如何在 JAXB 封送期间添加 XML 处理指令

Question

我想在序列化集合/数组属性时添加处理指令，以获得类似

<alice>
  <? array bob ?>
  <bob>edgar</bob>
  <bob>david</bob>
</alice>

JAXB 这可能吗？或者至少有一些特定的 JAXB 实现？

Answer 1

您可以利用 XMLStreamWriter 和 XmlAdapter 来执行此操作：

BobAdapter

关于 XmlAdapter 的注意事项：

• 它是有状态的并引用 XMLStreamWriter。稍后我们将利用 JAXB 的功能在 Marshaller 上设置有状态 XmlAdapter。
• 它从 List 转换为 List，我们在这里只是使用 XmlAdapter 来获取事件。
• marshal 方法是我们在 XMLStreamWriter 上调用 writeProcessingInstruction 的地方

：

package forum6931520;

import java.util.List;

import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.stream.XMLStreamWriter;

public class BobAdapter extends XmlAdapter<List<String>, List<String>> {

    private boolean first = true;
    private XMLStreamWriter xmlStreamWriter;

    public BobAdapter() {
    }

    public BobAdapter(XMLStreamWriter xmlStreamWriter) {
        this();
        this.xmlStreamWriter = xmlStreamWriter;
    }

    @Override
    public List<String> marshal(List<String> stringList) throws Exception {
        if(first) {
            xmlStreamWriter.writeProcessingInstruction("array", "bob");
            first = false;
        }
        return stringList;
    }

    @Override
    public List<String> unmarshal(List<String> stringList) throws Exception {
        return stringList;
    }

}

Alice

@XmlJavaTypeAdapter 注释用于将 XmlAdapter 与字段/属性链接：

package forum6931520;

import java.util.List;

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
public class Alice {

    private List<String> bob;

    @XmlJavaTypeAdapter(BobAdapter.class)
    public List<String> getBob() {
        return bob;
    }

    public void setBob(List<String> bob) {
        this.bob = bob;
    }

}

Demo

package forum6931520;

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamWriter;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Alice.class);

        File xml = new File("src/forum6931520/input.xml");
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Alice alice = (Alice) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        XMLOutputFactory xof = XMLOutputFactory.newFactory();
        XMLStreamWriter xmlStreamWriter = xof.createXMLStreamWriter(System.out);
        marshaller.setAdapter(new BobAdapter(xmlStreamWriter));
        marshaller.marshal(alice, xmlStreamWriter);
    }

}

input .xml

<?xml version="1.0" encoding="UTF-8"?>
<alice>
  <?array bob?>
  <bob>edgar</bob>
  <bob>david</bob>
</alice>

输出

<?xml version='1.0' encoding='UTF-8'?><alice><?array bob?><bob>edgar</bob><bob>david</bob></alice>

注意

此示例需要使用 EclipseLink JAXB (MOXy)，因为 JAXB RI 将抛出以下异常（我是 MOXy 的负责人）：

Exception in thread "main" com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 2 counts of IllegalAnnotationExceptions
java.util.List is an interface, and JAXB can't handle interfaces.
    this problem is related to the following location:
        at java.util.List
        at public java.util.List forum6931520.Alice.getBob()
        at forum6931520.Alice
java.util.List does not have a no-arg default constructor.
    this problem is related to the following location:
        at java.util.List
        at public java.util.List forum6931520.Alice.getBob()
        at forum6931520.Alice

了解更多信息

• http://blog.bdoughan.com/2010/07/xmladapter-jaxbs-secret-weapon.html
• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html

---

更新

我已输入增强请求 ( https://bugs.eclipse.org/354286) 添加对处理指令的本机支持。这最终将允许您在您的财产上指定以下内容：

@XmlProcessingInstruction(target="array", value="bob")
public List<String> getBob() {
    return bob;
}