当字符串为空但不为null时，如何防止在JAXB中将空标签编组

问题描述

|| 我试图使JAXB在字符串为空时不封送空标签。 我可以通过创建XMLAdapter来实现，其中将返回插入的空字符串null。但是以这种方式，我将不得不使用此适配器注释每个属性。 有什么方法可以使其更全球化？     

解决方法

EclipseLink JAXB（MOXy）将允许您在包装级别为

java.lang.String

指定

XmlAdapter

（我是MOXy技术负责人）： 包装信息

@XmlJavaTypeAdapter(value=StringAdapter.class,type=String.class)
package example;

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

字符串适配器

package example;

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

public class StringAdapter extends XmlAdapter<String,String> {

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

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

}

根

package example;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Root {

    private String foo;
    private String bar;

    public String getFoo() {
        return foo;
    }

    public void setFoo(String foo) {
        this.foo = foo;
    }

    public String getBar() {
        return bar;
    }

    public void setBar(String bar) {
        this.bar = bar;
    }

}

jaxb.properties 要将MOXy用作JAXB提供程序，您需要在与模型类相同的包中包含一个名为

jaxb.properties

的文件，并带有以下条目：

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

演示版

package example;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;

public class Demo {

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

        Root root = new Root();
        root.setFoo(\"\");
        root.setBar(\"\");

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

}

注意 由于我感觉是JAXB参考实现中的错误，如果您使用Java SE 6中包含的JAXB版本，则会引发以下异常：

Exception in thread \"main\" java.lang.NullPointerException
    at com.sun.xml.bind.v2.runtime.output.Encoded.setEscape(Encoded.java:107)
    at com.sun.xml.bind.v2.runtime.output.UTF8XmlOutput.doText(UTF8XmlOutput.java:315)
    at com.sun.xml.bind.v2.runtime.output.UTF8XmlOutput.text(UTF8XmlOutput.java:299)
    at com.sun.xml.bind.v2.runtime.output.IndentingUTF8XmlOutput.text(IndentingUTF8XmlOutput.java:153)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.leafElement(XMLSerializer.java:325)
    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:340)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:593)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:324)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:494)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:315)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:244)
    at javax.xml.bind.helpers.AbstractMarshallerImpl.marshal(AbstractMarshallerImpl.java:75)
    at example.Demo.main(Demo.java:18)