Java @XmlElement 和无用的“必需”参数
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@XmlElement and useless 'required' parameter
提问by
I put @XmlElement(name = "title",required = true)before the javabean property int some_property, and didn't assign a value to some_property. For some reason this property is not occurred in generated XML. So, please, explain the meaning of required
我将 @XmlElement(name = "title",required = true)放在 javabean 属性 int some_property 之前，并且没有为some_property 赋值。出于某种原因，此属性未出现在生成的 XML 中。所以，请解释required的含义
some meaningful parts of the code:
代码的一些有意义的部分：
@XmlRootElement(name = "book")
@XmlType(propOrder = { "author", "name", "publisher", "isbn" })
public class Book {
private String name;
private String author;
private String publisher;
private String isbn;
// If you like the variable name, e.g. "name", you can easily change this
// name for your XML-Output:
@XmlElement(name = "title",required = true)
public String getName() {
return name;
}
....
Somewhere int the Main:
某处主要的：
// create books
Book book1 = new Book();
book1.setIsbn("978-0060554736");
book1.setAuthor("Neil Strauss");
book1.setPublisher("Harpercollins");
bookList.add(book1);
Book book2 = new Book();
book2.setIsbn("978-3832180577");
book2.setName("Feuchtgebiete");
book2.setAuthor("Charlotte Roche");
book2.setPublisher("Dumont Buchverlag");
bookList.add(book2);
JAXBContext context = JAXBContext.newInstance(Bookstore.class);
Marshaller m = context.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
// Write to System.out
m.marshal(bookstore, System.out);
// Write to File
m.marshal(bookstore, new File(BOOKSTORE_XML));
// get variables from our xml file, created before
System.out.println();
System.out.println("Output from our XML File: ");
Unmarshaller um = context.createUnmarshaller();
Bookstore bookstore2 = (Bookstore) um.unmarshal(new FileReader(BOOKSTORE_XML));
ArrayList<Book> list = bookstore2.getBooksList();
采纳答案by bdoughan
What requiredDoes on @XmlElement
有什么required作用@XmlElement
The requiredproperty on the @XmlElementannotation affects the XML schema that is generated from Java classes.
在required对房地产@XmlElement注释影响从Java类生成的XML架构。
Domain Model (Root)
域模型（根）
Below is a simple Java model. Note how the barproperty has required=trueand the fooproperty does not.
下面是一个简单的 Java 模型。请注意该bar属性如何具有required=true和该foo属性没有。
import javax.xml.bind.annotation.*;
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
@XmlElement
private String foo;
@XmlElement(required=true)
private String bar;
@XmlElement(nillable=true)
private String baz;
}
Demo Code
演示代码
Below is some code that demonstrates how to generate an XML schema using the JAXBContext.
下面是一些演示如何使用 .xml 文件生成 XML 模式的代码JAXBContext。
import java.io.IOException;
import javax.xml.bind.*;
import javax.xml.transform.Result;
import javax.xml.transform.stream.StreamResult;
public class GenerateSchema {
public static void main(String[] args) throws Exception {
JAXBContext jc = JAXBContext.newInstance(Root.class);
jc.generateSchema(new SchemaOutputResolver() {
@Override
public Result createOutput(String namespaceUri,
String suggestedFileName) throws IOException {
StreamResult result = new StreamResult(System.out);
result.setSystemId(suggestedFileName);
return result;
}
});
}
}
Generated XML Schema
生成的 XML 模式
Below is the resulting XML schema note how the XML element corresponding to the foofield has minOccurs="0"while the XML element corresponding to the barfield (which was annotated with @XmlElement(required=true)does not. This is because the default minOccursis 1 meaning it's required.
下面是生成的 XML 模式注释，与该foo字段对应的 XML 元素如何，minOccurs="0"而与该bar字段对应的 XML 元素（注释为@XmlElement(required=true)没有。这是因为默认minOccurs值为 1 表示它是必需的。
<?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="root" type="root"/>
<xs:complexType name="root">
<xs:sequence>
<xs:element name="foo" type="xs:string" minOccurs="0"/>
<xs:element name="bar" type="xs:string"/>
<xs:element name="baz" type="xs:string" nillable="true" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
If You Want an Element for nullValues
如果您想要null值的元素
Domain Model (Root)
域模型（根）
The bazfield has been annotated with @XmlElement(nillable=true). If the value is null the resulting XML element will leverage the xsi:nilattribute. Without this annotation null values will be treated as absent nodes.
该baz字段已用 注释@XmlElement(nillable=true)。如果值为 null，则生成的 XML 元素将利用该xsi:nil属性。如果没有此注释，空值将被视为不存在的节点。
import javax.xml.bind.annotation.*;
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
@XmlElement
private String foo;
@XmlElement(required=true)
private String bar;
@XmlElement(nillable=true)
private String baz;
}
Demo Code
演示代码
import javax.xml.bind.*;
public class MarshalDemo {
public static void main(String[] args) throws Exception {
JAXBContext jc = JAXBContext.newInstance(Root.class);
Root root = new Root();
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(root, System.out);
}
}
Output
输出
Below is the resulting XML from running the demo code.
下面是运行演示代码的结果 XML。
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root>
<baz xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</root>
回答by michal
Could you show us a sample of your code and generated xml? According to docs:
您能否向我们展示您的代码示例和生成的 xml？根据文档：
If required is true, then Javabean property is mapped to an XML schema element declaration with minOccurs="1". maxOccurs is "1" for a single valued property and "unbounded" for a multivalued property.
如果 required 为 true，则 Javabean 属性将映射到 minOccurs="1" 的 XML 模式元素声明。maxOccurs 对于单值属性为“1”，对于多值属性为“unbounded”。
回答by Norw?
From: http://docs.oracle.com/javaee/5/api/javax/xml/bind/annotation/XmlElement.html
来自：http: //docs.oracle.com/javaee/5/api/javax/xml/bind/annotation/XmlElement.html
public abstract boolean required
Customize the element declaration to be required. If required() is true, then Javabean property is mapped to an XML schema element declaration with minOccurs="1". maxOccurs is "1" for a single valued property and "unbounded" for a multivalued property.
If required() is false, then the Javabean property is mapped to XML Schema element declaration with minOccurs="0". maxOccurs is "1" for a single valued property and "unbounded" for a multivalued property.
需要公共抽象布尔值
自定义需要的元素声明。如果 required() 为 true，则 Javabean 属性将映射到 minOccurs="1" 的 XML 模式元素声明。maxOccurs 对于单值属性为“1”，对于多值属性为“unbounded”。
如果 required() 为 false，则 Javabean 属性将映射到 minOccurs="0" 的 XML 模式元素声明。maxOccurs 对于单值属性为“1”，对于多值属性为“unbounded”。
Your property is mapped to an element that is (hopefully) declared as required in the schema. Your generated XML is not compliant to that particular schema, which is more or less what I expected from an instance that does not "play by the rules" it has laid out
您的属性映射到（希望）根据架构中的要求声明的元素。您生成的 XML 不符合该特定模式，这或多或少是我对不“遵守规则”的实例所期望的
