How to tell castor to marshall a null field to an

问题:

I'm marshalling an object that can have some field set to null. I'm using castor with a xml-mapping file for the configuration. The class I'm marshalling is like this:

class Entity {
    private int id;
    private String name;
    private String description; // THIS CAN BE NULL
    /* ... getters and setters follow ... */
}

...and a mapping file like this:

<mapping>
    <class name="Entity">
        <field name="id" type="integer"/>
        <field name="name" type="string"/>
        <field name="description" type="string"/>
    </class>
</mapping>

What I'm getting at the moment if the field is null (simplified example):

<entity>
   <id>123</id>
   <name>Some Name</name>
</entity>

while I want to have an empty tag in the resulting XML, even if the description field is null.

<entity>
   <id>123</id>
   <name>Some Name</name>
   <description /> <!-- open/close tags would be ok -->
</entity>

回答1:

One way to do this is with a GeneralizedFieldHandler. It's a bit of a hack but it will work for other fields that are Strings.

Example:

<mapping>
    <class name="Entity">
        <field name="id" type="integer"/>
        <field name="name" type="string"/>
        <field name="description" type="string" handler="NullHandler"/>
    </class>
</mapping>


public class NullHandler extends GeneralizedFieldHandler {

    @Override
    public Object convertUponGet( Object arg0 )
    {
        if( arg0 == null )
        {
            return "";
        }

        return arg0;
    }

    @Override
    public Object convertUponSet( Object arg0 )
    {
        return arg0;
    }

    @Override
    public Class getFieldType()
    {
        return String.class;
    }

}