xmlreader.html

xml开源解析代码.版本为libxml2-2.6.29,可支持GB3212.网络消息发送XML时很有用.

<pre>def processNode(reader):
    print "%d %d %s %d %s" % (reader.Depth(), reader.NodeType(),
                              reader.Name(), reader.IsEmptyElement(),
                              reader.Value())</pre>

<p>The result of the test is:</p>
<pre>0 1 doc 0 None
1 1 a 1 None
1 1 b 0 None
2 3 #text 0 some text
1 15 b 0 None
1 3 #text 0

1 1 c 1 None
0 15 doc 0 None</pre>

<p>There are a few things to note:</p>
<ul>
  <li>the increase of the depth value (first row) as children nodes are
    explored</li>
  <li>the text node child of the b element, of type 3 and its content</li>
  <li>the text node containing the line return between elements b and c</li>
  <li>that elements have the Value None (or NULL in C)</li>
</ul>

<p>The equivalent routine for <code>processNode()</code> as used by
<code>xmllint --stream --debug</code> is the following and can be found in
the xmllint.c module in the source distribution:</p>
<pre>static void processNode(xmlTextReaderPtr reader) {
    xmlChar *name, *value;

    name = xmlTextReaderName(reader);
    if (name == NULL)
        name = xmlStrdup(BAD_CAST "--");
    value = xmlTextReaderValue(reader);

    printf("%d %d %s %d",
            xmlTextReaderDepth(reader),
            xmlTextReaderNodeType(reader),
            name,
            xmlTextReaderIsEmptyElement(reader));
    xmlFree(name);
    if (value == NULL)
        printf("\n");
    else {
        printf(" %s\n", value);
        xmlFree(value);
    }
}</pre>

<h2><a name="Extracting1">Extracting information for the attributes</a></h2>

<p>The previous examples don't indicate how attributes are processed. The
simple test "<code>&lt;doc a="b"/&gt;</code>" provides the following
result:</p>
<pre>0 1 doc 1 None</pre>

<p>This proves that attribute nodes are not traversed by default. The
<em>HasAttributes</em> property allow to detect their presence. To check
their content the API has special instructions. Basically two kinds of operations
are possible:</p>
<ol>
  <li>to move the reader to the attribute nodes of the current element, in
    that case the cursor is positionned on the attribute node</li>
  <li>to directly query the element node for the attribute value</li>
</ol>

<p>In both case the attribute can be designed either by its position in the
list of attribute (<em>MoveToAttributeNo</em> or <em>GetAttributeNo</em>) or
by their name (and namespace):</p>
<ul>
  <li><em>GetAttributeNo</em>(no): provides the value of the attribute with
    the specified index no relative to the containing element.</li>
  <li><em>GetAttribute</em>(name): provides the value of the attribute with
    the specified qualified name.</li>
  <li>GetAttributeNs(localName, namespaceURI): provides the value of the
    attribute with the specified local name and namespace URI.</li>
  <li><em>MoveToAttributeNo</em>(no): moves the position of the current
    instance to the attribute with the specified index relative to the
    containing element.</li>
  <li><em>MoveToAttribute</em>(name): moves the position of the current
    instance to the attribute with the specified qualified name.</li>
  <li><em>MoveToAttributeNs</em>(localName, namespaceURI): moves the position
    of the current instance to the attribute with the specified local name
    and namespace URI.</li>
  <li><em>MoveToFirstAttribute</em>: moves the position of the current
    instance to the first attribute associated with the current node.</li>
  <li><em>MoveToNextAttribute</em>: moves the position of the current
    instance to the next attribute associated with the current node.</li>
  <li><em>MoveToElement</em>: moves the position of the current instance to
    the node that contains the current Attribute  node.</li>
</ul>

<p>After modifying the processNode() function to show attributes:</p>
<pre>def processNode(reader):
    print "%d %d %s %d %s" % (reader.Depth(), reader.NodeType(),
                              reader.Name(), reader.IsEmptyElement(),
                              reader.Value())
    if reader.NodeType() == 1: # Element
        while reader.MoveToNextAttribute():
            print "-- %d %d (%s) [%s]" % (reader.Depth(), reader.NodeType(),
                                          reader.Name(),reader.Value())</pre>

<p>The output for the same input document reflects the attribute:</p>
<pre>0 1 doc 1 None
-- 1 2 (a) [b]</pre>

<p>There are a couple of things to note on the attribute processing:</p>
<ul>
  <li>Their depth is the one of the carrying element plus one.</li>
  <li>Namespace declarations are seen as attributes, as in DOM.</li>
</ul>

