Reading HTML file to DOM tree using Java

JTidy, either by processing the stream to XHTML then using your favourite DOM implementation to re-parse, or using parseDOM if the limited DOM imp that gives you is enough.

Alternatively Neko.

Here is a link that might be useful. It's a list of Open Source HTML Parser in Java Open Source HTML Parsers in Java

Since HTML files are generally problematic, you'll need to first clean them up using a parser/scanner. I've used JTidy but never happily. NekoHTML works okay, but any of these tools are always just making a best guess of what is intended. You're effectively asking to let a program alter a document's markup until it conforms to a schema. That will likely cause structural (markup), style or content loss. It's unavoidable, and you won't really know what's missing unless you manually scan via a browser (and then you have to trust the browser too).

It really depends on your purpose — if you have thousands of ugly documents with tons of extraneous (non-HTML) markup, then a manual process is probably unreasonable. If your goal is accuracy on a few important documents, then manually fixing them is a reasonable proposition.

One approach is the manual process of repeatedly passing the source through a well-formed and/or validating parser, in an edit cycle using the error messages to eventually fix the broken markup. This does require some understanding of XML, but that's not a bad education to undertake.

With Java 5 the necessary XML features — called the JAXP API — are now built into Java itself; you don't need any external libraries.

You first obtain an instance of a DocumentBuilderFactory, set its features, create a DocumentBuilder (parser), then call its parse() method with an InputSource. InputSource has a number of possible constructors, with a StringReader used in the following example:

import javax.xml.parsers.*;
// ...

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setValidating(false);
dbf.setNamespaceAware(true);
dbf.setIgnoringComments(false);
dbf.setIgnoringElementContentWhitespace(false);
dbf.setExpandEntityReferences(false);
DocumentBuilder db = dbf.newDocumentBuilder();
return db.parse(new InputSource(new StringReader(source)));

This returns a DOM Document. If you don't mind using external libraries there's also the JDOM and XOM APIs, and while these have some advantages over the SAX and DOM APIs in JAXP, they do require non-Java libraries to be added. The DOM can be somewhat cumbersome, but after so many years of using it I don't really mind any longer.