Question

我正在编写一个将集成到API的应用程序，该API的端点返回XSD支持的XML。我的应用程序最初必须针对此应用程序的两个不同版本（可能在将来的版本中提供更多版本），而不必是动态的。启动应用程序时，用户将必须告诉应用程序要支持哪个API主版本。 XSD不在我的控制下，我不想编辑它们。

XSD生成相同名称的类，并且我已经在这里遇到了问题。我无法将XJC生成的两个ObjectFactory类都加载到JAXBContext中。我的解决方案现在是JAXBContexts的映射：

private static Map<Integer, Pair<Class<?>, JAXBContext>> contexts = new HashMap<>();

static {
    try {
        contexts.put(4, Pair.of(com.api.v4_2_0.ObjectFactory.class, JAXBContext.newInstance(com.api.v4_2_0.ObjectFactory.class)));
        contexts.put(3, Pair.of(com.api.v3_9_4.ObjectFactory.class, JAXBContext.newInstance(com.api.v3_9_4.ObjectFactory.class)));
    } catch (JAXBException e) {
        LOGGER.error("Failed to initialize JAXBContext", e);
    }
}

该对用于了解JAXBContext基于哪个类，因为我无法在运行时恢复该类。然后，要序列化对象，我使用了很多魔术反射，这些反射有效但感觉不正确：

public static String objectToString(final Object object, final Integer majorVersion) {
    try {
        final ByteArrayOutputStream os = new ByteArrayOutputStream();
        getMarshallerForMajorVersion(majorVersion).marshal(createJaxbElementFromObject(object, getObjectFactoryForMajorVersion(majorVersion)), os);
        return os.toString(UTF_8);
    } catch (JAXBException e) {
        throw SesameException.from("Failed to serialize object", e);
    }
}

private static Marshaller getMarshallerForMajorVersion(final Integer majorVersion) throws JAXBException {
    return getContextForMajorVersion(majorVersion).getRight().createMarshaller();
}

private static Class<?> getObjectFactoryForMajorVersion(final Integer majorVersion) {
    return getContextForMajorVersion(majorVersion).getLeft();
}

private static Pair<Class<?>, JAXBContext> getContextForMajorVersion(final Integer majorVersion) {
    if (contexts.containsKey(majorVersion)) {
        return contexts.get(majorVersion);
    }
    throw illegalArgument("No JAXBContext for API with major version %d", majorVersion);
}

private static JAXBElement<?> createJaxbElementFromObject(final Object object, final Class<?> objectFactory) {
    try {
        LOGGER.info("Attempting to find a JAXBElement producer for class {}", object.getClass());
        final Method method = findElementMethodInObjectFactory(object, objectFactory);
        return (JAXBElement<?>) method.invoke(objectFactory.getConstructor().newInstance(), object);
    } catch (IllegalAccessException | InvocationTargetException | NoSuchMethodException | InstantiationException e) {
        throw illegalArgument("Failed to construct JAXBElement for class %s", object.getClass().getName());
    }
}

private static Method findElementMethodInObjectFactory(final Object object, final Class<?> left) {
    return Arrays.stream(left.getMethods())
            .filter(m -> m.getReturnType().equals(JAXBElement.class))
            .filter(m -> m.getName().endsWith(object.getClass().getSimpleName()))
            .findFirst()
            .orElseThrow(() -> illegalArgument("Failed to find JAXBElement constructor for class %s", object.getClass().getName()));
}

这很好，但感觉很脆弱。

问题

更糟糕的是，必须使用泛型将XML反序列化为对象：

public static <T> T stringToObject(final String xml, final Class<T> toClass, final Integer majorVersion) {
    try {
        final Unmarshaller unmarshaller = getUnmarshallerForVersion(majorVersion);
        final JAXBElement<T> unmarshalledElement = (JAXBElement<T>) unmarshaller.unmarshal(new StringReader(xml));
        return toClass.cast(unmarshalledElement.getValue());
    } catch (JAXBException e) {
        throw SesameException.from(format("Failed to deserialize XML into %s", toClass.getCanonicalName()), e);
    }
}

