Question

我的问题与How to prevent marshalling empty tags in JAXB when string is empty but not null

非常相似

不同之处在于我无法将注释添加到package-info.java，因为我们的所有JAXB类型都是从每次构建的模式生成的。如果可能，我也更愿意不更改JAXB提供程序。

我想要实现的是设置一个空String不会创建该元素，但我需要为许多模式中的所有生成的JAXB类型设置它。有没有办法将它应用于所有生成的JAXB类中的所有String字段？

更新我已经设法通过进行以下更改来为模式中的所有字符串生成XML适配器：

在项目POM中，我将其添加到maven-jaxb2-plugin：

<bindingDirectory>src/main/resources</bindingDirectory>
<bindingIncludes>
    <include>bindings.xjb</include>
</bindingIncludes>

这是我的bindings.xjb文件：

<jxb:bindings xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:jxb="http://java.sun.com/xml/ns/jaxb" version="2.1">
    <jxb:globalBindings>
                    <jxb:javaType name="java.lang.String" xmlType="xs:token"
                        parseMethod="com.project.Formatter.parseString"
                        printMethod="com.project.Formatter.printString"/>
    </jxb:globalBindings>
</jxb:bindings>

格式化方法：

public static String printString(final String value)
{
    if (StringUtils.isBlank(value))
    {
        return null;
    }

    return value;
}

问题是这会导致JAXB内部出现空指针异常。这是堆栈跟踪：

Caused by: java.lang.NullPointerException
    at com.sun.xml.bind.v2.runtime.output.SAXOutput.text(SAXOutput.java:158)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.leafElement(XMLSerializer.java:321)
    at com.sun.xml.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:210)
    at com.sun.xml.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:209)
    at com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor$CompositeTransducedAccessorImpl.writeLeafElement(TransducedAccessor.java:250)
    at com.sun.xml.bind.v2.runtime.property.SingleElementLeafProperty.serializeBody(SingleElementLeafProperty.java:98)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:322)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:681)
    at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:150)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:322)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:681)
    at com.sun.xml.bind.v2.runtime.property.ArrayElementNodeProperty.serializeItem(ArrayElementNodeProperty.java:65)
    at com.sun.xml.bind.v2.runtime.property.ArrayElementProperty.serializeListBody(ArrayElementProperty.java:168)
    at com.sun.xml.bind.v2.runtime.property.ArrayERProperty.serializeBody(ArrayERProperty.java:152)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:322)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:681)
    at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:150)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:322)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:681)
    at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:150)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:322)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsXsiType(XMLSerializer.java:681)
    at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:150)
    at com.sun.xml.bind.v2.runtime.ElementBeanInfoImpl$1.serializeBody(ElementBeanInfoImpl.java:156)
    at com.sun.xml.bind.v2.runtime.ElementBeanInfoImpl$1.serializeBody(ElementBeanInfoImpl.java:185)
    at com.sun.xml.bind.v2.runtime.ElementBeanInfoImpl.serializeBody(ElementBeanInfoImpl.java:305)
    at com.sun.xml.bind.v2.runtime.ElementBeanInfoImpl.serializeRoot(ElementBeanInfoImpl.java:312)
    at com.sun.xml.bind.v2.runtime.ElementBeanInfoImpl.serializeRoot(ElementBeanInfoImpl.java:71)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:490)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:328)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:257)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:103)

此问题的原因归结为此方法：

com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor.CompositeTransducedAccessorImpl.hasValue(BeanT)

如果在运行任何适配器之前值，则上述方法将呈现该元素。

有没有办法覆盖JAXB中使用的Accessor，以便在确定是否呈现元素之前运行适配器？还有另一种方法可以达到我的目的吗？

Answer 1

注意：我是EclipseLink JAXB (MOXy)主管，是JAXB (JSR-222)专家组的成员。

你所做的是对的，你看到的错误是由于我认为是JAXB reference implementation中的错误。 JAXB RI应该能够处理从XmlAdapter返回的空值。这个用例适用于EclipseLink JAXB（MOXy），我将在下面举例说明。

<强> StringAdapter

下面是一个implmentation，它大致与您从XML模式生成Java模型后的内容大致相同（参见http://blog.bdoughan.com/2011/08/xml-schema-to-java-generating.html）。

package forum11894193;

import javax.xml.bind.annotation.adapters.XmlAdapter;

public class StringAdapter extends XmlAdapter<String, String> {

    @Override
    public String marshal(String string) throws Exception {
        if("".equals(string)) {
            return null;
        }
        return string;
    }

