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<html><head>
<!-- $Id: package.html,v 1.1 2003-02-01 02:10:18 cbj Exp $ -->
<title>foo</title>
</head><body>
<p> This is a Free Software
DOM Level 2 implementation, supporting these features:
"XML",
"Events", "MutationEvents",
"HTMLEvents" (won't generate them though),
"UIEvents" (also won't generate them),
"USER-Events" (a conformant extension),
and
"Traversal" (optional; no TreeWalker yet).
It includes JAXP bootstrapping support, addressing the issue that
DOM (Levels 1 and 2) doesn't include bootstrapping APIs.
It is intended to be a reasonable base both for
experimentation and supporting additional DOM modules as clean layers.
</p>
<p>Note that while DOM does not specify its behavior in the
face of concurrent access, this implementation does.
Specifically:
if only one thread at a time accesses a Document,
of if several threads cooperate for read-only access,
then no concurrency conflicts will occur.
If several threads mutate a given document
(or send events using it) at the same time,
there is currently no guarantee that
they won't interfere with each other.
</p>
<h3> Design Goals </h3>
<p> A number of DOM implementations are available in Java, including
commercial ones from Sun, IBM, Oracle, and DataChannel as well as
noncommercial ones from Docuverse, OpenXML, and Silfide. Why have
another? Some of the goals of this version: </p> <ul>
<li> DOM Level 2 support. This was the first generally available
implementation of DOM Level 2 in Java. </li>
<li> Free Software. This one is distributed under the GPL (with
"library exception") so it can be used with a different class of
application. </li>
<li> Second implementation syndrome. I can do it simpler this time
around ... and heck, writing it only takes a bit over a day once you
know your way around. (Debugging is of course a different story,
since W3C hasn't provided any conformance test suite and publicly
available suites have poor coverage.) </li>
<li> Sanity check the then-current Last Call DOM draft. Best to find
bugs early, when they're relatively fixable. Yes, bugs were found.
</li>
<li> Modularity. Most of the implementations mentioned above are part
of huge packages; take all (including bugs, of which some have far
too many), or take nothing. I prefer a menu approach, when possible.
This code is standalone, not beholden to any particular parser or XSL
or XPath code. </li>
<li> OK, I'm a hacker, I like to write code. </li>
</ul>
<p> This also works with the Gnu Compiler for Java (GCJ). GCJ promises
to be quite the environment for programming Java, both directly and from
C++ using the new CNI interfaces (which really use C++, unlike JNI). </p>
<h3> Open Issues </h3>
<p> At this writing: </p> <ul>
<li> An approximation of XML rules for legal names is used (Unicode
rules with minor tweaks) rather than the huge character tables in the
XML appendix. </li>
<li> See below for some restrictions on the mutation event
support ... some events aren't reported (and likely won't be). </li>
<li> There's no implementation (yet)
for the TreeWalker traversal API.</li>
<li> More testing and conformance work is needed. </li>
</ul>
<p> I ran a profiler a few times and remove some of the performance hotspots,
but it's not tuned. Reporting mutation events, in particular, is
rather costly -- it started at about a 40% penalty for appendNode calls,
I've got it down around 12%, but it'll be hard to shrink it much further.
The overall code size is relatively small, though you may want to be rid of
many of the unused DOM interface classes (HTML, CSS, and so on).</p>
<h2><a name="features">Features of this Package</a></h2>
<p> Starting with DOM Level 2, you can really see that DOM is constructed
as a bunch of optional modules around a core of either XML or HTML
functionality. Different implementations will support different optional
modules. This implementation provides a set of features that should be
useful if you're not depending on the HTML functionality (lots of convenience
functions that mostly don't buy much except API surface area) and user
interface support. That is, browsers will want more -- but what they
need should be cleanly layered over what's already here. </p>
<h3> Core Feature Set: "XML" </h3>
<p> This DOM implementation supports the "XML" feature set, which basically
gets you four things over the bare core (which you're officially not supposed
to implement except in conjunction with the "XML" or "HTML" feature). In
order of decreasing utility, those four things are: </p> <ol>
<li> ProcessingInstruction nodes. These are probably the most
valuable thing. Handy little buggers, in part because all the APIs
you need to use them are provided, and they're designed to let you
escape XML document structure rules in controlled ways.</li>
<li> CDATASection nodes. These are of of limited utility since CDATA
is just text that prints funny. These are of use to some sorts of
applications, though I encourage folk to not use them. </li>
<li> DocumentType nodes, and associated Notation and Entity nodes.
These appear to be useless. Briefly, these "Type" nodes expose no
typing information. They're only really usable to expose some lexical
structure that almost every application needs to ignore. (XML editors
might like to see them, but they need true typing information much more.)
I strongly encourage people not to use these. </li>
<li> EntityReference nodes can show up. These are actively annoying,
since they add an extra level of hierarchy, are the cause of most of
the complexity in attribute values, and their contents are immutable.
Avoid these.</li>
</ol>
<h3> Optional Feature Sets: "Events", and friends </h3>
<p> Events may be one of the more interesting new features in Level 2.
