274 lines
11 KiB
HTML
274 lines
11 KiB
HTML
<html>
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<body>
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<p>
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This is a Free Software DOM Level 3 implementation, supporting these features:
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<ul>
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<li>"XML"</li>
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<li>"Events"</li>
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<li>"MutationEvents"</li>
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<li>"HTMLEvents" (won't generate them though)</li>
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<li>"UIEvents" (also won't generate them)</li>
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<li>"USER-Events" (a conformant extension)</li>
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<li>"Traversal" (optional)</li>
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<li>"XPath"</li>
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<li>"LS" and "LS-Async"</li>
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</ul>
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It is intended to be a reasonable base both for
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experimentation and supporting additional DOM modules as clean layers.
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</p>
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<p>
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Note that while DOM does not specify its behavior in the
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face of concurrent access, this implementation does.
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Specifically:
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<ul>
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<li>If only one thread at a time accesses a Document,
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of if several threads cooperate for read-only access,
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then no concurrency conflicts will occur.</li>
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<li>If several threads mutate a given document
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(or send events using it) at the same time,
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there is currently no guarantee that
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they won't interfere with each other.</li>
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</ul>
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</p>
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<h3>Design Goals</h3>
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<p>
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A number of DOM implementations are available in Java, including
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commercial ones from Sun, IBM, Oracle, and DataChannel as well as
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noncommercial ones from Docuverse, OpenXML, and Silfide. Why have
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another? Some of the goals of this version:
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</p>
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<ul>
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<li>Advanced DOM support. This was the first generally available
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implementation of DOM Level 2 in Java, and one of the first Level 3
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and XPath implementations.</li>
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<li> Free Software. This one is distributed under the GPL (with
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"library exception") so it can be used with a different class of
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application.</li>
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<li>Second implementation syndrome. I can do it simpler this time
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around ... and heck, writing it only takes a bit over a day once you
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know your way around.</li>
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<li>Sanity check the then-current Last Call DOM draft. Best to find
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bugs early, when they're relatively fixable. Yes, bugs were found.</li>
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<li>Modularity. Most of the implementations mentioned above are part
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of huge packages; take all (including bugs, of which some have far
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too many), or take nothing. I prefer a menu approach, when possible.
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This code is standalone, not beholden to any particular parser or XSL
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or XPath code.</li>
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<li>OK, I'm a hacker, I like to write code.</li>
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</ul>
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<p>
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This also works with the GNU Compiler for Java (GCJ). GCJ promises
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to be quite the environment for programming Java, both directly and from
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C++ using the new CNI interfaces (which really use C++, unlike JNI). </p>
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<h3>Open Issues</h3>
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<p>At this writing:</p>
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<ul>
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<li>See below for some restrictions on the mutation event
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support ... some events aren't reported (and likely won't be).</li>
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<li>More testing and conformance work is needed.</li>
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<li>We need an XML Schema validator (actually we need validation in the DOM
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full stop).</li>
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</ul>
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<p>
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I ran a profiler a few times and remove some of the performance hotspots,
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but it's not tuned. Reporting mutation events, in particular, is
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rather costly -- it started at about a 40% penalty for appendNode calls,
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I've got it down around 12%, but it'll be hard to shrink it much further.
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The overall code size is relatively small, though you may want to be rid of
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many of the unused DOM interface classes (HTML, CSS, and so on).
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</p>
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<h2><a name="features">Features of this Package</a></h2>
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<p> Starting with DOM Level 2, you can really see that DOM is constructed
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as a bunch of optional modules around a core of either XML or HTML
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functionality. Different implementations will support different optional
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modules. This implementation provides a set of features that should be
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useful if you're not depending on the HTML functionality (lots of convenience
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functions that mostly don't buy much except API surface area) and user
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interface support. That is, browsers will want more -- but what they
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need should be cleanly layered over what's already here. </p>
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<h3> Core Feature Set: "XML" </h3>
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<p> This DOM implementation supports the "XML" feature set, which basically
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gets you four things over the bare core (which you're officially not supposed
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to implement except in conjunction with the "XML" or "HTML" feature). In
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order of decreasing utility, those four things are: </p> <ol>
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<li> ProcessingInstruction nodes. These are probably the most
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valuable thing. Handy little buggers, in part because all the APIs
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you need to use them are provided, and they're designed to let you
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escape XML document structure rules in controlled ways.</li>
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<li> CDATASection nodes. These are of of limited utility since CDATA
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is just text that prints funny. These are of use to some sorts of
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applications, though I encourage folk to not use them. </li>
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<li> DocumentType nodes, and associated Notation and Entity nodes.
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These appear to be useless. Briefly, these "Type" nodes expose no
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typing information. They're only really usable to expose some lexical
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structure that almost every application needs to ignore. (XML editors
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might like to see them, but they need true typing information much more.)
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I strongly encourage people not to use these. </li>
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<li> EntityReference nodes can show up. These are actively annoying,
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since they add an extra level of hierarchy, are the cause of most of
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the complexity in attribute values, and their contents are immutable.
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Avoid these.</li>
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</ol>
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<h3> Optional Feature Sets: "Events", and friends </h3>
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<p> Events may be one of the more interesting new features in Level 2.
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This package provides the core feature set and exposes mutation events.
