jaxb - 不可为 nillable 但为必需且为 null 值

Question

我刚和我的同事发现了一个关于 JAXB 的奇怪案例。

我用 @XmlElement(required = true) 声明了一个字段，默认为 nillable = false。

但我无意中试图编码一个值为 null 的实例。

而且我看到该元素只是被省略了。正常吗？这只是程序员的错吗？

import java.io.*;
import javax.xml.bind.*;
import javax.xml.bind.annotation.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;

@XmlAccessorType(XmlAccessType.NONE)
@XmlRootElement
public class Doughan {

    public static void main(final String[] args) throws JAXBException, IOException {

        final JAXBContext context =
            JAXBContext.newInstance(Doughan.class);

        final Marshaller marshaller = context.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);

        System.out.println("\n------------------------------------- not valid, is it?");
        marshaller.marshal(new Doughan(), System.out);

        System.out.println("\n------------------------------------------------- valid");
        marshaller.marshal(new Doughan("Blaise"), System.out);

        System.out.println("\n------------------------------------------------ schema");
        context.generateSchema(new SchemaOutputResolver(){
                @Override public Result createOutput(final String namespaceUri,
                                                     final String suggestedFileName)
                    throws IOException {
                    return new StreamResult(System.out) {
                        @Override public String getSystemId() {
                            return "Do I really have to do this?";
                        }
                    };
                }
            });
    }

    public Doughan() {
        this(null);
    }

    public Doughan(final String name) {
        super();
        this.name = name;
    }

    @XmlElement(required = true) // default; nillable = false
    private String name;

    @XmlElement(nillable = true, required = true)
    private String nill = null;
}

这是我得到的。

------------------------------------- not valid, is it?
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>

-------------------------------------- valid, isn't it?
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <name>Blaise</name>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>

------------------------------------------------ schema
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="doughan" type="doughan"/>
  <xs:complexType name="doughan">
    <xs:sequence>
      <xs:element name="name" type="xs:string"/>
      <xs:element name="nill" type="xs:string" nillable="true"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

xmllint

$ head -n100 doughan.xsd doughan1.xml doughan2.xml
==> doughan.xsd <==
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="doughan" type="doughan"/>
  <xs:complexType name="doughan">
    <xs:sequence>
      <xs:element name="name" type="xs:string"/>
      <xs:element name="nill" type="xs:string" nillable="true"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>
==> doughan1.xml <==
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>
==> doughan2.xml <==
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <name>Blaise</name>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>

$ xmllint --schema doughan.xsd doughan1.xml
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>
Element 'nill': This element is not expected. Expected is ( name ).
doughan1.xml fails to validate

$ xmllint --schema doughan.xsd doughan2.xml
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<doughan>
    <name>Blaise</name>
    <nill xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</doughan>
doughan2.xml validates

$

敬上。

Answer 1

是的，如果元素未使用 @XmlElement(nillable=true) 注释，则 XML 文档中将省略空值。

了解更多信息

• http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html

注意:我猜你是在用这个问题针对我 :)。