<h2><a name="Validating">Validating a document</a></h2>

<p>Libxml2 implementation adds some extra features on top of the XmlTextReader
API. The main one is the ability to DTD validate the parsed document
progressively. This is simply the activation of the associated feature of the
parser used by the reader structure. There are a few options available
defined as the enum xmlParserProperties in the libxml/xmlreader.h header
file:</p>
<ul>
  <li>XML_PARSER_LOADDTD: force loading the DTD (without validating)</li>
  <li>XML_PARSER_DEFAULTATTRS: force attribute defaulting (this also imply
    loading the DTD)</li>
  <li>XML_PARSER_VALIDATE: activate DTD validation (this also imply loading
    the DTD)</li>
  <li>XML_PARSER_SUBST_ENTITIES: substitute entities on the fly, entity
    reference nodes are not generated and are replaced by their expanded
    content.</li>
  <li>more settings might be added, those were the one available at the 2.5.0
    release...</li>
</ul>

<p>The GetParserProp() and SetParserProp() methods can then be used to get
and set the values of those parser properties of the reader. For example</p>
<pre>def parseAndValidate(file):
    reader = libxml2.newTextReaderFilename(file)
    reader.SetParserProp(libxml2.PARSER_VALIDATE, 1)
    ret = reader.Read()
    while ret == 1:
        ret = reader.Read()
    if ret != 0:
        print "Error parsing and validating %s" % (file)</pre>

<p>This routine will parse and validate the file. Error messages can be
captured by registering an error handler. See python/tests/reader2.py for
more complete Python examples. At the C level the equivalent call to cativate
the validation feature is just:</p>
<pre>ret = xmlTextReaderSetParserProp(reader, XML_PARSER_VALIDATE, 1)</pre>

<p>and a return value of 0 indicates success.</p>

<h2><a name="Entities">Entities substitution</a></h2>

<p>By default the xmlReader will report entities as such and not replace them
with their content. This default behaviour can however be overriden using:</p>

<p><code>reader.SetParserProp(libxml2.PARSER_SUBST_ENTITIES,1)</code></p>

<h2><a name="L1142">Relax-NG Validation</a></h2>

<p style="font-size: 10pt">Introduced in version 2.5.7</p>

<p>Libxml2 can now validate the document being read using the xmlReader using
Relax-NG schemas. While the Relax NG validator can't always work in a
streamable mode, only subsets which cannot be reduced to regular expressions
need to have their subtree expanded for validation. In practice it means
that, unless the schemas for the top level element content is not expressable
as a regexp, only chunk of the document needs to be parsed while
validating.</p>

<p>The steps to do so are:</p>
<ul>
  <li>create a reader working on a document as usual</li>
  <li>before any call to read associate it to a Relax NG schemas, either the
    preparsed schemas or the URL to the schemas to use</li>
  <li>errors will be reported the usual way, and the validity status can be
    obtained using the IsValid() interface of the reader like for DTDs.</li>
</ul>

<p>Example, assuming the reader has already being created and that the schema
string contains the Relax-NG schemas:</p>
<pre><code>rngp = libxml2.relaxNGNewMemParserCtxt(schema, len(schema))<br>
rngs = rngp.relaxNGParse()<br>
reader.RelaxNGSetSchema(rngs)<br>
ret = reader.Read()<br>
while ret == 1:<br>
    ret = reader.Read()<br>
if ret != 0:<br>
    print "Error parsing the document"<br>
if reader.IsValid() != 1:<br>
    print "Document failed to validate"</code><br>
</pre>

<p>See <code>reader6.py</code> in the sources or documentation for a complete
example.</p>

<h2><a name="Mixing">Mixing the reader and tree or XPath operations</a></h2>

<p style="font-size: 10pt">Introduced in version 2.5.7</p>

<p>While the reader is a streaming interface, its underlying implementation
is based on the DOM builder of libxml2. As a result it is relatively simple
to mix operations based on both models under some constraints. To do so the
reader has an Expand() operation allowing to grow the subtree under the
current node. It returns a pointer to a standard node which can be
manipulated in the usual ways. The node will get all its ancestors and the
full subtree available. Usual operations like XPath queries can be used on
that reduced view of the document. Here is an example extracted from
reader5.py in the sources which extract and prints the bibliography for the
"Dragon" compiler book from the XML 1.0 recommendation:</p>
<pre>f = open('../../test/valid/REC-xml-19980210.xml')
input = libxml2.inputBuffer(f)
reader = input.newTextReader("REC")
res=""
while reader.Read():
    while reader.Name() == 'bibl':
        node = reader.Expand()            # expand the subtree
        if node.xpathEval("@id = 'Aho'"): # use XPath on it
            res = res + node.serialize()
        if reader.Next() != 1:            # skip the subtree
            break;</pre>

<p>Note, however that the node instance returned by the Expand() call is only
valid until the next Read() operation. The Expand() operation does not
affects the Read() ones, however usually once processed the full subtree is
not useful anymore, and the Next() operation allows to skip it completely and
process to the successor or return 0 if the document end is reached.</p>

<p><a href="mailto:[e-mail]">Daniel Veillard</a></p>

<p>$Id: xmlreader.html 3320 2005-10-18 19:11:55Z veillard $</p>

<p></p>
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