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<title>HTML Parser Frequently Asked Questions</title>
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Derrick Oswald" />
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<div class="section"><h2>Frequently Asked Questions</h2>
<ul>
<li><a href="#encodingchangeexception">Why am I getting an EncodingChangeException?</a></li>
<li><a href="#post">How can I use POST to fetch a page?</a></li>
<li><a href="#timeout">Is there a way to force a timeout for delinquent pages?</a></li>
<li><a href="#composite">Why aren't <P>, <B>, <I> etc. tags fully nested?</a></li>
<li><a href="#quiet">How can I block parser messages from appearing on stdout?</a></li>
<li><a href="#empty">How does the parser deal with tags like <tag/>?</a></li>
<li><a href="#jsp">How is JSP parsed using the parser?</a></li>
<li><a href="#byte">How do you find the byte offset from the beginning of a document for a tag?</a></li>
</ul>
<a name="encodingchangeexception"></a>
<div class="section"><h3>Why am I getting an EncodingChangeException?</h3>
An EncodingChangeException is thrown to let you, the user, know that some nodes
already handed out by the parser are incorrect according to an encoding directive
in a <META> tag.
<p>When a <META> tag with an encoding directive is encountered, the parser
rescans the input up to the current position using the new encoding.
If a different character results from interpreting the bytes with the
new encoding, the exception is thrown.
</p>
<p>
If you are supplying the parser with your own input, as from a file,
be sure to set the encoding if it is not the default (ISO-8859-1).
You can do this on the Page, Lexer, or Parser objects.
</p>
<p>
If the parser is fetching the data for you, the problem is with the
HTTP server, which should have sent the correct encoding as part of the
Content-Type header string.
Given that you have no control over the server, the only solution is
to reattempt the parse with the new encoding.
</p>
<p>After the exception is thrown, the parser has set it's encoding to the
new value, so you should be able to just reset and reparse, see for
example the handling in StringBean:
<div class="source"><pre>
try
{
... parser.parse (...) throws an EncodingChangeException...
}
catch (EncodingChangeException ece)
{
... do whatever necessary to reset your state here
try
{
// reset the parser
parser.reset ();
// try again with the encoding now in force
parser.parse (...);
}
catch (ParserException pe)
{
}
}
catch (ParserException pe)
{
}
</pre></div>
</p>
</div>
<a name="post"></a>
<div class="section"><h3>How can I use POST to fetch a page?</h3>
<p>The standard HTTP request submitted by the parser is a GET.
The usual request submitted by a form is a POST.
</p>
<p>To illustrate how to use POST with the parser, we'll submit a form to the WHOIS
database of the American Registry for Internet Numbers (ARIN).<br></br>
<i>Note: there is an equivalent GET form at http://ws.arin.net/whois</i>.<br></br>
<i>See also:</i>.
<ul>
<li>RIPE http://www.ripe.net/perl/whois</li>
<li>APNIC http://www.apnic.net/apnic-bin/whois.pl</li>
<li>LACNIC http://lacnic.net/cgi-bin/lacnic/whois</li>
</ul>
<p>On the ARIN web site, the page <a href="http://ws.arin.net/cgi-bin/whois.pl">http://ws.arin.net/cgi-bin/whois.pl</a>
has the following FORM that asks for an IP address and returns the registry details:
</p>
<div class="source"><pre>
<form name="thisform" method="POST" action="/cgi-bin/whois.pl">
<font face="arial,verdana,helvetica" size="2"> Search for : </font>
<input type="text" Name="queryinput" size="20">
<input type="submit"><br>
</form>
</pre></div>
<p>From this we determine that the <tt>METHOD</tt> is <tt>POST</tt> and the form
should be submitted to <tt>/cgi-bin/whois.pl</tt>. This absolute URL is relative to the
page it is found on, so the form should be submitted to <tt>http://ws.arin.net/cgi-bin/whois.pl</tt>
when the <tt>Submit</tt> input is clicked. The only <tt>INPUT</tt> element other than the
<tt>Submit</tt> is a single <tt>text</tt> field named <tt>queryinput</tt> that takes 20 or fewer characters.
