问题描述

在我们的应用程序中有一个相当普遍的模式.我们在 Xml 中配置一组(或列表)对象，它们都实现了一个通用接口.在启动时，应用程序读取 Xml 并使用 JAXB 创建/配置对象列表.我从来没有想过(在多次阅读各种帖子之后)只使用 JAXB 来做到这一点的正确方法".

There is a fairly common pattern in our applications. We configure a configure a set (or list) of objects in Xml, which all implement a common interface. On start-up, the application reads the Xml and uses JAXB to create/configure a List of objects. I have never figured out (after reading various posts many times) the "right way" to do this using only JAXB.

例如，我们有一个接口Fee，以及多个具体的实现类，它们有一些共同的属性，也有一些不同的属性，以及非常不同的行为.我们用来配置应用程序使用的费用列表的 XML 是:

For example, we have an interface Fee, and multiple concrete implementing classes which have some common properties, as well as some diverging properties, and very different behaviors. The Xml we use to configure the List of Fees used by application is:

<fees>
   <fee type="Commission" name="commission" rate="0.000125" />
   <fee type="FINRAPerShare" name="FINRA" rate="0.000119" />
   <fee type="SEC" name="SEC" rate="0.0000224" />
   <fee type="Route" name="ROUTES">
       <routes>
        <route>
            <name>NYSE</name>
            <rates>
                <billing code="2" rate="-.0014" normalized="A" />
                <billing code="1" rate=".0029" normalized="R" />
            </rates>
        </route>        
        </routes>
          ...
    </fee>
  </fees>

在上面的 XML 中，每个 <fee> 元素对应一个 Fee 接口的具体子类.type 属性提供有关要实例化哪种类型的信息，然后一旦实例化，JAXB 解组器就会应用剩余 Xml 中的属性.

In the above Xml, each <fee> element corresponds to a concrete subclass of a Fee interface. The type attribute gives information about which type to instantiate, and then once it is instantiated, the JAXB unmarshalling applies the properties from the remaining Xml.

我总是不得不求助于这样的事情:

I always have to resort to doing something like this:

private void addFees(TradeFeeCalculator calculator) throws Exception {
    NodeList feeElements = configDocument.getElementsByTagName("fee");
    for (int i = 0; i < feeElements.getLength(); i++) {
        Element feeElement = (Element) feeElements.item(i);
        TradeFee fee = createFee(feeElement);
        calculator.add(fee);
    }
}

private TradeFee createFee(Element feeElement) {
    try {
        String type = feeElement.getAttribute("type");
        LOG.info("createFee(): creating TradeFee for type=" + type);
        Class<?> clazz = getClassFromType(type);
        TradeFee fee = (TradeFee) JAXBConfigurator.createAndConfigure(clazz, feeElement);
        return fee;
    } catch (Exception e) {
        throw new RuntimeException("Trade Fees are misconfigured, xml which caused this=" + XmlUtils.toString(feeElement), e);
    }
}

在上面的代码中，JAXBConfigurator 只是用于解组的 JAXB 对象的简单包装器:

In the above code, the JAXBConfigurator is just a simple wrapper around the JAXB objects for unmarshalling:

public static Object createAndConfigure(Class<?> clazz, Node startNode) {
    try {
        JAXBContext context = JAXBContext.newInstance(clazz);
        Unmarshaller unmarshaller = context.createUnmarshaller();
        @SuppressWarnings("rawtypes")
        JAXBElement configElement = unmarshaller.unmarshal(startNode, clazz);
        return configElement.getValue();
    } catch (JAXBException e) {
        throw new RuntimeException(e);
    }
}

在上述代码的最后，我们得到一个列表，其中包含在 Xml 中配置的任何类型.

At the end, of the above code, we get a List which contains whichever types were configured in the Xml.

有没有办法让 JAXB 自动执行此操作，而无需编写代码来迭代上述元素?

Is there a way to get JAXB to do this automatically without having to write the code to iterate the elements as above?

推荐答案

注意:我是EclipseLink JAXB (MOXy) 领导和 JAXB (JSR-222) 专家组.

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

如果您使用 MOXy 作为您的 JAXB 提供程序，那么您可以使用 MOXy 的 @XmlPaths 注释来扩展标准 JAXB @XmlElements 注释以执行以下操作:

If you are using MOXy as your JAXB provider then you could use the MOXy's @XmlPaths annotation to extend the standard JAXB @XmlElements annotation to do the following:

费用

import java.util.List;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.*;

@XmlRootElement
public class Fees {

    @XmlElements({
        @XmlElement(type=Commission.class),
        @XmlElement(type=FINRAPerShare.class),
        @XmlElement(type=SEC.class),
        @XmlElement(type=Route.class)
    })
    @XmlPaths({
        @XmlPath("fee[@type='Commission']"),
        @XmlPath("fee[@type='FINRAPerShare']"),
        @XmlPath("fee[@type='SEC']"),
        @XmlPath("fee[@type='Route']")
    })
    private List<Fee> fees;

}

佣金

Fee接口的实现会被正常注释.

The implementations of the Fee interface would be annotated normally.

import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
public class Commission implements Fee {

    @XmlAttribute
    private String name;

    @XmlAttribute
    private String rate;

}

更多信息

• http://blog.bdoughan.com/2011/03/map-to-element-based-on-attribute-value.html
• http://blog.bdoughan.com/2010/10/jaxb-and-xsd-choice-xmlelements.html
• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html