Java 如何使用 JAXB 实例化一个空元素
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How to instantiate an empty element with JAXB
提问by doekman
I use JAXB to create XML messages. The XML I need to create is (for the sake of simplicity):
我使用 JAXB 来创建 XML 消息。我需要创建的 XML 是（为了简单起见）：
<request>
<header/>
</request>
My code looks like this:
我的代码如下所示：
import javax.xml.bind.annotation.*;
@XmlRootElement(name = "request")
public class Request {
private String header;
@XmlElement(required=true)
public String getHeader() {
return header;
}
public void setHeader(String header) {
this.header=header;
}
}
The problem: the headerelement is not displayed (header is null). When header is set to an empty string, the following is displayed:
问题：header元素未显示（标题为null）。当 header 设置为空字符串时，将显示以下内容：
<request>
<header></header>
</request>
When I use as type Objectinstead of String, the result is even worse:
当我使用 as 类型Object而不是 时String，结果更糟：
<request>
<header xsi:type="xs:string"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"></header>
</request>
BTW: I'm using this codeto create the XML string.
顺便说一句：我正在使用此代码来创建 XML 字符串。
Is it possible to get an empty tag?
是否有可能获得一个空标签？
采纳答案by Tom Hawtin - tackline
In XML, <header/>and <header></header>are the same thing. If you really want the former, then use a prettifier. javax.xml.transform.TransformerFactory.newTransformer()will probably do that for you.
在 XML 中，<header/>和<header></header>是一回事。如果您真的想要前者，请使用美化剂。javax.xml.transform.TransformerFactory.newTransformer()可能会为你做到这一点。
回答by Chris Dail
An empty tag for a String object is essentially the empty string.
String 对象的空标签本质上是空字符串。
If you call the following, you will get what you are looking for:
如果您拨打以下电话，您将得到您要找的东西：
request.setHeader("")
I should also note that in XML the following two declarations of a header are idential. Both of these have no child text nodes. These are essentially the same and will be treated the same by all XML parsers:
我还应该注意，在 XML 中，标题的以下两个声明是相同的。这两个都没有子文本节点。这些本质上是相同的，并且所有 XML 解析器都将对其进行相同的处理：
<header></header>
<header/>
回答by Jin Kwon
As @Tom Hawtin - tackline said
正如@Tom Hawtin-tackline 所说
<header/>and <header></header>is same. Parsers will give you "".
<header/>并且<header></header>是一样的。解析器会给你“”。
You have to to put nillableon your header annotation
你必须把nillable你的标题注释
@XmlElement(nillable=true, required=true)
public String getHeader() {
return header;
}
I hope this code will generate following XML for nullvalue.
我希望这段代码将生成以下 XMLnull值。
import javax.xml.bind.*;
import javax.xml.bind.annotation.*;
@XmlRootElement
public class Request {
public static void main(String[] args) throws JAXBException {
final Request request = new Request();
final JAXBContext context = JAXBContext.newInstance(Request.class);
final Marshaller marshaller = context.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT,
Boolean.TRUE);
marshaller.marshal(request, System.out);
System.out.flush();
}
@XmlElement(nillable=true, required=true)
private String header;
}
prints
印刷
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<request>
<header xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</request>
回答by Mark D
I wanted the same thing, effectively <header/>rather than <header></header>during the xml serialization process.
我想要同样的东西，有效地<header/>而不是<header></header>在 xml 序列化过程中。
Since a null value - rather than an empty string - will produce the correct result, I modified my setter method to set the value explicitly to null:
由于空值 - 而不是空字符串 - 会产生正确的结果，我修改了我的 setter 方法以将值显式设置为空：
public void setHeader(String header) {
this.header = "".equals(header) ? null : header;
}