This package provides the core feature set and exposes mutation events.
No gooey events though; if you want that, write a layered implementation! </p>
<p> Three mutation events aren't currently generated:</p> <ul>
<li> <em>DOMSubtreeModified</em> is poorly specified. Think of this
as generating one such event around the time of finalization, which
is a fully conformant implementation. This implementation is exactly
as useful as that one. </li>
<li> <em>DOMNodeRemovedFromDocument</em> and
<em>DOMNodeInsertedIntoDocument</em> are supposed to get sent to
every node in a subtree that gets removed or inserted (respectively).
This can be <em>extremely costly</em>, and the removal and insertion
processing is already significantly slower due to event reporting.
It's much easier, and more efficient, to have a listener higher in the
tree watch removal and insertion events through the bubbling or capture
mechanisms, than it is to watch for these two events.</li>
</ul>
<p> In addition, certain kinds of attribute modification aren't reported.
A fix is known, but it couldn't report the previous value of the attribute.
More work could fix all of this (as well as reduce the generally high cost
of childful attributes), but that's not been done yet. </p>
<p> Also, note that it is a <em>Bad Thing™</em> to have the listener
for a mutation event change the ancestry for the target of that event.
Or to prevent mutation events from bubbling to where they're needed.
Just don't do those, OK? </p>
<p> As an experimental feature (named "USER-Events"), you can provide
your own "user" events. Just name them anything starting with "USER-"
and you're set. Dispatch them through, bubbling, capturing, or what
ever takes your fancy. One important thing you can't currently do is
pass any data (like an object) with those events. Maybe later there
will be a "UserEvent" interface letting you get some substantial use
out of this mechanism even if you're not "inside" of a DOM package.</p>
<p> You can create and send HTML events. Ditto UIEvents. Since DOM
doesn't require a UI, it's the UI's job to send them; perhaps that's
part of your application. </p>
<p><em>This package may be built without the ability to report mutation
events, gaining a significant speedup in DOM construction time. However,
if that is done then certain other features -- notably node iterators
and getElementsByTagname -- will not be available.</em>
<h3> Optional Feature: "Traversal" </h3>
<p> Each DOM node has all you need to walk to everything connected
to that node. Lightweight, efficient utilities are easily layered on
top of just the core APIs. </p>
<p> Traversal APIs are an optional part of DOM Level 2, providing
a not-so-lightweight way to walk over DOM trees, if your application
didn't already have such utilities for use with data represented via
DOM. Implementing this helped debug the (optional) event and mutation
event subsystems, so it's provided here. </p>
<p> At this writing, the "TreeWalker" interface isn't implemented. </p>
<h2><a name='avoid'>DOM Functionality to Avoid</a></h2>
<p> For what appear to be a combination of historical and "committee
logic" reasons, DOM has a number of <em>features which I strongly advise
you to avoid using</em> in your library and application code. These
include the following types of DOM nodes; see the documentation for the
implementation class for more information: <ul>
<li> CDATASection
(<a href='DomCDATA.html'>DomCDATA</a> class)
... use normal Text nodes instead, so you don't have to make
every algorithm recognize multiple types of character data
<li> DocumentType
(<a href='DomDoctype.html'>DomDocType</a> class)
... if this held actual typing information, it might be useful
<li> Entity
(<a href='DomEntity.html'>DomEntity</a> class)
... neither parsed nor unparsed entities work well in DOM; it
won't even tell you which attributes identify unparsed entities
<li> EntityReference
(<a href='DomEntityReference.html'>DomEntityReference</a> class)
... permitted implementation variances are extreme, all children
are readonly, and these can interact poorly with namespaces
<li> Notation
(<a href='DomNotation.html'>DomNotation</a> class)
... only really usable with unparsed entities (which aren't well
supported; see above) or perhaps with PIs after the DTD, not with
NOTATION attributes
</ul>
<p> If you really need to use unparsed entities or notations, use SAX;
it offers better support for all DTD-related functionality.
It also exposes actual
document typing information (such as element content models).</p>
<p> Also, when accessing attribute values, use methods that provide their
values as single strings, rather than those which expose value substructure
(Text and EntityReference nodes). (See the <a href='DomAttr.html'>DomAttr</a>
documentation for more information.) </p>
<p> Note that many of these features were provided as partial support for
editor functionality (including the incomplete DTD access). Full editor
functionality requires access to potentially malformed lexical structure,
at the level of unparsed tokens and below. Access at such levels is so
complex that using it in non-editor applications sacrifices all the
benefits of XML; editor aplications need extremely specialized APIs. </p>
<p> (This isn't a slam against DTDs, note; only against the broken support
for them in DOM. Even despite inclusion of some dubious SGML legacy features
such as notations and unparsed entities,
and the ongoing proliferation of alternative schema and validation tools,
DTDs are still the most widely adopted tool
to constrain XML document structure.
Alternative schemes generally focus on data transfer style
applications; open document architectures comparable to
DocBook 4.0 don't yet exist in the schema world.
Feel free to use DTDs; just don't expect DOM to help you.) </p>
</body></html>
|
