java - Why does JAXB need a no arg constructor for marshalling?

Question

If you try to marshal a class which references a complex type that does not have a no-arg constructor, such as:

import java.sql.Date;

@XmlRootElement(name = "Foo")
@XmlAccessorType(XmlAccessType.FIELD)
public class Foo {
    int i;
    Date d; //java.sql.Date does not have a no-arg constructor
}

with the JAXB implementation that is part of Java, as follows:

    Foo foo = new Foo();
    JAXBContext jc = JAXBContext.newInstance(Foo.class);
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    Marshaller marshaller = jc.createMarshaller();
    marshaller.marshal(foo, baos);

JAXB will throw a

com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 1 counts of IllegalAnnotationExceptions java.sql.Date does not have a no-arg default constructor

Now, I understand why JAXB needs a no-arg constructor on unmarshalling - because it needs to instantiate the object. But why does JAXB need a no-arg constructor while marshalling?

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?

Am I missing something or are these just bad implementation choices in Java's JAXB implementation?

Answer 1

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

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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".

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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
}