Java/JAXB：根据属性将 Xml 解组为特定子类
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/2992234/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Java/JAXB: Unmarshall Xml to specific subclass based on an attribute
提问by Frothy
Is it possible to use JAXB to unmarshall xml to a specific Java class based on an attribute of the xml?
是否可以使用 JAXB 根据 xml 的属性将 xml 解组为特定的 Java 类？
<shapes>
<shape type="square" points="4" square-specific-attribute="foo" />
<shape type="triangle" points="3" triangle-specific-attribute="bar" />
</shapes>
I would like to have a List of Shape objects containing a triangle and a square, each with their own shape-specific attribute. IE:
我想要一个包含三角形和正方形的 Shape 对象列表，每个对象都有自己的特定于形状的属性。IE：
abstract class Shape {
int points;
//...etc
}
class Square extends Shape {
String square-specific-attribute;
//...etc
}
class Triangle extends Shape {
String triangle-specific-attribute;
//...etc
}
I'm currently just putting all attributes in one big "Shape" class and it's less than ideal.
我目前只是将所有属性放在一个大的“形状”类中，这不太理想。
I could get this to work if the shapes were properly named xml elements, but unfortunately I don't have control of the xml I'm retrieving.
如果形状被正确命名为 xml 元素，我可以让它工作，但不幸的是我无法控制我正在检索的 xml。
Thanks!
谢谢！
采纳答案by bdoughan
JAXB is a spec, specific implementations will provide extension points to do things such as this. If you are using EclipseLink JAXB (MOXy)you could modify the Shape class as follows:
JAXB 是一个规范，具体的实现将提供扩展点来做这样的事情。如果您使用EclipseLink JAXB (MOXy)，您可以按如下方式修改 Shape 类：
import javax.xml.bind.annotation.XmlAttribute;
import org.eclipse.persistence.oxm.annotations.XmlCustomizer;
@XmlCustomizer(ShapeCustomizer.class)
public abstract class Shape {
int points;
@XmlAttribute
public int getPoints() {
return points;
}
public void setPoints(int points) {
this.points = points;
}
}
Then using the MOXy @XMLCustomizer you could access the InheritancePolicy and change the class indicator field from "@xsi:type" to just "type":
然后使用 MOXy @XMLCustomizer 您可以访问 InheritancePolicy 并将类指示符字段从“@xsi:type”更改为“type”：
import org.eclipse.persistence.config.DescriptorCustomizer;
import org.eclipse.persistence.descriptors.ClassDescriptor;
public class ShapeCustomizer implements DescriptorCustomizer {
@Override
public void customize(ClassDescriptor descriptor) throws Exception {
descriptor.getInheritancePolicy().setClassIndicatorFieldName("@type");
}
}
You will need to ensure that you have a jaxb.properties file in with you model classes (Shape, Square, etc) with the following entry specifying the EclipseLink MOXy JAXB implementation:
您需要确保在模型类（Shape、Square 等）中有一个 jaxb.properties 文件，并使用以下条目指定 EclipseLink MOXy JAXB 实现：
javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory
Below is the rest of the model classes:
以下是其余的模型类：
Shapes
形状
import java.util.ArrayList;
import java.util.List;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
public class Shapes {
private List<Shape> shape = new ArrayList<Shape>();;
public List<Shape> getShape() {
return shape;
}
public void setShape(List<Shape> shape) {
this.shape = shape;
}
}
Square
正方形
import javax.xml.bind.annotation.XmlAttribute;
public class Square extends Shape {
private String squareSpecificAttribute;
@XmlAttribute(name="square-specific-attribute")
public String getSquareSpecificAttribute() {
return squareSpecificAttribute;
}
public void setSquareSpecificAttribute(String s) {
this.squareSpecificAttribute = s;
}
}
Triangle
三角形
import javax.xml.bind.annotation.XmlAttribute;
public class Triangle extends Shape {
private String triangleSpecificAttribute;
@XmlAttribute(name="triangle-specific-attribute")
public String getTriangleSpecificAttribute() {
return triangleSpecificAttribute;
}
public void setTriangleSpecificAttribute(String t) {
this.triangleSpecificAttribute = t;
}
}
Below is a demo program to check that everything works:
下面是一个演示程序，用于检查一切是否正常：
import java.io.StringReader;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
public class Demo {
public static void main(String[] args) throws Exception {
JAXBContext jaxbContext = JAXBContext.newInstance(Shapes.class, Triangle.class, Square.class);
StringReader xml = new StringReader("<shapes><shape square-specific-attribute='square stuff' type='square'><points>4</points></shape><shape triangle-specific-attribute='triangle stuff' type='triangle'><points>3</points></shape></shapes>");
Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
Shapes root = (Shapes) unmarshaller.unmarshal(xml);
Marshaller marshaller = jaxbContext.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(root, System.out);
}
}
I hope this helps.
我希望这有帮助。
For more information on EclipseLink MOXy see:
有关 EclipseLink MOXy 的更多信息，请参阅：
EDIT
编辑
In EclipseLink 2.2 we're making this easier to configure, check out the following article for more information:
在 EclipseLink 2.2 中，我们使它更容易配置，查看以下文章了解更多信息：
回答by skaffman
No, I'm afraid that's not an option, JAXB isn't that flexible.
不，恐怕这不是一种选择，JAXB 不是那么灵活。
The best I can suggest is that you put a method on the Shapeclass which instantiates the "correct" type based on the attribute. The client code would invoke that factory method to obtain it.
我能建议的最好方法是在Shape类上放置一个方法，该方法根据属性实例化“正确”类型。客户端代码将调用该工厂方法来获取它。
Best I can come up with, sorry.
我能想到的最好的，对不起。
回答by Quotidian
AFAIK, you'll have to write an XmlAdapterwhich knows how to handle the marshal/unmarshalling of the Shape.
AFAIK，您必须编写一个XmlAdapter，它知道如何处理 Shape 的编组/解组。
回答by Barmak
The annotation @XmlElementsenables you to specify which tag corresponds with which subclass.
注释@XmlElements使您能够指定哪个标签对应于哪个子类。
@XmlElements({
@XmlElement(name="square", type=Square.class),
@XmlElement(name="triangle", type=Triangle.class)
})
public List<Shape> getShape() {
return shape;
}
Also see javadoc for @XmlElements
另请参阅@XmlElements 的javadoc
回答by Molis
There is @XmlSeeAlso annotation to tell to bind subclasses.
有 @XmlSeeAlso 注释告诉绑定子类。
For example, with the following class definitions:
例如，使用以下类定义：
class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}
The user would be required to create JAXBContext as JAXBContext.newInstance(Dog.class,Cat.class) (Animal will be automatically picked up since Dog and Cat refers to it.)
用户需要将 JAXBContext 创建为 JAXBContext.newInstance(Dog.class,Cat.class) （因为 Dog 和 Cat 引用了 Animal ，所以会自动拾取它。）
XmlSeeAlso annotation would allow you to write:
XmlSeeAlso 注释将允许您编写：
@XmlSeeAlso({Dog.class,Cat.class})
class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}
