JAX-WS, JAXB and Generics

When  JAXB 2.1 came along it introduced a new annotation namely @XmlSeeAlso. It resolved the problem where at runtime, JAX-WS would only know about classes bound by JAXB and not sub classes of those which have been bound.

By using @XmlSeeAlso on a class, sub types of the bound class can be listed thereby allowing them to be bound also and therefore reachable by JAX-WS when marshalling and unmarshalling.

It can also be used to overcome the problem of using generics in a JAXB annotated class as the following simple example will describe. Given the below LogFile class, it will only be marshalled/unmarshalled if the T at runtime is bound.

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class LogFile<T> {

    @XmlElement(name="logFileLine")
    private List<T> logFileLines;

    public LogFile() {
        logFileLines = new ArrayList<T>();
    }

    public LogFile(List<T> logFileLines) {
        this.logFileLines = logFileLines;
    }

    public List<T> getLogFileLines() {
        return logFileLines;
    }

    public void setLogFileLines(List<T> logFileLines) {
        this.logFileLines = logFileLines;
    }

    public void addLogFileLine(T logFileLine) {
        this.logFileLines.add(logFileLine);
    }

}

If LogFile contains a collection of eg Strings then this will work fine but if T is a class not bound, eg com.city81.domain.LogLine then an error similar to below will be thrown:

javax.xml.bind.MarshalException - with linked exception: [javax.xml.bind.JAXBException: class com.city81.domain.LogLine nor any of its super class is known to this context.]

To resolve this, the class LogLine needs to be included the @XmlSeeAlso annotation.

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso(LogLine.class)
public class LogFile<T> {

    ....

}

This does of course mean you need to know what potential classes can be T before runtime but at least it means you can use Generics on JAXB annotated classes.