JAXB 避免保存默认值

2022-01-19 00:00:00 marshalling java jaxb unmarshalling

有没有办法让 JAXB 不保存哪些值是 @Element 注释中指定的默认值的字段，然后在从 XML 加载 null 或空元素时将值设置为它?一个例子:

Is there any way to make JAXB not save fields which values are the default values specified in the @Element annotations, and then make set the value to it when loading elements from XML that are null or empties? An example:

class Example
{
    @XmlElement(defaultValue="default1")
    String prop1;
}

Example example = new Example();
example.setProp1("default1");
jaxbMarshaller.marshal(example, aFile);

应该生成:

<example/>

加载时

Example example = (Example) jaxbUnMarshaller.unmarshal(aFile);
assertTrue(example.getProp1().equals("default1"));

我正在尝试这样做是为了生成一个干净的 XML 配置文件，并使其更具可读性和更小的尺寸.

I am trying to do this in order to generate a clean XML configuration file, and make it better readable and smaller size.

提前致谢.

推荐答案

您可以通过利用 XmlAccessorType(XmlAccessType.FIELD) 并将逻辑放入 get/set 方法中来执行以下操作:

You could do something like the following by leveraging XmlAccessorType(XmlAccessType.FIELD) and putting logic in the get/set methods:

示例

package forum8885011;

import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class Example {

    private static final String PROP1_DEFAULT = "default1";
    private static final String PROP2_DEFAULT = "123";

    @XmlElement(defaultValue=PROP1_DEFAULT)
    String prop1;

    @XmlElement(defaultValue=PROP2_DEFAULT)
    Integer prop2;

    public String getProp1() {
        if(null == prop1) {
            return PROP1_DEFAULT;
        }
        return prop1;
    }

    public void setProp1(String value) {
        if(PROP1_DEFAULT.equals(value)) {
            prop1 = null;
        } else {
            prop1 = value;
        }
    }

    public int getProp2() {
        if(null == prop2) {
            return Integer.valueOf(PROP2_DEFAULT);
        }
        return prop2;
    }

    public void setProp2(int value) {
        if(PROP2_DEFAULT.equals(String.valueOf(value))) {
            prop2 = null;
        } else {
            prop2 = value;
        }
    }

}

演示

package forum8885011;

import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Example.class);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        Example example = new Example();
        example.setProp1("default1");
        example.setProp2(123);
        System.out.println(example.getProp1());
        System.out.println(example.getProp2());
        marshaller.marshal(example, System.out);

        example.setProp1("FOO");
        example.setProp2(456);
        System.out.println(example.getProp1());
        System.out.println(example.getProp2());
        marshaller.marshal(example, System.out);
    }

}

输出

default1
123
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<example/>
FOO
456
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<example>
    <prop1>FOO</prop1>
    <prop2>456</prop2>
</example>

更多信息

  • http://blog.bdoughan.com/2011/06/using-jaxbs-xmlaccessortype-to.html

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