Java JAXB 空元素解组
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JAXB empty element unmarshalling
提问by ikos23
The problem is in the following :
问题在于：
I get the soap response with empty element inside (e.g. ... <someDate /> ...)
and as a result exception is being throwed when JAXB wants to parse this element
instead to set the appropriate field with nullvalue.
我得到里面有空元素的soap响应（例如... <someDate /> ...），结果当JAXB想要解析这个元素而不是用null值设置适当的字段时抛出异常。
How to configure JAXB to treat empty elements as null ? Can we do this with JAXB only (not using some third-party workarounds)
如何配置 JAXB 以将空元素视为 null ？我们可以仅使用 JAXB 来执行此操作吗（不使用某些第三方解决方法）
采纳答案by bdoughan
Base Problem
基本问题
Empty Stringis not a valid value for the xsd:datetype. To be valid with the XML schema an optional element should be represented as an absent node.,
EmptyString不是该xsd:date类型的有效值。要对 XML 模式有效，可选元素应表示为不存在的节点。，
Why the Base Problem is Impacting You
为什么基本问题会影响你
All JAXB implementations will recognize that empty Stringis not a valid value for xsd:date. They do this by reporting it to an instance of ValidationEventHandler. You can see this yourself by doing the following:
所有 JAXB 实现都会认识到 emptyString不是 的有效值xsd:date。他们通过将其报告给ValidationEventHandler. 您可以通过执行以下操作自行查看：
Unmarshaller unmarshaller = jc.createUnmarshaller();
unmarshaller.setEventHandler(new ValidationEventHandler() {
@Override
public boolean handleEvent(ValidationEvent event) {
System.out.println(event);
return true;
}
});
The implementation of JAX-WS you are using, leverages EclipseLink MOXyas the JAXB provider. And in the version you are using MOXy will by default throw an exception when a ValidationEventof severity ERRORis encountered instead of FATAL_ERRORlike the reference implementation. This has since been fixed in the following bug:
您正在使用的 JAX-WS 实现利用EclipseLink MOXy作为 JAXB 提供者。在您使用的版本中，MOXy 默认会在遇到ValidationEvent严重性时抛出异常，ERROR而不是FATAL_ERROR像参考实现那样。此问题已在以下错误中得到修复：
Work Around
解决
If you are using the JAXB APIs directly you could simply override the default ValidationEventHandler. In a JAX-WS environment a XmlAdaptercan be used to provide custom conversion logic. We will leverage an XmlAdapterto override how the conversion to/from Dateis handled.
如果您直接使用 JAXB API，您可以简单地覆盖默认的ValidationEventHandler. 在 JAX-WS 环境中，aXmlAdapter可用于提供自定义转换逻辑。我们将利用 anXmlAdapter来覆盖转换到/从Date的处理方式。
XmlAdapter (DateAdapter)
XmlAdapter (DateAdapter)
import java.text.SimpleDateFormat;
import java.util.Date;
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class DateAdapter extends XmlAdapter<String, Date>{
private SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
@Override
public Date unmarshal(String v) throws Exception {
if(v.length() == 0) {
return null;
}
return dateFormat.parse(v);
}
@Override
public String marshal(Date v) throws Exception {
if(null == v) {
return null;
}
return dateFormat.format(v);
}
}
Java Model (Root)
Java 模型（根）
The XmlAdapteris referenced using the @XmlJavaTypeAdapterannotation. If you wish this XmlAdapterto apply to all instances of Dateyou can register it at the package level (see: http://blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html).
在XmlAdapter使用引用的@XmlJavaTypeAdapter注释。如果您希望这XmlAdapter适用于Date您可以在包级别注册它的所有实例（请参阅：http: //blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html）。
import java.util.Date;
import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
@XmlSchemaType(name = "date")
@XmlJavaTypeAdapter(value=DateAdapter.class, type=Date.class)
private Date abc;
@XmlSchemaType(name="date")
@XmlJavaTypeAdapter(value=DateAdapter.class, type=Date.class)
private Date qwe;
}
Demo Code
演示代码
Below is a standalone example you can run to see that everything works.
下面是一个独立的示例，您可以运行以查看一切正常。
jaxb.properties
jaxb.properties
In a standalone example to use MOXy as your JAXB provider you need to include a file called jaxb.propetiesin the same package as your domain model with the following entry (see: http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html).
在使用 MOXy 作为 JAXB 提供程序的独立示例中，您需要包含一个jaxb.propeties在与域模型相同的包中调用的文件，其中包含以下条目（请参阅：http: //blog.bdoughan.com/2011/05/specifying-eclipselink -moxy-as-your.html）。
javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory
input.xml
输入文件
<?xml version="1.0" encoding="UTF-8"?>
<root>
<abc></abc>
<qwe>2013-09-05</qwe>
</root>
Demo
演示
import java.io.File;
import javax.xml.bind.*;
public class Demo {
public static void main(String[] args) throws Exception {
JAXBContext jc = JAXBContext.newInstance(Root.class);
Unmarshaller unmarshaller = jc.createUnmarshaller();
File xml = new File("src/forum18617998/input.xml");
Root root = (Root) unmarshaller.unmarshal(xml);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(root, System.out);
}
}
Output
输出
Note that in the marshalled XML the Datefield that was null was marshalled as an absent element (see: http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html).
请注意，在编组的 XML 中Date，空字段被编组为不存在的元素（请参阅：http: //blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html）。
<?xml version="1.0" encoding="UTF-8"?>
<root>
<qwe>2013-09-05</qwe>
</root>
