如何阅读带有 java 注释的 xsi:type

Question

我想将基于 jaxb 的 xml-file 读入我的面向对象结构。

假设这是我的 xml-file:

    <?xml version="1.0" encoding="utf-8" standalone="yes"?>
    <children xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
        <child xsi:type="girl">
            <age>12</age>
            <isdancing>true</isdancing>
        </child>
        <child xsi:type="boy">
            <age>10</age>
            <issoccerplayer>true</issoccerplayer>
        </child>
    </children>

children 是某种包含多个 child 元素的包装器元素。 child可以是xsi:type指定的男孩或女孩。这两个 classes 有一些共同的元素（如 age）和一些不同的（不包括）元素（如 isdancing 或 是足球运动员)

要读取文件，我有这个方法：

    public static void main( String[] args ) throws JAXBException
    {
        JAXBContext jaxbContext;
        jaxbContext = JAXBContext.newInstance(Children.class);             
        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
        File file = new File("C:/test.xml");
        if (!file.exists()) System.out.println("File does not exist");

        Children children = (Children) jaxbUnmarshaller.unmarshal(file);
        System.out.println(children.toString());
    }

我的 Children class 看起来像这样：

    @XmlRootElement(name="children")
    @XmlAccessorType(XmlAccessType.FIELD)
    public class Children {

        @XmlElement(name="child")
        private List<Child> childrenList;

        public List<Child> getChildren() { return childrenList; }
        public void setChildren(List<Child> children) {this.childrenList = children;}

    @Override
        public String toString() {
            return ReflectionToStringBuilder.toString(this);
        }
    }

我的 Child class 看起来像这样：

    @XmlAccessorType(XmlAccessType.FIELD)
    public class Child {

    @XmlAttribute(name="xsi:type")
    private XsiType xsiType;

    private int age;

    @XmlElement(name = "isdancing")
    private boolean isDancing;

    @XmlElement(name = "issoccerplayer")
    private boolean isSoccerPlayer;

        //Getter and setter for all fields

    @Override
    public String toString() {
        return ReflectionToStringBuilder.toString(this);
        }
    }

我的 XsiType class 看起来像这样：

    @XmlAccessorType(XmlAccessType.FIELD)
    public class XsiType {

        @XmlAttribute(name="xsi:type")
        private String name;

        @XmlValue
        private String value;

        public String getName() { return name; }
        public void setName(String name) { this.name = name; }
        public String getValue() { return value; 
        public void setValue(String value) { this.value = value; }
    }

在我的 pom.xml 中，我包含了以下依赖项：

    <dependencies>
        <dependency>
            <groupId>javax.xml.bind</groupId>
            <artifactId>jaxb-api</artifactId>
            <version>2.3.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.9</version>
        </dependency>
    </dependencies>

我现在的问题是，输出正常，但是 Child-class 的元素 xsiType 始终为 null，否则它会以 IllegalAnnotationExceptions 结束，这与 XmlTest.model.Child.xsiType

所以我预计设置任何类型的@Xml-Annotation 都会出错。有人可以帮我找出错误吗？

目标是迭代 children 的列表并在运行时（基于 xsiType）决定这是女孩还是男孩。

谢谢

Answer 1

您不需要 XsiType class。您可以只使用 String 代替。

在你的Childclass xsiType 属性应如下所示。

@XmlAttribute(name = "type", namespace = "http://www.w3.org/2001/XMLSchema-instance")
private String xsiType;

注意：在@XmlAttribute注释中

使用name = "type"（不带前缀xsi:）
指定 namespace 参数，如您的 XML 通过 xmlns:xsi="..."

顺便说一句：
而不是键入字符串 "http://www.w3.org/2001/XMLSchema-instance" 你最好使用常量 XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI。所以你的改进代码会像这样：

@XmlAttribute(name = "type", namespace = XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI)
private String xsiType;

Answer 2

xsi类型通常用于表示对具体类型的引用。 Jaxb 可以使用 xsi 类型而无需进一步的解决方法。

创建 Boy 和扩展 Children 的 Girl class。（您可能需要使用 @XmlType 调整类型名称）。这样，所有带有 xsi:type=Girl 的元素都将绑定到 class Girl

@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso({ Boy.class, Girl.class }) // Either use @XmlSeeAlso to register classes in the JaxbContext
                                       //  or add them to the context directly
public class Child {

    private int age;

    @XmlElement(name = "isdancing")
    private boolean isDancing;