// And calling this from another class
private com.api.v4_2_0.SomeClass.class toSomeClass(final HttpResponse<String> response) {
    return XmlUtil.stringToObject(response.body(), com.api.v4_2_0.SomeClass.class, apiMajorVersion); // <--- I can't beforehand use this package since major version might be 3.
}

现在（据我所知），我无法使用泛型并将其根据所使用的API的主要版本映射到正确包中的正确类。

我也尝试过使用抽象基类，并且只为每个版本提供一个单独的ObjectFactory，但这仍然给我在问题部分中描述的问题。我不知道如何返回该类的正确版本：

private com.api.v4_2_0.SomeClass.class toSomeClass(final HttpResponse<String> response) {
    return version4XmlUtil.stringToObject(response.body(), com.api.v4_2_0.SomeClass.class); // <--- I can't beforehand use this package since major version might be 3.
}

如何构造代码来解决此问题？哪些模式有用？我会完全走错路了吗？

Answer 1

EclipseLink JAXB（MOXy）的@XmlPath和外部绑定文件扩展名可以提供帮助。

我在下面引用的博客将单个对象模型映射到两个不同的XML模式。它通过使用标准JAXB和MOXy扩展注释的组合来映射第一个API来实现此目的。

package blog.weather;

import java.util.List;

import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

import org.eclipse.persistence.oxm.annotations.XmlPath;

@XmlRootElement(name="xml_api_reply")
@XmlType(propOrder={"location", "currentCondition", "currentTemperature", "forecast"})
@XmlAccessorType(XmlAccessType.FIELD)
public class WeatherReport {

    @XmlPath("weather/forecast_information/city/@data")
    private String location;

    @XmlPath("weather/current_conditions/temp_f/@data")
    private int currentTemperature;

    @XmlPath("weather/current_conditions/condition/@data")
    private String currentCondition;

    @XmlPath("weather/forecast_conditions")
    private List<Forecast> forecast;

}

然后...

package blog.weather;

import org.eclipse.persistence.oxm.annotations.XmlPath;

public class Forecast {

    @XmlPath("day_of_week/@data")
    private String dayOfTheWeek;

    @XmlPath("low/@data")
    private int low;

    @XmlPath("high/@data")
    private int high;

    @XmlPath("condition/@data")
    private String condition;

}

您不能通过注释创建到对象模型的辅助映射集，因此其他的可以利用MOXy的XML元数据，从而覆盖了第二个API。

<?xml version="1.0"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="blog.weather"
    xml-mapping-metadata-complete="true">
    <xml-schema element-form-default="QUALIFIED">
        <xml-ns prefix="yweather" namespace-uri="http://xml.weather.yahoo.com/ns/rss/1.0"/>
    </xml-schema>
    <java-types>
        <java-type name="WeatherReport" xml-accessor-type="FIELD">
            <xml-root-element name="rss"/>
            <xml-type prop-order="location currentTemperature currentCondition forecast"/>
            <java-attributes>
                <xml-attribute java-attribute="location" xml-path="channel/yweather:location/@city"/>
                <xml-attribute java-attribute="currentTemperature" name="channel/item/yweather:condition/@temp"/>
                <xml-attribute java-attribute="currentCondition" name="channel/item/yweather:condition/@text"/>
                <xml-element java-attribute="forecast" name="channel/item/yweather:forecast"/>
            </java-attributes>
        </java-type>
        <java-type name="Forecast" xml-accessor-type="FIELD">
            <java-attributes>
                <xml-attribute java-attribute="dayOfTheWeek" name="day"/>
                <xml-attribute java-attribute="low"/>
                <xml-attribute java-attribute="high"/>
                <xml-attribute java-attribute="condition" name="text"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

默认行为是MOXy的映射文档用于扩充模型上指定的所有注释。但是，取决于设置xml-mapping-metadata-complete标志的方式，XML元数据可以完全替换，或简单扩充（默认），即注释提供的元数据。

试穿以获取尺寸，让我知道您的想法：

Mapping Objects to Multiple XML Schemas Using EclipseLink MOXy http://blog.bdoughan.com/2011/09/mapping-objects-to-multiple-xml-schemas.html

处理XSD生成的类的多个版本

1 个答案: