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## Package org.htmlparser.lexer

The lexer package is the base level I/O subsystem.

See:
Description

Class Summary
Cursor | A bookmark in a page.
Lexer | This class parses the HTML stream into nodes.
Page | Represents the contents of an HTML page.
PageIndex | A sorted array of integers, the positions of the first characters of each line.
Source | A buffered source of characters.
Stream | Provides for asynchronous fetching from a stream.

## Package org.htmlparser.lexer Description

The lexer package is the base level I/O subsystem.

The lexer package is responsible for reading characters from the HTML source and identifying the node lexemes. For example, the HTML code below would return the list of nodes shown:

    <html><head><title>Humoresque</title></head>
    <body bgcolor='silver'>
    Passengers will please refrain
    from flushing toilets while the train
    is standing in the station. I love you!
    <p>
    We encourage constipation
    while the train is in the station
    If the train can't go
    then why should you.
    </body>
    </html>

1. line 0, offset 0, to line 0, offset 6, html tag
2. line 0, offset 6, to line 0, offset 12, head tag
3. line 0, offset 12, to line 0, offset 19, title tag
4. line 0, offset 19, to line 0, offset 29, string node "Humoresque"
5. line 0, offset 29, to line 0, offset 37, end title tag
6. line 0, offset 37, to line 0, offset 44, end head tag
7. line 0, offset 44, to line 0, offset 45, string node "\n"
8. line 1, offset 0, to line 1, offset 23, body tag
9. line 1, offset 23, to line 4, offset 40, string node "\nPassengers...you!\n"
10. line 5, offset 0, to line 5, offset 2, paragraph tag
11. line 5, offset 3, to line 9, offset 21, string node "\nWe...you.\n"
12. line 10, offset 0, to line 10, offset 7, end body tag
13. line 10, offset 8, to line 10, offset 9, string "\n"
14. line 11, offset 0, to line 11, offset 7, html tag
15. line 11, offset 7, to line 11, offset 8, string node "\n"

Stream, Source, Page and Lexer

The package is arranged in four levels, `Stream`, `Source` `Page` and `Lexer` in the order of lowest to highest. A `Stream` is raw bytes from the URLConnection or file. It has no intelligence. A `Source` is raw characters, hence it knows about the encoding scheme used and can be reset if a different encoding is detected after partially reading in the text. A `Page` provides characters from the source while maintaining the index of line numbers, and hence can be thought of as an array of strings corresponding to source file lines, but it doesn't actually store any text, relying on the buffering within the `Source` instead. The `Lexer` contains the actual lexeme parsing code. It reads characters from the page, keeping track of where it is with a `Cursor` and creates the array of nodes using various state machines.

The following are some design goals and 'invariants' within the package, if you are attempting to understand or modify it. Things that differ substantially from previous implementations are highlighted in bold.

Contiguous Nodes
  Adjacent nodes have no characters between them. The list of nodes forms an uninterrupted chain that, by start and end definitions, completely covers the characters that were read from the HTML source.
Text Fidelity
  Besides complete coverage, the nodes do not initially contain copies of the text, but instead simply contain offsets into a single large buffer that contains the text read from the HTML source. Even within tags, the attributes list can contain whitespace, thus there is no lost whitespace or text formatting either outside or within tags. Upper and lower case text is preserved.
Line Endings
  End of line characters are just whitespace. There is no distinction made between end of line characters (or pairs of characters on Windows) and other whitespace. The text is not read in line by line so nodes (tags) can easily span multiple lines with no special processing. Line endings are not transformed between platforms, i.e. Unix line endings are not converted to Windows line endings by this level. Each node will has a starting and ending location, which the page can use to extract the text. To facilitate formatting error and log messages the page can turn these offsets into row and column numbers. In general ignore line breaks in the source if at all possible.
One Parser, One Scan
  The Lexer has the following state machines corresponding (roughly) to the four parsers it replaces (StringParser, RemarkNodeParser, TagParser & AttributeParser):
  - in text - parseString()
  - in comment - parseRemark()
  - in tag - parseTag()
  By integrating the four state machines into one, a single pass over the text is all that's needed for a low level parse of the HTML source. In previous implementations, the attributes were parsed on a second scan after the initial tag was extracted. (Actually, for error conditions, the lexer can back up a node to handle missing end tags etc.).
Two Jars
  For elementary operations at the node level, a minimalist jar file containing only the lexer and base tag classes is split out from the larger `htmlparser.jar`. In this way, simple parsing and output is handled with a jar file that is under 45 kilobytes, but anything beyond peephole manipulation, i.e. closing tag detection and other semantic reasoning will need the full set of scanners, nodes and ancillary classes, which now stands at 210 kilobytes.

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| © 2004 Somik Raha Mar 14, 2004
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HTML Parser is an open source library released under LGPL.