    @XmlElement(name = "issoccerplayer")
    private boolean isSoccerPlayer;

    // Getter and setter for all fields

}

@XmlType(name = "boy") // can be omitted if default value matches with the default value
public class Boy extends Child {

}

@XmlType(name = "girl")
public class Girl extends Child {

}

完整的独立示例：

package jaxb;

import java.io.File;
import java.io.StringReader;
import java.util.List;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSeeAlso;
import javax.xml.bind.annotation.XmlType;

public class Inheritance {

    public static void main(String[] args) throws JAXBException {
        JAXBContext jaxbContext;
        jaxbContext = JAXBContext.newInstance(Children.class);
        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

        String x = "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\"?>\r\n"
                + "    <children xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\">\r\n"
                + "        <child xsi:type=\"girl\">\r\n" + "            <age>12</age>\r\n"
                + "            <isdancing>true</isdancing>\r\n" + "        </child>\r\n"
                + "        <child xsi:type=\"boy\">\r\n" + "            <age>10</age>\r\n"
                + "            <issoccerplayer>true</issoccerplayer>\r\n" + "        </child>\r\n" + "    </children>";

        Children children = (Children) jaxbUnmarshaller.unmarshal(new StringReader(x));
        System.out.println(children.getChildren().toString());
    }

    @XmlRootElement(name = "children")
    @XmlAccessorType(XmlAccessType.FIELD)
    public static class Children {

        @XmlElement(name = "child")
        private List<Child> childrenList;

        public List<Child> getChildren() {
            return childrenList;
        }

        public void setChildren(List<Child> children) {
            this.childrenList = children;
        }

    }

    @XmlAccessorType(XmlAccessType.FIELD)
    @XmlSeeAlso({ Boy.class, Girl.class })
    public static class Child {

        private int age;

        @XmlElement(name = "isdancing")
        private boolean isDancing;

        @XmlElement(name = "issoccerplayer")
        private boolean isSoccerPlayer;

        // Getter and setter for all fields

    }

    @XmlType(name = "boy")
    public static class Boy extends Child {

    }

    @XmlType(name = "girl")
    public static class Girl extends Child {

    }
}

Answer 3

第二种方法的干净解决方案（基于单独的 class 文件）：

public class App
{
    public static void main(String[] args) throws JAXBException
    {
        JAXBContext jaxbContext = JAXBContext.newInstance(Children.class);             
        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
        File file = new File("C:/test2.xml");
        Children children = (Children) jaxbUnmarshaller.unmarshal(file);

        for (Child c : children.getChildren()) {
            if (c instanceof Boy) {
                System.out.println(((Boy)c).toString());
            } else if (c instanceof Girl){
                System.out.println(((Girl)c).toString());
            }
        }
    }
}

Children.java

@XmlRootElement(name="children")
@XmlAccessorType(XmlAccessType.FIELD)
public class Children {

    @XmlElement(name="child")
    private List<Child> childrenList;

    public List<Child> getChildren() { return childrenList; }
    public void setChildren(List<Child> children) {this.childrenList = children;}

    @Override
    public String toString() { return ReflectionToStringBuilder.toString(this); }
}

Boy.java

@XmlType(name="boy")
public class Boy extends Child {

    @XmlElement(name = "issoccerplayer")
    private boolean isSoccerPlayer;

    public boolean isSoccerPlayer() { return isSoccerPlayer; }
    public void setSoccerPlayer(boolean isSoccerPlayer) { this.isSoccerPlayer = isSoccerPlayer; }

    @Override
    public String toString() { return ReflectionToStringBuilder.toString(this); }
}

Girl.java

@XmlType(name="girl")
public class Girl extends Child {

    @XmlElement(name = "isdancing")
    private boolean isDancing;

    public boolean isDancing() { return isDancing; }
    public void setDancing(boolean isDancing) { this.isDancing = isDancing; }

    @Override
    public String toString() { return ReflectionToStringBuilder.toString(this); }
}

Child.java

@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso({ Boy.class, Girl.class }) 
public abstract class Child {

    private int age;

    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}

输出应该是：

de.home.myproject.XmlTest.model.Girl@12edcd21[isDancing=true,age=12]
de.home.myproject.XmlTest.model.Boy@27bc2616[isSoccerPlayer=true,age=10]

如何阅读带有 java 注释的 xsi:type

How to read xsi:type with java annotations

java

xml

jaxb

xml-namespaces