validationconsumer.java

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/*
 * Copyright (C) 1999-2001 David Brownell
 * 
 * This file is part of GNU JAXP, a library.
 *
 * GNU JAXP is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * GNU JAXP is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * As a special exception, if you link this library with other files to
 * produce an executable, this library does not by itself cause the
 * resulting executable to be covered by the GNU General Public License.
 * This exception does not however invalidate any other reasons why the
 * executable file might be covered by the GNU General Public License. 
 */

package gnu.xml.pipeline;

import java.io.*;

import java.util.EmptyStackException;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.Vector;

import org.xml.sax.*;
import org.xml.sax.ext.*;
import org.xml.sax.helpers.XMLReaderFactory;


/**
 * This class checks SAX2 events to report validity errors; it works as
 * both a filter and a terminus on an event pipeline.  It relies on the
 * producer of SAX events to:  </p> <ol>
 *
 *	<li> Conform to the specification of a non-validating XML parser that
 *	reads all external entities, reported using SAX2 events. </li>
 *
 *	<li> Report ignorable whitespace as such (through the ContentHandler
 *	interface).  This is, strictly speaking, optional for nonvalidating
 *	XML processors.  </li>
 *
 *	<li> Make SAX2 DeclHandler callbacks, with default
 *	attribute values already normalized (and without "&lt;").</li>
 *
 *	<li> Make SAX2 LexicalHandler startDTD() and endDTD ()
 *	callbacks. </li>
 *
 *	<li> Act as if the <em>(URI)/namespace-prefixes</em> property were
 *	set to true, by providing XML 1.0 names and all <code>xmlns*</code>
 *	attributes (rather than omitting either or both). </li>
 *
 *	</ol>
 *
 * <p> At this writing, the major SAX2 parsers (such as &AElig;lfred2,
 * Crimson, and Xerces) meet these requirements, and this validation
 * module is used by the optional &AElig;lfred2 validation support.
 * </p>
 *
 * <p> Note that because this is a layered validator, it has to duplicate some
 * work that the parser is doing; there are also other cost to layering.
 * However, <em>because of layering it doesn't need a parser</em> in order
 * to work! You can use it with anything that generates SAX events, such
 * as an application component that wants to detect invalid content in
 * a changed area without validating an entire document, or which wants to
 * ensure that it doesn't write invalid data to a communications partner.</p>
 *
 * <p> Also, note that because this is a layered validator, the line numbers
 * reported for some errors may seem strange.  For example, if an element does
 * not permit character content, the validator
 * will use the locator provided to it.
 * That might reflect the last character of a <em>characters</em> event
 * callback, rather than the first non-whitespace character. </p>
 *
 * <hr />
 *
 * <!--
 * <p> Of interest is the fact that unlike most currently known XML validators,
 * this one can report some cases of non-determinism in element content models.
 * It is a compile-time option, enabled by default.  This will only report
 * such XML errors if they relate to content actually appearing in a document;
 * content models aren't aggressively scanned for non-deterministic structure.
 * Documents which trigger such non-deterministic transitions may be handled
 * differently by different validating parsers, without losing conformance
 * to the XML specification. </p>
 * -->
 *
 * <p> Current limitations of the validation performed are in roughly three
 * categories.  </p>
 *
 * <p> The first category represents constraints which demand violations
 * of software layering:  exposing lexical details, one of the first things
 * that <em>application</em> programming interfaces (APIs) hide.  These
 * invariably relate to XML entity handling, and to historical oddities
 * of the XML validation semantics.  Curiously,
 * recent (Autumn 1999) conformance testing showed that these constraints are
 * among those handled worst by existing XML validating parsers.  Arguments
 * have been made that each of these VCs should be turned into WFCs (most
 * of them) or discarded (popular for the standalone declaration); in short,
 * that these are bugs in the XML specification (not all via SGML): </p><ul>
 *
 *	<li> The <em>Proper Declaration/PE Nesting</em> and
 *	<em>Proper Group/PE Nesting</em> VCs can't be tested because they
 *	require access to particularly low level lexical level information.
 *	In essence, the reason XML isn't a simple thing to parse is that
 *	it's not a context free grammar, and these constraints elevate that
 *	SGML-derived context sensitivity to the level of a semantic rule.
 *
 *	<li> The <em>Standalone Document Declaration</em> VC can't be
 *	tested.  This is for two reasons.  First, this flag isn't made
 *	available through SAX2.  Second, it also requires breaking that
 *	lexical layering boundary.  (If you ever wondered why classes
 *	in compiler construction or language design barely mention the
 *	existence of context-sensitive grammars, it's because of messy
 *	issues like these.)
 *
 *	<li> The <em>Entity Declared</em> VC can't be tested, because it
 *	also requires breaking that lexical layering boundary!  There's also
 *	another issue: the VC wording (and seemingly intent) is ambiguous.
 *	(This is still true in the "Second edition" XML spec.)
 *	Since there is a WFC of the same name, everyone's life would be
 *	easier if references to undeclared parsed entities were always well
 *	formedness errors, regardless of whether they're parameter entities
 *	or not.  (Note that nonvalidating parsers are not required
 *	to report all such well formedness errors if they don't read external
 *	parameter entities, although currently most XML parsers read them
 *	in an attempt to avoid problems from inconsistent parser behavior.)
 *
 *	</ul>
 *
 * <p> The second category of limitations on this validation represent
 * constraints associated with information that is not guaranteed to be
 * available (or in one case, <em>is guaranteed not to be available</em>,
 * through the SAX2 API: </p><ul>
 *
 *	<li> The <em>Unique Element Type Declaration</em> VC may not be
 *	reportable, if the underlying parser happens not to expose
 *	multiple declarations.   (&AElig;lfred2 reports these validity
 *	errors directly.)</li>
 *
 *	<li> Similarly, the <em>Unique Notation Name</em> VC, added in the
 *	14-January-2000 XML spec errata to restrict typing models used by
 *	elements, may not be reportable.  (&AElig;lfred reports these
 *	validity errors directly.) </li>
 *
 *	</ul>
 *
 * <p> A third category relates to ease of implementation.  (Think of this
 * as "bugs".)  The most notable issue here is character handling.  Rather
 * than attempting to implement the voluminous character tables in the XML
 * specification (Appendix B), Unicode rules are used directly from
 * the java.lang.Character class.  Recent JVMs have begun to diverge from
 * the original specification for that class (Unicode 2.0), meaning that
 * different JVMs may handle that aspect of conformance differently.
 * </p>
 *
 * <p> Note that for some of the validity errors that SAX2 does not
 * expose, a nonvalidating parser is permitted (by the XML specification)
 * to report validity errors.  When used with a parser that does so for
 * the validity constraints mentioned above (or any other SAX2 event
 * stream producer that does the same thing), overall conformance is
 * substantially improved.
 *
 * @see gnu.xml.aelfred2.SAXDriver
 * @see gnu.xml.aelfred2.XmlReader
 *
 * @author David Brownell
 */
public final class ValidationConsumer extends EventFilter
{
    // report error if we happen to notice a non-deterministic choice?
    // we won't report buggy content models; just buggy instances
    private static final boolean	warnNonDeterministic = false;

    // for tracking active content models
    private String		rootName;
    private Stack		contentStack = new Stack ();

    // flags for "saved DTD" processing
    private boolean		disableDeclarations;
    private boolean		disableReset;

    //
    // most VCs get tested when we see element start tags.  the per-element
    // info (including attributes) recorded here duplicates that found inside
    // many nonvalidating parsers, hence dual lookups etc ... that's why a
    // layered validator isn't going to be as fast as a non-layered one.
    //

    // key = element name; value = ElementInfo
    private Hashtable		elements = new Hashtable ();

    // some VCs relate to ID/IDREF/IDREFS attributes
    // key = id; value = boolean true (defd) or false (refd)
    private Hashtable		ids = new Hashtable ();

    // we just record declared notation and unparsed entity names.
    // the implementation here is simple/slow; these features
    // are seldom used, one hopes they'll wither away soon
    private Vector		notations = new Vector (5, 5);
    private Vector		nDeferred = new Vector (5, 5);
    private Vector		unparsed = new Vector (5, 5);
    private Vector		uDeferred = new Vector (5, 5);
	
	// note: DocBk 3.1.7 XML defines over 2 dozen notations,
	// used when defining unparsed entities for graphics
	// (and maybe in other places)

    

    /**
     * Creates a pipeline terminus which consumes all events passed to
     * it; this will report validity errors as if they were fatal errors,
     * unless an error handler is assigned.
     *
     * @see #setErrorHandler
     */
	// constructor used by PipelineFactory
	    // ... and want one taking system ID of an external subset
    public ValidationConsumer ()
    {
	this (null);
    }

    /**
     * Creates a pipeline filter which reports validity errors and then
     * passes events on to the next consumer if they were not fatal.
     *
     * @see #setErrorHandler
     */
	// constructor used by PipelineFactory
	    // ... and want one taking system ID of an external subset
	    // (which won't send declaration events)
    public ValidationConsumer (EventConsumer next)
    {
	super (next);

	setContentHandler (this);
	setDTDHandler (this);
	try { setProperty (DECL_HANDLER, this); }
	catch (Exception e) { /* "can't happen" */ }
	try { setProperty (LEXICAL_HANDLER, this); }
	catch (Exception e) { /* "can't happen" */ }
    }

    
    private static final String	fakeRootName
	= ":Nobody:in:their_Right.Mind_would:use:this-name:1x:";
    
    /**
     * Creates a validation consumer which is preloaded with the DTD provided.
     * It does this by constructing a document with that DTD, then parsing
     * that document and recording its DTD declarations.  Then it arranges
     * not to modify that information.
     *
     * <p> The resulting validation consumer will only validate against
     * the specified DTD, regardless of whether some other DTD is found
     * in a document being parsed.
     *
     * @param rootName The name of the required root element; if this is
     *	null, any root element name will be accepted.
     * @param publicId If non-null and there is a non-null systemId, this
     *	identifier provides an alternate access identifier for the DTD's
     *	external subset.
     * @param systemId If non-null, this is a URI (normally URL) that
     *	may be used to access the DTD's external subset.
     * @param internalSubset If non-null, holds literal markup declarations
     *	comprising the DTD's internal subset.
     * @param resolver If non-null, this will be provided to the parser for
     *	use when resolving parameter entities (including any external subset).
     * @param resolver If non-null, this will be provided to the parser for
     *	use when resolving parameter entities (including any external subset).
     * @param minimalElement If non-null, a minimal valid document.
     *
     * @exception SAXNotSupportedException If the default SAX parser does
     *	not support the standard lexical or declaration handlers.
     * @exception SAXParseException If the specified DTD has either
     *	well-formedness or validity errors
     * @exception IOException If the specified DTD can't be read for
     *	some reason
     */
    public ValidationConsumer (
	String		rootName,
	String		publicId,
	String		systemId,
	String		internalSubset,
	EntityResolver	resolver,
	String		minimalDocument
    ) throws SAXException, IOException
    {
	this (null);

	disableReset = true;
	if (rootName == null)
	    rootName = fakeRootName;

	//
	// Synthesize document with that DTD; is it possible to do
	// better for the declaration of the root element?
	//
	// NOTE:  can't use SAX2 to write internal subsets.
	//
	StringWriter	writer = new StringWriter ();

	writer.write ("<!DOCTYPE ");
	writer.write (rootName);
	if (systemId != null) {
	    writer.write ("\n  ");
	    if (publicId != null) {
		writer.write ("PUBLIC '");
		writer.write (publicId);
		writer.write ("'\n\t'");
	    } else
		writer.write ("SYSTEM '");
	    writer.write (systemId);
	    writer.write ("'");
	}
	writer.write (" [ ");
	if (rootName == fakeRootName) {
	    writer.write ("\n<!ELEMENT ");
	    writer.write (rootName);
	    writer.write (" EMPTY>");
	}
	if (internalSubset != null)
	    writer.write (internalSubset);
	writer.write ("\n ]>");

	if (minimalDocument != null) {
	    writer.write ("\n");
	    writer.write (minimalDocument);
	    writer.write ("\n");
	} else {
	    writer.write (" <");
	    writer.write (rootName);
	    writer.write ("/>\n");
	}
	minimalDocument = writer.toString ();

