Only the org.xml.sax
package is guaranteed to be part of every SAX distribution.
The helpers don't depend on the extensions, to
preserve this notion of "core plus options."
In practice, it's a rare distribution that doesn't include all
of these packages, and SAX2 parsers without support for the
extension handlers are likely to support only XML subsets.
JAXP 1.1 (and hence JDK 1.4) includes all three packages.
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1.4. A Short History of SAX |  | 1.6. Some Popular SAX2 Parser Distributions |