QDox Attributes is a package that allows you to specify metadata attributes on code elements using Javadoc tags, then query those attributes at runtime. While it is by no means the only package of its kind, it does have some features that others lack (see below). It will in due course be obsoleted by the JSR 175 effort, but if you need to use attributes now it provides a viable alternative.
Download the QDox Attributes package v0.6 (includes source code)
Requirements: JDK 1.4 only at runtime, JDK 1.4 and QDox (included) at compile time