static getClassFromForeignClassPath(String foreignClassPath) {
  // return class
}

I also have a TargetedMessage class to capture meta info regarding the message itself:

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class TargetedMessage {
    private String sender;
    private Baz baz;
}

What annotation would be required to unmarshal the XML properly? I've tried using an XmlAdapter on the Baz field in the TargetedMessage class, but the ValueType parameter is always null.

@XmlAttribute doesn't appear as though it could work either, as the value of the class attributes are not the same as their corresponding local class names.

Answer 1

You have at least two options. First, you can map the "class" attribute to an enumeration of strings and implement instantiation logic in the #getBaz() method of your TargetedMessage. The XSD would be as shown below (rename types as necessary):

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.example.org/XMLSchema" xmlns:tns="http://www.example.org/XMLSchema" elementFormDefault="qualified">

    <include schemaLocation=""></include>

    <simpleType name="simpleTypeClass">
        <restriction base="string">
            <enumeration value="class.path.from.external.application.Foo"></enumeration>
            <enumeration value="class.path.from.external.application.Bar"></enumeration>
        </restriction>
    </simpleType>

    <complexType name="payloadType">
        <sequence>
            <element name="id" type="string"></element>
        </sequence>
        <attribute name="class" type="tns:simpleTypeClass"></attribute>
    </complexType>

    <complexType name="targetedMessageType">
        <sequence>
            <element name="sender" type="string"></element>
            <element name="payload" type="tns:payloadType"></element>
        </sequence>
    </complexType>
</schema>

Second, you can transform your initial XML, so as the "payload" element is renamed according to its "class" attribute content. For example, it could be "payloadFoo", "payloadBar", etc. Then, you can map it directly to the desired classes. The respective XSD is shown below.

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.example.org/XMLSchema" xmlns:tns="http://www.example.org/XMLSchema" elementFormDefault="qualified">

    <simpleType name="simpleTypeFoo">
        <restriction base="string">
            <enumeration value="class.path.from.external.application.Foo"></enumeration>
        </restriction>
    </simpleType>

    <simpleType name="simpleTypeBar">
        <restriction base="string">
            <enumeration value="class.path.from.external.application.Bar"></enumeration>
        </restriction>
    </simpleType>

    <complexType name="payloadTypeFoo">
        <sequence>
            <element name="id" type="string"></element>
        </sequence>
        <attribute name="class" type="tns:simpleTypeFoo"></attribute>
    </complexType>

    <complexType name="payloadTypeBar">
        <sequence>
            <element name="id" type="string"></element>
        </sequence>
        <attribute name="class" type="tns:simpleTypeBar"></attribute>
    </complexType>

    <complexType name="payloadType">
        <choice>
            <element name="payloadFoo" type="tns:payloadTypeFoo" minOccurs="0" maxOccurs="1"></element>
            <element name="payloadBar" type="tns:payloadTypeBar" minOccurs="0" maxOccurs="1"></element>
        </choice>
    </complexType>

    <complexType name="targetedMessageType">
        <sequence>
            <element name="sender" type="string"></element>
            <element name="payload" type="tns:payloadType"></element>
        </sequence>
    </complexType>
</schema>