java - JAXB 将空字符串全局编码为空

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，因为我们所有的 JAXB 类型都是在每次构建时从模式生成的。如果可能的话，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字符串不会创建元素，但我需要为从许多模式生成的所有 JAXB 类型设置它。有没有办法将其应用于所有生成的 JAXB 类中的所有字符串字段？

更新通过进行以下更改，我设法让 XML 适配器为模式中的所有字符串生成:

在项目 POM 中，我将其添加到 maven-jaxb2-plugin 中:

<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>

以及格式化方法:

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

    return value;
}

问题在于，这会导致 JAXB 深处出现 Null Pointer Exception。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)

这个问题的原因归结为这个方法:

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

如果值不为 null 在任何适配器运行之前，上述方法将呈现元素。

有什么方法可以覆盖 JAXB 中使用的访问器，以便在确定是否呈现元素之前运行适配器？还有其他方法可以实现我想要的吗？

Answer 1

注意:我是 EclipseLink JAXB (MOXy) JAXB (JSR-222) 的领导和成员专家组。

您所做的是正确的，您看到的错误是由于我认为是 JAXB reference implementation 中的错误所致. JAXB RI 应该能够处理从 XmlAdapter 返回的空值。此用例适用于 EclipseLink JAXB (MOXy)，我将在下面通过示例进行演示。

字符串适配器

下面是一个实现，它大致完成了从 XML 模式生成 Java 模型后将获得的实现(请参阅 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;
    }

}

包信息

由于您正在注册一个全局适配器，它将从一个 package-info 类中引用，如下所示(参见: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.*;

根

下面是一个带有一些 String 字段的示例域类。由于 XmlAdapter 是在包级别注册的，它将应用于该包中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;

}

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 ""，然后将其编码为 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 会导致 NPE。堆栈跟踪是不同的，但很可能我们使用不同的编码方法。我还使用了 JDK 中包含的 JAXB RI 版本，它被重新打包到 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>