Other types of input element are described in <a href="http://www.w3.org/TR/html4/interact/forms.html">http://www.w3.org/TR/html4/interact/forms.html</a>.
</p>
<p>The basic operation is to pass a fully prepared <tt>HttpURLConnection</tt> connected
to the <tt>POST</tt> target URL into the <tt>Parser</tt>, either in the constructor or
via the <tt>setConnection()</tt> method. To condition the connection, use the
<tt>setRequestMethod()</tt> method to set the <tt>POST</tt> operation, and the
<tt>setRequestProperty()</tt> and other explicit method calls.
Then write the input field(s) as an ampersand concatenation
(<tt>"input1=value1&input2=value2&..."</tt>) into the <tt>PrintWriter</tt>
obtained by a call to <tt>getOutputStream()</tt>.
</p>
<p>The following sample program illustrates the principles using a <tt>StringBean</tt>,
but the same code could be used with a <tt>Parser</tt> by replacing the last three lines
in the <tt>try</tt> block with:
</p>
<div class="source"><pre>
parser = new Parser ();
parser.setConnection (connection);
// ... do parser operations
</pre></div>
<p></p>
<div class="source"><pre>
import java.io.PrintWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import org.htmlparser.beans.StringBean;
/**
* WhoIs.java
* Use POST to get information about an IP address from ws.arin.net.
* Created on April 29, 2006, 11:06 PM
*/
public class WhoIs
{
String mText; // text extracted from the response to the POST request
/**
* Creates a new instance of WhoIs.
*/
public WhoIs (String ipaddress)
{
URL url;
HttpURLConnection connection;
StringBuffer buffer;
PrintWriter out;
StringBean bean;
try
{
// from the 'action' (relative to the refering page)
url = new URL ("http://ws.arin.net/cgi-bin/whois.pl");
connection = (HttpURLConnection)url.openConnection ();
connection.setRequestMethod ("POST");
connection.setDoOutput (true);
connection.setDoInput (true);
connection.setUseCaches (false);
// more or less of these may be required
// see Request Header Definitions: http://www.ietf.org/rfc/rfc2616.txt
connection.setRequestProperty ("Accept-Charset", "*");
connection.setRequestProperty ("Referer", "http://ws.arin.net/cgi-bin/whois.pl");
connection.setRequestProperty ("User-Agent", "WhoIs.java/1.0");
buffer = new StringBuffer (1024);
// 'input' fields separated by ampersands (&)
buffer.append ("queryinput=");
buffer.append (ipaddress);
// etc.
out = new PrintWriter (connection.getOutputStream ());
out.print (buffer);
out.close ();
bean = new StringBean ();
bean.setConnection (connection);
mText = bean.getStrings ();
}
catch (Exception e)
{
mText = e.getMessage ();
}
}
public String getText ()
{
return (mText);
}
/**
* Program mainline.
* @param args The ip address (dot notation) to look up.
*/
public static void main (String[] args)
{
if (0 >= args.length)
System.out.println ("Usage: java WhoIs <ipaddress>");
else
System.out.println (new WhoIs (args[0]).getText ());
}
}
</pre></div>
</div>
<a name="timeout"></a>
<div class="section"><h3>Is there a way to force a timeout for delinquent pages?</h3>
<p>If you are using the Sun jvm, try using:
<div class="source"><pre>
System.setProperty ("sun.net.client.defaultReadTimeout", "7000");
System.setProperty ("sun.net.client.defaultConnectTimeout", "7000");
</pre></div>
in the mainline before starting your main application processing.
</p>
<p>This sets the socket timeouts to 7 seconds, but you will need
to catch the I/O exceptions.
</p>
</div>
<a name="composite"></a>
<div class="section"><h3>Why aren't <P>, <B>, <I> etc. tags fully nested?</h3>
<p>Authors are sometimes lazy and often fail to close some tags as
required by the HTML standard. This causes some problems for the parser.
</p>
<p>For this heuristic reason, not all possible tags are registered as composite tags,
which is what generates the 'parent/child' nesting relationship.
It is considered better to have a valid, less nested parse than a possibly invalid parse.
