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):

• tags (parseTags(boolean) method): whether tags should be parsed;
• attributes (parseAttributes(boolean) and methods): whether attributes should be parsed (of course, setting this flag is useless if you are not parsing tags); note that setting this flag will just activate the attribute parsing feature, but you must also register every attribute whose value you want to obtain.
• text (parseText(boolean)method): whether text should be parsed; if this flag is set, the parser will call the Callback.characters(char[], int, int, boolean) method for every text chunk found.
• CDATA sections (parseCDATA(boolean)method): whether CDATA sections (stylesheets & scripts) should be parsed; if this flag is set, the parser will call the Callback.cdata(Element,char[],int,int) method for every CDATA section found.

Invoking the parser

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

Field Summary

Fields

Modifier and Type	Field and Description
protected it.unimi.dsi.fastutil.objects.Reference2ObjectMap<Attribute,MutableString>	attrMap A map from attributes to attribute values.
protected Callback	callback The callback of this parser.
protected static TextPattern	CLOSED_CDATA Closed section (conditional, CDATA, etc.).
protected static TextPattern	CLOSED_COMMENT Closed comment.
protected static TextPattern	CLOSED_PERCENT Closed ASP or similar tag.
protected static TextPattern	CLOSED_PIC Closed processing instruction.
protected static TextPattern	CLOSED_SECTION Closed section (conditional, etc.).
ParsingFactory	factory The parsing factory used by this parser.
protected static int	HEXADECIMAL The base for non-decimal entity.
protected char	lastEntity The character represented by the last scanned entity.
protected static int	MAX_DEC_ENTITY_LENGTH The maximum number of digits of a decimal numeric entity.
protected static int	MAX_ENTITY_VALUE The maximum Unicode value accepted for a numeric entity.
protected static int	MAX_HEX_ENTITY_LENGTH The maximum number of digits of a hexadecimal numeric entity.
protected static char[]	NONSPACE_WHITESPACE An array containing the non-space whitespace.
protected boolean	parseAttributes Whether we should parse attributes.
protected boolean	parseCDATA Whether we should invoke the CDATA section handler.
it.unimi.dsi.fastutil.objects.ReferenceSet<Attribute>	parsedAttributes An externally visible, immutable subset of attributes whose values will be actually parsed.
protected it.unimi.dsi.fastutil.objects.ReferenceArraySet<Attribute>	parsedAttrs The subset of attributes whose values will be actually parsed (if, of course, parseAttributesis true).
protected boolean	parseTags Whether we should parse tags.
protected boolean	parseText Whether we should invoke the text handler.
protected static TextPattern	SCRIPT_CLOSE_TAG_PATTERN Closing tag for a script element.
protected static char[]	SPACE An array, parallel to NONSPACE_WHITESPACE, containing spaces.
protected static int	STATE_BEFORE_END_TAG_NAME Scanning a closing tag.
protected static int	STATE_BEFORE_START_TAG_NAME Scanning attribute name/value pairs.
protected static int	STATE_IN_END_TAG Scanning a closing tag.
protected static int	STATE_IN_START_TAG Scanning attribute name/value pairs.
protected static int	STATE_TEXT Scanning text..
protected static TextPattern	STYLE_CLOSE_TAG_PATTERN Closing tag for a style element.

Constructor Summary

Constructors

Constructor and Description
BulletParser() Creates a new bullet parser using the default factory HTMLFactory.INSTANCE.
BulletParser(ParsingFactory factory) Creates a new bullet parser.

Method Summary

Methods

Modifier and Type	Method and Description
protected char	entity2Char(MutableString name) Returns the character corresponding to a given entity name.
protected int	handleMarkup(char[] text, int pos, int end) Handles markup.
protected int	handleProcessingInstruction(char[] text, int pos, int end) Handles processing instruction, ASP tags etc.
void	parse(char[] text) Analyze the text document to extract information.
void	parse(char[] text, int offset, int length) Analyze the text document to extract information.
BulletParser	parseAttribute(Attribute attribute) Adds the given attribute to the set of attributes to be parsed.
boolean	parseAttributes() Returns whether this parser will parse attributes.
BulletParser	parseAttributes(boolean parseAttributes) Sets the attribute parsing flag.
boolean	parseCDATA() Returns whether this parser will invoke the CDATA-section handler.
BulletParser	parseCDATA(boolean parseCDATA) Sets the CDATA-section handler flag.
boolean	parseTags() Returns whether this parser will parse tags and invoke element handlers.
BulletParser	parseTags(boolean parseTags) Sets whether this parser will parse tags and invoke element handlers.
boolean	parseText() Returns whether this parser will invoke the text handler.
BulletParser	parseText(boolean parseText) Sets the text handler flag.
protected void	replaceEntities(MutableString s, MutableString entity, boolean loose) Replaces entities with the corresponding characters.
protected int	scanEntity(char[] a, int offset, int length, boolean loose, MutableString entity) Searches for the end of an entity.
BulletParser	setCallback(Callback callback) Sets the callback for this parser, resetting at the same time all parsing flags.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

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

protected it.unimi.dsi.fastutil.objects.Reference2ObjectMap<Attribute,MutableString> 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 it.unimi.dsi.fastutil.objects.ReferenceArraySet<Attribute> parsedAttrs

The subset of attributes whose values will be actually parsed (if, of course, parseAttributesis true).

parsedAttributes

public it.unimi.dsi.fastutil.objects.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 Detail

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 Detail

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.