The HttpSessionListener and HttpSessionAttributeListener are configured in the deployment descriptor. Therefore, even if a session attribute implements these interfaces, the sessionDestroyed() and attributeRemoved() methods will not be called on that attribute.
I don't know what the context that paragraph comes from, but I think it means it's not enough that the class implements the interfaces, it also needs to be registered in the web.xml in order for the listener to work.
SCJP 1.4, SCWCD 1.4, SCBCD 1.3
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