    @Override
    public String unmarshal(String string) throws Exception {
        return string;
    }

}

<强>包信息

由于您正在注册全局适配器，因此将从package-info类引用它，如下所示（请参阅：http://blog.bdoughan.com/2012/02/jaxb-and-package-level-xmladapters.html）。

@XmlJavaTypeAdapters({
    @XmlJavaTypeAdapter(value=StringAdapter.class, type=String.class)
})
package forum11894193;

import javax.xml.bind.annotation.adapters.*;

<强>根

以下是一个包含几个String字段的示例域类。由于XmlAdapter已在包级别注册，因此它将应用于该包中的所有映射的字符串字段/属性。

package forum11894193;

import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {

    String a;
    String b;
    String c;

}

<强>演示

在下面的演示代码中，我们将创建一个Root的实例，将几个字段设置为""，然后将其编组为XML。

package forum11894193;

import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);

        Root root = new Root();
        root.a = "";
        root.b = "b";
        root.c = "";

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(root, System.out);
    }

}

使用JAXB RI输出

在此示例中使用JAXB RI会生成NPE。堆栈跟踪是不同的，但我们最有可能使用不同的编组方法。我还使用JDK中包含的JAXB RI版本，该版本重新打包到com.sun.xml.internal.bind.v2。

Exception in thread "main" java.lang.NullPointerException
    at com.sun.xml.internal.bind.v2.runtime.output.Encoded.setEscape(Encoded.java:96)
    at com.sun.xml.internal.bind.v2.runtime.output.UTF8XmlOutput.doText(UTF8XmlOutput.java:294)
    at com.sun.xml.internal.bind.v2.runtime.output.UTF8XmlOutput.text(UTF8XmlOutput.java:283)
    at com.sun.xml.internal.bind.v2.runtime.output.IndentingUTF8XmlOutput.text(IndentingUTF8XmlOutput.java:141)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.leafElement(XMLSerializer.java:293)
    at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:179)
    at com.sun.xml.internal.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$1.writeLeafElement(RuntimeBuiltinLeafInfoImpl.java:166)
    at com.sun.xml.internal.bind.v2.runtime.reflect.TransducedAccessor$CompositeTransducedAccessorImpl.writeLeafElement(TransducedAccessor.java:239)
    at com.sun.xml.internal.bind.v2.runtime.property.SingleElementLeafProperty.serializeBody(SingleElementLeafProperty.java:87)
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:306)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:561)
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:290)
    at com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:462)
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:314)
    at com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:243)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:75)
    at forum11894193.Demo.main(Demo.java:17)

使用EclipseLink JAXB（MOXy）输出

当MOXy用作JAXB提供程序时，您将获得所需的输出。有关将MOXy指定为JAXB提供程序的信息，请参阅：http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html。

<?xml version="1.0" encoding="UTF-8"?>
<root>
   <b>b</b>
</root>

Answer 2

空字符串仍然是值。这就是元素创建的原因。如上所述，如果字符串为空，在setter方法中将变量设置为null？

另外，请检查此帖子 JAXB: how to make JAXB NOT to unmarshal empty string to 0

Answer 3

将以下代码用于Marshall课程

public static String toXml(Object o, Class clazz, boolean isFormatted, boolean isEmptyNodes) {
        try {
            Map<String, Object> properties = new HashMap<String, Object>(1);
            if(isEmptyNodes) {
                SessionEventListener sessionEventListener = new NullPolicySessionEventListener();
                properties.put(JAXBContextProperties.SESSION_EVENT_LISTENER, sessionEventListener);
            } else {
                SessionEventListener sessionEventListener = new DiscardEmptyTagSessionEventListener();
                properties.put(JAXBContextProperties.SESSION_EVENT_LISTENER, sessionEventListener);
            }

            // Create a JaxBContext
            JAXBContext jc = JAXBContext.newInstance(new Class[] {clazz}, properties);
            StringWriter sw = new StringWriter();

            // Create the UnMarshaller Object using the JaxB Context
            Marshaller marshaller = jc.createMarshaller();

            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, isFormatted);
            // remove the xml version line from the output
            marshaller.setProperty("com.sun.xml.bind.xmlDeclaration", Boolean.FALSE);
            // Marshal the employee object to XML and print the output to console
            marshaller.marshal(o, sw);

            return sw.toString();
        } catch (JAXBException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
}

JAXB Marshal空字符串全局为空

3 个答案: