当字符串为空但不为null时，如何防止在JAXB中编组空标记 [英] How to prevent marshalling empty tags in JAXB when string is empty but not null

问题描述

我正在尝试使字符串为空时JAXB不会封送空标记。
我可以通过创建XmlAdapter来创建它，其中将返回insted空字符串null。但就这样，我将不得不用这个适配器注释每个属性。

I am trying to make JAXB do not marshal empty tags when string is empty. I can make it by creating XmlAdapter where insted empty string null would be returned. But in that way, I will have to annotate each attribute with this adapter.

有没有办法让它更全球化？

Is there any way to make it more global?

推荐答案

EclipseLink JAXB（MOXy）允许你为 java.lang指定 XmlAdapter 包裹级别的.String （我是MOXy技术主管）：

EclipseLink JAXB (MOXy) will let you specify the XmlAdapter for java.lang.String at the package level (I'm the MOXy tech lead):

package-info

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

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

StringAdapter

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;
    }

}

Root

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，您需要在文件中包含一个名为 jaxb.properties 的文件与您的模型类相同的包，包含以下条目：

To use MOXy as your JAXB provider you need to include a file named jaxb.properties in the same package as your model classes with the following entry:

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

演示

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

由于我认为JAXB参考实现中存在错误，如果您使用Java SE 6中包含的JAXB版本，则会引发以下异常：

Due to what I feel is a bug in the JAXB reference implementation, the following exception is thrown if you use the version of JAXB included in Java SE 6:

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)

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