Why does JAXB need a no arg constructor for marshalling?

When a JAXB (JSR-222) implementation initializes its metadata it ensures that it can support both marshalling and unmarshalling.

For POJO classes that do not have a no-arg constructor you can use a type level XmlAdapter to handle it:

• http://blog.bdoughan.com/2010/12/jaxb-and-immutable-objects.html

java.sql.Date is not supported by default (although in EclipseLink JAXB (MOXy) it is). This can also be handled using an XmlAdapter specified via @XmlJavaTypeAdapter at field, property, or package level:

• http://blog.bdoughan.com/2011/05/jaxb-and-joda-time-dates-and-times.html
• http://blog.bdoughan.com/2011/01/jaxb-and-datetime-properties.html

Also, another nit, why does Java’s JAXB implementation throw an
exception if the field is null, and isn’t going to be marshalled
anyway?

What exception are you seeing? Normally when a field is null it is not included in the XML result, unless it is annotated with @XmlElement(nillable=true) in which case the element will include xsi:nil="true".

UPDATE

You could do the following:

SqlDateAdapter

Below is an XmlAdapter that will convert from the java.sql.Date that your JAXB implementation doesn’t know how to handle to a java.util.Date which it does:

package forum9268074;

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

public class SqlDateAdapter extends XmlAdapter<java.util.Date, java.sql.Date> {

    @Override
    public java.util.Date marshal(java.sql.Date sqlDate) throws Exception {
        if(null == sqlDate) {
            return null;
        }
        return new java.util.Date(sqlDate.getTime());
    }

    @Override
    public java.sql.Date unmarshal(java.util.Date utilDate) throws Exception {
        if(null == utilDate) {
            return null;
        }
        return new java.sql.Date(utilDate.getTime());
    }

}

Foo

The XmlAdapter is registered via the @XmlJavaTypeAdapter annotation:

package forum9268074;

import java.sql.Date;
import javax.xml.bind.annotation.*;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement(name = "Foo")
@XmlAccessorType(XmlAccessType.FIELD)
public class Foo {
    int i;

    @XmlJavaTypeAdapter(SqlDateAdapter.class)
    Date d; //java.sql.Date does not have a no-arg constructor
}