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|SAXDriver||An enhanced SAX2 version of Microstar's Ælfred XML parser.|
|XmlParser||Parse XML documents and return parse events through call-backs.|
|XmlReader||This SAX2 parser optionally layers a validator over the Ælfred2 SAX2 parser.|
This package contains Ælfred2, which includes an enhanced SAX2-compatible version of the Ælfred non-validating XML parser, a modular (and hence optional) DTD validating parser, and modular (and hence optional) JAXP glue to those. Use these like any other SAX2 parsers.
Some of the documentation below was modified from the original Ælfred README.txt file. All of it has been updated.
Ælfred is a Java-based XML parser originally from Microstar Software Limited (no longer in existence) and more or less placed into the public domain.
In most Java applets and applications, XML should not be the central feature; instead, XML is the means to another end, such as loading configuration information, reading meta-data, or parsing transactions.
When an XML parser is only a single component of a much larger program, it cannot be large, slow, or resource-intensive. With Java applets, in particular, code size is a significant issue. The standard modem is still not operating at 56 Kbaud, or sometimes even with data compression. Assuming an uncompressed 28.8 Kbaud modem, only about 3 KBytes can be downloaded in one second; compression often doubles that speed, but a V.90 modem may not provide another doubling. When used with embedded processors, similar size concerns apply.
Ælfred is designed for easy and efficient use over the Internet, based on the following principles:
As you can see from this list, Ælfred is designed for production use, but neither validation nor perfect conformance was a requirement. Good validating parsers exist, including one in this package, and you should use them as appropriate. (See conformance reviews available at http://www.xml.com)
One of the main goals of Ælfred2 was to significantly improve conformance, while not significantly affecting the other goals stated above. Since the only use of this parser is with SAX, some classes could be removed, and so the overall size of Ælfred was actually reduced. Subsequent performance work produced a notable speedup (over twenty percent on larger files). That is, the tradeoffs between speed, size, and conformance were re-targeted towards conformance and support of newer APIs (SAX2), with a a positive performance impact.
The role anticipated for this version of Ælfred is as a lightweight Free Software SAX parser that can be used in essentially every Java program where the handful of conformance violations (noted below) are acceptable. That certainly includes applets, and nowadays one must also mention embedded systems as being even more size-critical. At this writing, all parsers that are more conformant are significantly larger, even when counting the optional validation support in this version of Ælfred.
Ælfred the Great (AElfred in ASCII) was King of Wessex, and some say of King of England, at the time of his death in 899 AD. Ælfred introduced a wide-spread literacy program in the hope that his people would learn to read English, at least, if Latin was too difficult for them. This Ælfred hopes to bring another sort of literacy to Java, using XML, at least, if full SGML is too difficult.
The initial Æ ligature ("AE)" is also a reminder that XML is not limited to ASCII.
The Ælfred parser currently builds in support for a handful of input encodings. Of course these include UTF-8 and UTF-16, which all XML parsers are required to support:
If you use any encoding other than UTF-8 or UTF-16 you should make sure to label your data appropriately:
<?xml version="1.0" encoding="ISO-8859-15"?>
Encodings accessed through
are now fully supported for both external labels (such as MIME types)
and internal types (as shown above).
There is one limitation in the support for internal labels:
the encodings must be derived from the US-ASCII encoding,
the EBCDIC family of encodings is not recognized.
Note that Java defines its
own encoding names, which don't always correspond to the standard
Internet encoding names defined by the IETF/IANA, and that Java
may even require use of nonstandard encoding names.
such problems; some of them can be worked around in this parser,
and many can be worked around by using external labels.
Note that if you are using the Euro symbol with an fixed length eight bit encoding, you should probably be using the encoding label iso-8859-15 or, with a Microsoft OS, cp-1252. Of course, UTF-8 and UTF-16 handle the Euro symbol directly.
Known conformance issues should be of negligible importance for most applications, and include:
When tested against the July 12, 1999 version of the OASIS XML Conformance test suite, an earlier version passed 1057 of 1067 tests. That contrasts with the original version, which passed 867. The current parser is top-ranked in terms of conformance, as is its validating sibling (which has some additional conformance violations imposed on it by SAX2 API deficiencies as well as some of the more curious SGML layering artifacts found in the XML specification).
The XML 1.0 specification itself was not without problems, and after some delays the W3C has come out with a revised "second edition" specification. While that doesn't resolve all the problems identified the XML specification, many of the most egregious problems have been resolved. (You still need to drink magic Kool-Aid before some DTD-related issues make sense.) To the extent possible, this parser conforms to that second edition specification, and does well against corrected versions of the OASIS/NIST XML conformance test cases. See http://xmlconf.sourceforge.net for more information about SAX2/XML conformance testing.
As noted above, the original distribution was more or less public domain. The license had the constraint that modifications be clearly documented, as has been done here.
This version is Copyright (C) 1999,2000,2001 The Free Software Foundation, and all the modifications are distributed under the GNU General Public License (GPL). It is subject to the "Library Exception", supporting use in some environments (such as embedded systems where dynamic linking may not be available) by proprietary code without necessarily requiring all code to be licensed under the GPL.
As noted above, Microstar has not updated this parser since the summer of 1998, when it released version 1.2a on its web site. This release is intended to benefit the developer community by refocusing the API on SAX2, and improving conformance to the extent that most developers should not need to use another XML parser.
The code has been cleaned up (referring to the XML 1.0 spec in all the production numbers in comments, rather than some preliminary draft, for one example) and has been sped up a bit as well. JAXP support has been added, although developers are still strongly encouraged to use the SAX2 APIs directly.
The original version of Ælfred did not support the SAX2 APIs.
This version supports the SAX2 APIs, exposing the standard boolean feature descriptors. It supports the "DeclHandler" property to provide access to all DTD declarations not already exposed through the SAX1 API. The "LexicalHandler" property is supported, exposing entity boundaries (including the unnamed external subset) and things like comments and CDATA boundaries. SAX1 compatibility is currently provided.
In the 'pipeline' package in this same software distribution is an XML Validation component using any full SAX2 event stream (including all document type declarations) to validate. There is now a XmlReader class which combines that class and this enhanced Ælfred parser, creating an optionally validating SAX2 parser.
As noted in the documentation for that validating component, certain validity constraints can't reliably be tested by a layered validator. These include all constraints relying on layering violations (exposing XML at the level of tokens or below, required since XML isn't a context-free grammar), some that SAX2 doesn't support, and a few others. The resulting validating parser is conformant enough for most applications that aren't doing strange SGML tricks with DTDs. Moreover, that validating filter can be used without a parser ... any application component that emits SAX event streams can DTD-validate its output on demand.
You'll have noticed that the original version of Ælfred had small size as a top goal. Ælfred2 normally includes a DTD validation layer, but you can package without that. Similarly, JAXP factory support is available but optional. Then the main added cost due to this revision are for supporting the SAX2 API itself; DTD validation is as cleanly layered as allowed by SAX2.
Bugs fixed in Ælfred2 include:
Other bugs may also have been fixed.
For better overall validation support, some of the validity constraints that can't be verified using the SAX2 event stream are now reported directly by Ælfred2.
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