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

    • STATE_TEXT

      protected static final int STATE_TEXT
      Scanning text..
      See Also:
      Constant Field Values
    • STATE_BEFORE_START_TAG_NAME

      protected static final int STATE_BEFORE_START_TAG_NAME
      Scanning attribute name/value pairs.
      See Also:
      Constant Field Values
    • STATE_BEFORE_END_TAG_NAME

      protected static final int STATE_BEFORE_END_TAG_NAME
      Scanning a closing tag.
      See Also:
      Constant Field Values
    • STATE_IN_START_TAG

      protected static final int STATE_IN_START_TAG
      Scanning attribute name/value pairs.
      See Also:
      Constant Field Values
    • STATE_IN_END_TAG

      protected static final int STATE_IN_END_TAG
      Scanning a closing tag.
      See Also:
      Constant Field Values
    • MAX_ENTITY_VALUE

      protected static final int MAX_ENTITY_VALUE
      The maximum Unicode value accepted for a numeric entity.
      See Also:
      Constant Field Values
    • HEXADECIMAL

      protected static final int HEXADECIMAL
      The base for non-decimal entity.
      See Also:
      Constant Field Values
    • MAX_HEX_ENTITY_LENGTH

      protected static final int MAX_HEX_ENTITY_LENGTH
      The maximum number of digits of a hexadecimal numeric entity.
      See Also:
      Constant Field Values
    • MAX_DEC_ENTITY_LENGTH

      protected static final int MAX_DEC_ENTITY_LENGTH
      The maximum number of digits of a decimal numeric entity.
      See Also:
      Constant Field Values
    • SCRIPT_CLOSE_TAG_PATTERN

      protected static final TextPattern SCRIPT_CLOSE_TAG_PATTERN
      Closing tag for a script element.
    • STYLE_CLOSE_TAG_PATTERN

      protected static final TextPattern STYLE_CLOSE_TAG_PATTERN
      Closing tag for a style element.
    • NONSPACE_WHITESPACE

      protected static final char[] NONSPACE_WHITESPACE
      An array containing the non-space whitespace.
    • SPACE

      protected static final char[] SPACE
      An array, parallel to NONSPACE_WHITESPACE, containing spaces.
    • CLOSED_COMMENT

      protected static final TextPattern CLOSED_COMMENT
      Closed comment. It should be "-->", but mistakes are common.
    • CLOSED_PERCENT

      protected static final TextPattern CLOSED_PERCENT
      Closed ASP or similar tag.
    • CLOSED_PIC

      protected static final TextPattern CLOSED_PIC
      Closed processing instruction.
    • CLOSED_SECTION

      protected static final TextPattern CLOSED_SECTION
      Closed section (conditional, etc.).
    • CLOSED_CDATA

      protected static final TextPattern CLOSED_CDATA
      Closed section (conditional, CDATA, etc.).
    • factory

      public final ParsingFactory factory
      The parsing factory used by this parser.
    • callback

      protected Callback callback
      The callback of this parser.
    • attrMap

      A map from attributes to attribute values.
    • parseText

      protected boolean parseText
      Whether we should invoke the text handler.
    • parseCDATA

      protected boolean parseCDATA
      Whether we should invoke the CDATA section handler.
    • parseTags

      protected boolean parseTags
      Whether we should parse tags.
    • parseAttributes

      protected boolean parseAttributes
      Whether we should parse attributes.
    • parsedAttrs

      protected ReferenceArraySet<Attribute> parsedAttrs
      The subset of attributes whose values will be actually parsed (if, of course, parseAttributesis true).
    • parsedAttributes

      public ReferenceSet<Attribute> parsedAttributes
      An externally visible, immutable subset of attributes whose values will be actually parsed.
    • lastEntity

      protected char lastEntity
      The character represented by the last scanned entity.
  • Constructor Details

    • BulletParser

      public BulletParser​(ParsingFactory factory)
      Creates a new bullet parser.
    • BulletParser

      public BulletParser()
      Creates a new bullet parser using the default factory HTMLFactory.INSTANCE.
  • 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 &amp;. 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.