JavaMail: Implement DataSource in javax.activation package

In my java program I need to send some generated contents as e-mail
attachment. But I found that the javax.activation only comes with a
FileDataSource that implements DataSource. This can be used to send a file:

DataSource source = new FileDataSource(filename);

 How do I implement DataSource so that I have something like
InputStreamDataSource then  I can send some generated contents in my java
program as e-mail attachment?

Javax.activation doc:
http://java.sun.com/products/javabeans/glasgow/javadocs/javax/activation/pac
kage-summary.html

Thanks.
Bruce