	//
	// OK, load it
	//
	XMLReader	producer;

	producer = XMLReaderFactory.createXMLReader ();
	bind (producer, this);

	if (resolver != null)
	    producer.setEntityResolver (resolver);

	InputSource	in;
	
	in = new InputSource (new StringReader (minimalDocument));
	producer.parse (in);

	disableDeclarations = true;
	if (rootName == fakeRootName)
	    this.rootName = null;
    }

    private void resetState ()
    {
	if (!disableReset) {
	    rootName = null;
	    contentStack.removeAllElements ();
	    elements.clear ();
	    ids.clear ();

	    notations.removeAllElements ();
	    nDeferred.removeAllElements ();
	    unparsed.removeAllElements ();
	    uDeferred.removeAllElements ();
	}
    }


    private void warning (String description)
    throws SAXException
    {
	ErrorHandler		errHandler = getErrorHandler ();
	Locator			locator = getDocumentLocator ();
	SAXParseException	err;

	if (errHandler == null)
	    return;

	if (locator == null)
	    err = new SAXParseException (description, null, null, -1, -1);
	else
	    err = new SAXParseException (description, locator);
	errHandler.warning (err);
    }

    // package private (for ChildrenRecognizer)
    private void error (String description)
    throws SAXException
    {
	ErrorHandler		errHandler = getErrorHandler ();
	Locator			locator = getDocumentLocator ();
	SAXParseException	err;

	if (locator == null)
	    err = new SAXParseException (description, null, null, -1, -1);
	else
	    err = new SAXParseException (description, locator);
	if (errHandler != null)
	    errHandler.error (err);
	else	// else we always treat it as fatal!
	    throw err;
    }

    private void fatalError (String description)
    throws SAXException
    {
	ErrorHandler		errHandler = getErrorHandler ();
	Locator			locator = getDocumentLocator ();
	SAXParseException	err;

	if (locator != null)
	    err = new SAXParseException (description, locator);
	else
	    err = new SAXParseException (description, null, null, -1, -1);
	if (errHandler != null)
	    errHandler.fatalError (err);
	// we always treat this as fatal, regardless of the handler
	throw err;
    }


    private static boolean isExtender (char c)
    {
	// [88] Extender ::= ...
	return c == 0x00b7 || c == 0x02d0 || c == 0x02d1 || c == 0x0387
	       || c == 0x0640 || c == 0x0e46 || c == 0x0ec6 || c == 0x3005
	       || (c >= 0x3031 && c <= 0x3035)
	       || (c >= 0x309d && c <= 0x309e)
	       || (c >= 0x30fc && c <= 0x30fe);
    }


    // use augmented Unicode rules, not full XML rules
    private boolean isName (String name, String context, String id)
    throws SAXException
    {
	char	buf [] = name.toCharArray ();
	boolean	pass = true;

	if (!Character.isUnicodeIdentifierStart (buf [0])
		&& ":_".indexOf (buf [0]) == -1)
	    pass = false;
	else {
	    int max = buf.length;
	    for (int i = 1; pass && i < max; i++) {
		char c = buf [i];
		if (!Character.isUnicodeIdentifierPart (c)
			&& ":-_.".indexOf (c) == -1
			&& !isExtender (c))
		    pass = false;
	    }
	}

	if (!pass)
	    error ("In " + context + " for " + id
		+ ", '" + name + "' is not a name");
	return pass;	// true == OK
    }