</p>
<p>You are free to add whatever nodes you like as composite nodes using the
prototypical node factory paradigm. First create your class that derives from
<tt>CompositeTagNode</tt> (copy and modify one of the existing tags that is most
like your desired tag):
</p>
<div class="source"><pre>
public class BoldTag extends CompositeTag
{
private static final String[] mIds = new String[] {"B"};
public BoldTag ()
{
}
public String[] getIds ()
{
return (mIds);
}
public String[] getEnders ()
{
return (mIds);
}
public String[] getEndTagEnders ()
{
return (new String[0]);
}
}
</pre></div>
<p>Then, register an instance of your node with a PrototypicalNodeFactory:
</p>
<div class="source"><pre>
PrototypicalNodeFactory factory = new PrototypicalNodeFactory ();
factory.registerTag (new BoldTag ());
parser.setNodeFactory (factory);
</pre></div>
<p>The problem becomes detecting when the tag doesn't have a </B> like
it should, so getEnders() and getEndTagEnders() should probably have a
longer list of tag names. Enders are the tag names that force an end
tag to be generated, while EndTagEnders are the end tags (</xxx>) that force an end
tag to be generated.
</p>
</div>
<a name="quiet"></a>
<div class="section"><h3>How can I block parser messages from appearing on stdout?</h3>
<p>The parser sends warning and error messages to standard output by default.
You might want to block these messages. To achieve this, use a different feedback object:
</p>
<div class="source"><pre>
Parser parser = new Parser ("http://...", new DefaultParserFeedback (DefaultParserFeedback.QUIET));
</pre></div>
<p>The <tt>Parser</tt> class has a static member with just such a construction:
</p>
<div class="source"><pre>
Parser parser = new Parser ("http://...", Parser.DEVNULL);
</pre></div>
<p>You can also switch the feedback to DEBUG mode, to get extra details.
</p>
<div class="source"><pre>
Parser parser = new Parser ("http://...", new DefaultParserFeedback (DefaultParserFeedback.DEBUG));
</pre></div>
<p>
To handle the feedback yourself, implement the <tt>ParserFeedback</tt>,
interface by implementing <tt>info()</tt>, <tt>warning()</tt> and <tt>error()</tt>.
</p>
</div>
<a name="empty"></a>
<div class="section"><h3>How does the parser deal with tags like <tag/>?</h3>
<p>
The parser handles tags ending with a slash as a normal <tt>Tag</tt> object.
The <tt>Tag</tt> interface has a method - <tt>isEmptyXmlTag()</tt> which
returns <tt>true</tt> if is this such an empty xml tag (has no end tag).
</p>
</div>
<a name="jsp"></a>
<div class="section"><h3>How is JSP parsed using the parser?</h3>
<p>There is a <tt>JspTag</tt> class that handles "%", "%=" and "%@" tags,
<em>but not within tags or remarks</em>.
So, the Jsp tag within the tag <tt><input type='<%= MyType %>'></tt> would
not be returned as a tag, but would instead be part of the text of the 'type' attribute,
but the same tag within the text of the page would be returned as a <tt>JspTag</tt> tag.
</p>
</div>
<a name="byte"></a>
<div class="section"><h3>How do you find the byte offset from the beginning of a document for a tag?</h3>
<p>Character positions are much easier to obtain than byte positions.
Each tag returned by the parser or lexer has methods <tt>getStartPosition()</tt> and
<tt>getEndPosition()</tt> which return the starting and ending character positions.
</p>
<p>These can be converted to line and column numbers in a hypothetical text file using
<tt>row()</tt> and <tt>column()</tt> methods on the <tt>Page</tt> object:
</p>
<div class="source"><pre>
Page page = parser.getLexer ().getPage ();
int row = page.row (tag.getStartPosition ()); // note: zero based
int column = page.column (tag.getStartPosition ());
</pre></div>
<p>Converting a character position into a byte position is dependant on the character
encoding used. For the ISO-8859-1 encoding, the correspondence is one byte per character,
but for other encodings, often more than one byte is used per character. Perhaps the
only safe way is to write all the characters, up to the character position of interest,
to a suitably encoded writer on a stream, flush the writer and then examine the
byte position of the underlying stream.
</p>
</div>
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