Question

A class is defined with the following JAXB annotation:

class Course {
@XmlElement (name = "book")
List<Book> requiredBooks = new ArrayList<Book>();

When unmarshalling an XML document that contains this

<course>
  ...
  <book/>
</course>

I end up with a Book added to the list, with all of its attributes set to null. I don’t control the XML input. How can I prevent this empty book from being added? I tried intercepting in set..() or add..() methods, but turns out JAXB bypasses setters when dealing with collections. Any suggestions?

Answer 1

Here’s a solution that does work. Again, elegance was left behind.

There are two callbacks defined by JAXB: beforeUnmarshal and afterUnmarshal. By implementing afterUnmarshal to clean up the list, I was able to achieve the desired result.

  class Course
  {
    @XmlElement (name = "book")
    List<Book> requiredBooks = new ArrayList<Book>();  
    ...
    void afterUnmarshal(Unmarshaller aUnmarshaller, Object aParent)
    {
        if (requiredBooks != null)
        {
            Iterator<Book> iterator = requiredBooks.iterator();
            while (iterator.hasNext())
            {
                Book book = iterator.next();
                if (StringUtils.isEmpty(book.getTitle()))
                {
                    // a book without title is considered invalid
                    iterator.remove();
                }
            }
        }
    }
  }

While this works, by biggest issue with it is the absence of an interface for implementing afterUnmarshal. It’s looked up by JAXB using reflection, but I think it would’ve been convenient (and would reduce debugging/maintenance) if JAXB simply supplied interfaces and/or absract implementations. As it is, what if the signature of afterUnmarshal changes in the future? This code will just mysteriously stop working as it’s supposed to.