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No gooey events though; if you want that, write a layered implementation! </p>
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<p> Three mutation events aren't currently generated:</p> <ul>
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<li> <em>DOMSubtreeModified</em> is poorly specified. Think of this
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as generating one such event around the time of finalization, which
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is a fully conformant implementation. This implementation is exactly
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as useful as that one. </li>
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<li> <em>DOMNodeRemovedFromDocument</em> and
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<em>DOMNodeInsertedIntoDocument</em> are supposed to get sent to
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every node in a subtree that gets removed or inserted (respectively).
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This can be <em>extremely costly</em>, and the removal and insertion
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processing is already significantly slower due to event reporting.
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It's much easier, and more efficient, to have a listener higher in the
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tree watch removal and insertion events through the bubbling or capture
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mechanisms, than it is to watch for these two events.</li>
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</ul>
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<p> In addition, certain kinds of attribute modification aren't reported.
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A fix is known, but it couldn't report the previous value of the attribute.
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More work could fix all of this (as well as reduce the generally high cost
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of childful attributes), but that's not been done yet. </p>
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<p> Also, note that it is a <em>Bad Thing™</em> to have the listener
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for a mutation event change the ancestry for the target of that event.
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Or to prevent mutation events from bubbling to where they're needed.
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Just don't do those, OK? </p>
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<p> As an experimental feature (named "USER-Events"), you can provide
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your own "user" events. Just name them anything starting with "USER-"
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and you're set. Dispatch them through, bubbling, capturing, or what
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ever takes your fancy. One important thing you can't currently do is
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pass any data (like an object) with those events. Maybe later there
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will be a "UserEvent" interface letting you get some substantial use
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out of this mechanism even if you're not "inside" of a DOM package.</p>
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<p> You can create and send HTML events. Ditto UIEvents. Since DOM
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doesn't require a UI, it's the UI's job to send them; perhaps that's
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part of your application. </p>
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<p><em>This package may be built without the ability to report mutation
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events, gaining a significant speedup in DOM construction time. However,
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if that is done then certain other features -- notably node iterators
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and getElementsByTagname -- will not be available.</em>
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<h3> Optional Feature: "Traversal" </h3>
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<p> Each DOM node has all you need to walk to everything connected
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to that node. Lightweight, efficient utilities are easily layered on
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top of just the core APIs. </p>
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<p> Traversal APIs are an optional part of DOM Level 2, providing
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a not-so-lightweight way to walk over DOM trees, if your application
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didn't already have such utilities for use with data represented via
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DOM. Implementing this helped debug the (optional) event and mutation
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event subsystems, so it's provided here. </p>
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<p> At this writing, the "TreeWalker" interface isn't implemented. </p>
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<h2><a name='avoid'>DOM Functionality to Avoid</a></h2>
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<p> For what appear to be a combination of historical and "committee
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logic" reasons, DOM has a number of <em>features which I strongly advise
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you to avoid using</em> in your library and application code. These
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include the following types of DOM nodes; see the documentation for the
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implementation class for more information: <ul>
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<li> CDATASection
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(<a href='DomCDATA.html'>DomCDATA</a> class)
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... use normal Text nodes instead, so you don't have to make
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every algorithm recognize multiple types of character data
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<li> DocumentType
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(<a href='DomDoctype.html'>DomDocType</a> class)
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... if this held actual typing information, it might be useful
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<li> Entity
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(<a href='DomEntity.html'>DomEntity</a> class)
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... neither parsed nor unparsed entities work well in DOM; it
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won't even tell you which attributes identify unparsed entities
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<li> EntityReference
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(<a href='DomEntityReference.html'>DomEntityReference</a> class)
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... permitted implementation variances are extreme, all children
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are readonly, and these can interact poorly with namespaces
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<li> Notation
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(<a href='DomNotation.html'>DomNotation</a> class)
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... only really usable with unparsed entities (which aren't well
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supported; see above) or perhaps with PIs after the DTD, not with
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NOTATION attributes
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</ul>
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<p> If you really need to use unparsed entities or notations, use SAX;
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it offers better support for all DTD-related functionality.
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It also exposes actual
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document typing information (such as element content models).</p>
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<p> Also, when accessing attribute values, use methods that provide their
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values as single strings, rather than those which expose value substructure
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(Text and EntityReference nodes). (See the <a href='DomAttr.html'>DomAttr</a>
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documentation for more information.) </p>
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<p> Note that many of these features were provided as partial support for
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editor functionality (including the incomplete DTD access). Full editor
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functionality requires access to potentially malformed lexical structure,
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at the level of unparsed tokens and below. Access at such levels is so
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complex that using it in non-editor applications sacrifices all the
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benefits of XML; editor aplications need extremely specialized APIs. </p>
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<p> (This isn't a slam against DTDs, note; only against the broken support
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for them in DOM. Even despite inclusion of some dubious SGML legacy features
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such as notations and unparsed entities,
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and the ongoing proliferation of alternative schema and validation tools,
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DTDs are still the most widely adopted tool
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to constrain XML document structure.
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Alternative schemes generally focus on data transfer style
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applications; open document architectures comparable to
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DocBook 4.0 don't yet exist in the schema world.
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Feel free to use DTDs; just don't expect DOM to help you.) </p>
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</body>
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</html>
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