Package it.unimi.dsi.parser

Class BulletParser

java.lang.Object
it.unimi.dsi.parser.BulletParser
public class BulletParser
extends Object
A fast, lightweight, on-demand (X)HTML parser.

The bullet parser has been written with two specific goals in mind: web crawling and targeted data extraction from massive web data sets. To be usable in such environments, a parser must obey a number of restrictions:

  • it should avoid excessive object creation (which, for instance, forbids a significant usage of Java strings);
  • it should tolerate invalid syntax and recover reasonably; in fact, it should never throw exceptions;
  • it should perform actual parsing only on a settable feature subset: there is no reason to parse the attributes of a P element while searching for links;
  • it should parse HTML as a regular language, and leave context-free properties (e.g., stack maintenance and repair) to suitably designed callbacks.

Thus, in fact the bullet parser is not a parser. It is a bunch of spaghetti code that analyses a stream of characters pretending that it is an (X)HTML document. It has a very defensive attitude against the stream character it is parsing, but at the same time it is forgiving with all typical (X)HTML mistakes.

The bullet parser is officially StringFree™. MutableStrings are used for internal processing, and Java strings are used only to return attribute values. All internal maps are reference-based maps from fastutil, which helps to accelerate further the parsing process.

HTML data

The bullet parser uses attributes and methods of HTMLFactory, Element, Attribute and Entity. Thus, for instance, whenever an element is to be passed around it is one of the shared objects contained in Element (e.g., Element.BODY).

Callbacks

The result of the parsing process is the invocation of a callback. The callback interface of the bullet parser remembers closely SAX2, but it has some additional methods targeted at (X)HTML, such as Callback.cdata(it.unimi.dsi.parser.Element,char[],int,int), which returns characters found in a CDATA section (e.g., a stylesheet).

Each callback must configure the parser, by requesting to perform the analysis and the callbacks it requires. A callback that wants to extract and tokenise text, for instance, will certainly require parseText(true), but not parseTags(true). On the other hand, a callback wishing to extract links will require to parse selectively certain attribute types.

A more precise description follows.

Writing callbacks

The first important issue is what has to be required to the parser. A newly created parser does not invoke any callback. It is up to every callback to add features so that it can do its job. Remember that since many callbacks can be composed, you must always add features, never remove them, and moreover your callbacks must be ready to be invoked with features they did not request (e.g., attribute types added by another callback).

The following parse features may be configured; most of them are just boolean features, a.k.a. flags: unless otherwise specified, by default all flags are set to false (e.g., by the default the parser will not parse tags):

Invoking the parser

After setting the parser callback, you just call parse(char[], int, int).

  • Field Details

  • Constructor Details

  • Method Details

    • parseText

      public boolean parseText()
      Returns whether this parser will invoke the text handler.
      Returns:
      whether this parser will invoke the text handler.
      See Also:
      parseText(boolean)

    • parseText

      public BulletParser parseText​(boolean parseText)
      Sets the text handler flag.
      Parameters:
      parseText - the new value.
      Returns:
      this parser.

    • parseCDATA

      public boolean parseCDATA()
      Returns whether this parser will invoke the CDATA-section handler.
      Returns:
      whether this parser will invoke the CDATA-section handler.
      See Also:
      parseCDATA(boolean)

    • parseCDATA

      public BulletParser parseCDATA​(boolean parseCDATA)
      Sets the CDATA-section handler flag.
      Parameters:
      parseCDATA - the new value.
      Returns:
      this parser.

    • parseTags

      public boolean parseTags()
      Returns whether this parser will parse tags and invoke element handlers.
      Returns:
      whether this parser will parse tags and invoke element handlers.
      See Also:
      parseTags(boolean)

    • parseTags

      public BulletParser parseTags​(boolean parseTags)
      Sets whether this parser will parse tags and invoke element handlers.
      Parameters:
      parseTags - the new value.
      Returns:
      this parser.

    • parseAttributes

      public boolean parseAttributes()
      Returns whether this parser will parse attributes.
      Returns:
      whether this parser will parse attributes.
      See Also:
      parseAttributes(boolean)

    • parseAttributes

      public BulletParser parseAttributes​(boolean parseAttributes)
      Sets the attribute parsing flag.
      Parameters:
      parseAttributes - the new value for the flag.
      Returns:
      this parser.

    • parseAttribute

      public BulletParser parseAttribute​(Attribute attribute)
      Adds the given attribute to the set of attributes to be parsed.
      Parameters:
      attribute - an attribute that should be parsed.
      Returns:
      this parser.
      Throws:
      IllegalStateException - if parseAttributes(true) has not been invoked on this parser.

    • setCallback

      public BulletParser setCallback​(Callback callback)
      Sets the callback for this parser, resetting at the same time all parsing flags.
      Parameters:
      callback - the new callback.
      Returns:
      this parser.

    • entity2Char

      protected char entity2Char​(MutableString name)
      Returns the character corresponding to a given entity name.
      Parameters:
      name - the name of an entity.
      Returns:
      the character corresponding to the entity, or an ASCII NUL if no entity with that name was found.

    • scanEntity

      protected int scanEntity​(char[] a, int offset, int length, boolean loose, MutableString entity)
      Searches for the end of an entity.

      This method will search for the end of an entity starting at the given offset (the offset must correspond to the ampersand).

      Real-world HTML pages often contain hundreds of misplaced ampersands, due to the unfortunate idea of using the ampersand as query separator (please use the comma in new code!). All such ampersand should be specified as &. If named entities are delimited using a transition from alphabetical to non-alphabetical characters, we can easily get false positives. If the parameter loose is false, named entities can be delimited only by whitespace or by a comma.

      Parameters:
      a - a character array containing the entity.
      offset - the offset at which the entity starts (the offset must point at the ampersand).
      length - an upper bound to the maximum returned position.
      loose - if true, named entities can be terminated by any non-alphabetical character (instead of whitespace or comma).
      entity - a support mutable string used to query ParsingFactory.getEntity(MutableString).
      Returns:
      the position of the last character of the entity, or -1 if no entity was found.

    • replaceEntities

      protected void replaceEntities​(MutableString s, MutableString entity, boolean loose)
      Replaces entities with the corresponding characters.

      This method will modify the mutable string s so that all legal occurrences of entities are replaced by the corresponding character.

      Parameters:
      s - a mutable string whose entities will be replaced by the corresponding characters.
      entity - a support mutable string used by scanEntity(char[], int, int, boolean, MutableString).
      loose - a parameter that will be passed to scanEntity(char[], int, int, boolean, MutableString).

    • handleMarkup

      protected int handleMarkup​(char[] text, int pos, int end)
      Handles markup.
      Parameters:
      text - the text.
      pos - the first character in the markup after <!.
      end - the end of text.
      Returns:
      the position of the first character after the markup.

    • handleProcessingInstruction

      protected int handleProcessingInstruction​(char[] text, int pos, int end)
      Handles processing instruction, ASP tags etc.
      Parameters:
      text - the text.
      pos - the first character in the markup after <%.
      end - the end of text.
      Returns:
      the position of the first character after the processing instruction.

    • parse

      public void parse​(char[] text)
      Analyze the text document to extract information.
      Parameters:
      text - a char array of text to be parsed.

    • parse

      public void parse​(char[] text, int offset, int length)
      Analyze the text document to extract information.
      Parameters:
      text - a char array of text to be parsed.
      offset - the offset in the array from which the parsing will begin.
      length - the number of characters to be parsed.