Interoperable Web Services and Null Values

The meaning of a null is “value not set”. Apparently it is a difference whether you say the credit limit is zero or not set. Therefore sometimes it is necessary to convey null values over Web Services.
Luckily Web Services provide the means to express null values.
Assume the following Web Service method:

public String getName()
{
    return null;
}

The reponse in the SOAP Message uses the xsi:nil=”true” to express the null value on the wire.

This is fine as long as we use certain types as e.g. strings.
But if we use types which have a different representation on different plattform we might run into problems. Even though they are represented by a legal XML Schema type.
For instance the java types java.util.Date and java.util.Calendar are reference types on the Java platform.
The equivalent type System.DateTime is a value type on the .NET plattform which can’t be null.
If a reference type set to null is returned from a Java Web Service it causes a System.FormatException on the .NET plattform.
As the SOAP reponse is valid it seems to be a bug in the .NET implementation.

To avoid the problem wrap the reference type in an complex type as follows:


public class ServiceCalendar
{
    public ServiceCalendar(Calendar date)
    {
        this.date=date;
    }

    public Calendar date;
}

public class Service
{
    java.util.Calendar cal= new GregorianCalendar();

    public ServiceCalendar getDate()
    {
        return new ServiceCalendar(cal); // or return null;
    }
}

Enjoy

1 comment to Interoperable Web Services and Null Values

  • Hi – thanks for this entry! It’s kinda funny that I went through this on our site’s back end in order to help with a serious problem. So this post is appreciated! Anyway… nice blog – I’m subscribed to your RSS feed now so thanks again!

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