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diff --git a/libjava/classpath/gnu/xml/dom/package.html b/libjava/classpath/gnu/xml/dom/package.html new file mode 100644 index 000000000..fbc864a4d --- /dev/null +++ b/libjava/classpath/gnu/xml/dom/package.html @@ -0,0 +1,273 @@ +<html> +<body> + +<p> +This is a Free Software DOM Level 3 implementation, supporting these features: +<ul> +<li>"XML"</li> +<li>"Events"</li> +<li>"MutationEvents"</li> +<li>"HTMLEvents" (won't generate them though)</li> +<li>"UIEvents" (also won't generate them)</li> +<li>"USER-Events" (a conformant extension)</li> +<li>"Traversal" (optional)</li> +<li>"XPath"</li> +<li>"LS" and "LS-Async"</li> +</ul> +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: +<ul> +<li>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.</li> +<li>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.</li> +</ul> +</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>Advanced DOM support. This was the first generally available +implementation of DOM Level 2 in Java, and one of the first Level 3 +and XPath implementations.</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.</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>See below for some restrictions on the mutation event +support ... some events aren't reported (and likely won't be).</li> + +<li>More testing and conformance work is needed.</li> + +<li>We need an XML Schema validator (actually we need validation in the DOM +full stop).</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> + |