How to add XML processing instructions during JAXB marshal

You could leverage an XMLStreamWriter and an XmlAdapter to do this:

BobAdapter

Things to note about the XmlAdapter:

It's stateful and references an XMLStreamWriter. We will later leverage JAXB's ability to set a stateful XmlAdapter on a Marshaller.
It converts from a List<String> to a List<String>, we're just using an XmlAdapter here to get an event.
The marshal method is where we call writeProcessingInstruction on the XMLStreamWriter:

package forum6931520;

import java.util.List;

import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.stream.XMLStreamWriter;

public class BobAdapter extends XmlAdapter<List<String>, List<String>> {

    private boolean first = true;
    private XMLStreamWriter xmlStreamWriter;

    public BobAdapter() {
    }

    public BobAdapter(XMLStreamWriter xmlStreamWriter) {
        this();
        this.xmlStreamWriter = xmlStreamWriter;
    }

    @Override
    public List<String> marshal(List<String> stringList) throws Exception {
        if(first) {
            xmlStreamWriter.writeProcessingInstruction("array", "bob");
            first = false;
        }
        return stringList;
    }

    @Override
    public List<String> unmarshal(List<String> stringList) throws Exception {
        return stringList;
    }

}

Alice

The @XmlJavaTypeAdapter annotation is used to link the XmlAdapter with the field/property:

package forum6931520;

import java.util.List;

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

@XmlRootElement
public class Alice {

    private List<String> bob;

    @XmlJavaTypeAdapter(BobAdapter.class)
    public List<String> getBob() {
        return bob;
    }

    public void setBob(List<String> bob) {
        this.bob = bob;
    }

}

Demo

package forum6931520;

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamWriter;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Alice.class);

        File xml = new File("src/forum6931520/input.xml");
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Alice alice = (Alice) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        XMLOutputFactory xof = XMLOutputFactory.newFactory();
        XMLStreamWriter xmlStreamWriter = xof.createXMLStreamWriter(System.out);
        marshaller.setAdapter(new BobAdapter(xmlStreamWriter));
        marshaller.marshal(alice, xmlStreamWriter);
    }

}

input.xml

<?xml version="1.0" encoding="UTF-8"?>
<alice>
  <?array bob?>
  <bob>edgar</bob>
  <bob>david</bob>
</alice>

Output

<?xml version='1.0' encoding='UTF-8'?><alice><?array bob?><bob>edgar</bob><bob>david</bob></alice>

Note

This example needs to be run with EclipseLink JAXB (MOXy) as the JAXB RI will throw the following exception (I'm the MOXy lead):

Exception in thread "main" com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 2 counts of IllegalAnnotationExceptions
java.util.List is an interface, and JAXB can't handle interfaces.
    this problem is related to the following location:
        at java.util.List
        at public java.util.List forum6931520.Alice.getBob()
        at forum6931520.Alice
java.util.List does not have a no-arg default constructor.
    this problem is related to the following location:
        at java.util.List
        at public java.util.List forum6931520.Alice.getBob()
        at forum6931520.Alice

For More Information

UPDATE

I have entered an enhancement request (https://bugs.eclipse.org/354286) to add native support for processing instructions. This would eventually allow you to specify the following on your property:

@XmlProcessingInstruction(target="array", value="bob")
public List<String> getBob() {
    return bob;
}

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