SDPL 2011 3: XML APIs and SAX 1 3. XML Processor APIs 3. XML Processor APIs How can (Java) applications How can (Java) applications manipulate structured (XML) manipulate structured (XML) documents? documents? – An overview of XML processor An overview of XML processor interfaces interfaces 3.1 SAX: an event-based interface 3.1 SAX: an event-based interface 3.2 DOM: an object-based interface 3.2 DOM: an object-based interface 3.3 JAXP: Java API for XML Processing 3.3 JAXP: Java API for XML Processing 3.4 StAX: Streaming API for XML 3.4 StAX: Streaming API for XML
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SDPL 2011 3: XML APIs and SAX 1
3. XML Processor APIs3. XML Processor APIs
How can (Java) applications manipulate How can (Java) applications manipulate structured (XML) documents?structured (XML) documents?– An overview of XML processor interfacesAn overview of XML processor interfaces
3.1 SAX: an event-based interface3.1 SAX: an event-based interface
3.2 DOM: an object-based interface3.2 DOM: an object-based interface
3.3 JAXP: Java API for XML Processing3.3 JAXP: Java API for XML Processing
3.4 StAX: Streaming API for XML3.4 StAX: Streaming API for XML
Each XML application contains a parserEach XML application contains a parser– editors, browsers editors, browsers – transformation/style engines, DB loaders, ...transformation/style engines, DB loaders, ...
XML parsers have become standard tools of XML parsers have become standard tools of application development frameworksapplication development frameworks– Standard Java (since JDK 1.4) provides default Standard Java (since JDK 1.4) provides default
parsers (built on Apache Xerces) via JAXPparsers (built on Apache Xerces) via JAXP
(See, e.g., Leventhal, Lewis & Fuchs: Designing XML (See, e.g., Leventhal, Lewis & Fuchs: Designing XML Internet Applications, Chapter 10, and Internet Applications, Chapter 10, and D. Megginson: D. Megginson: Events vs. Trees))
SDPL 2011 3: XML APIs and SAX 3
Tasks of a ParserTasks of a Parser
Document instance decompositionDocument instance decomposition– into into "XML Information Set""XML Information Set" of elements, attributes, of elements, attributes,
– well-formedness checking well-formedness checking » syntactical correctness of XML markupsyntactical correctness of XML markup
– validation (against a DTD or Schema)validation (against a DTD or Schema) Access to contents of the DTD (if supported)Access to contents of the DTD (if supported)
– SAX 2.0 Extensions provide info of declarations: SAX 2.0 Extensions provide info of declarations: element type names and their content model element type names and their content model expressionsexpressions
I: Event-based (streaming) interfacesI: Event-based (streaming) interfaces– Command line and ESIS interfacesCommand line and ESIS interfaces
» Element Structure Information Set, traditional interface to Element Structure Information Set, traditional interface to stand-alone SGML parsersstand-alone SGML parsers
Application implements a set of Application implements a set of call-back methodscall-back methods for acting on parse eventsfor acting on parse events– "Observer design pattern""Observer design pattern"– parameters qualify events further, with parameters qualify events further, with
» element type nameelement type name
» names and values of attributesnames and values of attributes
» values of content strings, …values of content strings, …
Idea behind ‘‘Idea behind ‘‘SAXSAX’’ (Simple API for XML)’’ (Simple API for XML)– an industry standard API for XML parsersan industry standard API for XML parsers– kind-of “kind-of “SSerial erial AAccess ccess XXML”ML”
The parser provides document contents as a The parser provides document contents as a stream of eventsstream of events, which the application can , which the application can iterateiterate over over– the application actively "the application actively "pullspulls" the events" the events
» in push processing, the application reacts on themin push processing, the application reacts on them
– leads often to simpler codeleads often to simpler code– allows to cancel the parsing more easily, or parse allows to cancel the parsing more easily, or parse
Idea behind ‘‘Idea behind ‘‘StAXStAX’’ (Streaming API for XML)’’ (Streaming API for XML)– a standard Java API for XML parsersa standard Java API for XML parsers
SDPL 2011 3: XML APIs and SAX
A Pull-Parsing ApplicationA Pull-Parsing Application
Tree-based / Object Model InterfacesTree-based / Object Model Interfaces
Application interacts with Application interacts with – a parser object, which builds ... a parser object, which builds ... – a a document objectdocument object consisting of consisting of documentdocument, ,
elements, attributes, textelements, attributes, text, …, … Abstraction level higher than in event based Abstraction level higher than in event based
interfaces; more powerful access interfaces; more powerful access – to descendants, following siblings, …to descendants, following siblings, …
Drawback: Higher memory consumptionDrawback: Higher memory consumption– > used mainly in client applications > used mainly in client applications
(to implement document manipulation by user)(to implement document manipulation by user)
3.1 The SAX Event Callback API3.1 The SAX Event Callback API
A de-facto industry standardA de-facto industry standard– Developed by members of the xml-dev mailing listDeveloped by members of the xml-dev mailing list– Version 1.0 in May 1998, Vers. 2.0 in May 2000Version 1.0 in May 1998, Vers. 2.0 in May 2000– NotNot a parser, but a common a parser, but a common interfaceinterface for different for different
parsers (like, say, JDBC is a common interface to parsers (like, say, JDBC is a common interface to various RDBs)various RDBs)
Supported directly by major XML parsersSupported directly by major XML parsers– many Java based, and free; Examples:many Java based, and free; Examples:
Apache Xerces, Oracle's XML Parser for Java; Apache Xerces, Oracle's XML Parser for Java; MSXML (in IE 5+), James Clark's XPMSXML (in IE 5+), James Clark's XP
SDPL 2011 3: XML APIs and SAX 13
SAX 2.0 InterfacesSAX 2.0 Interfaces
Co-operation of an application and a parser Co-operation of an application and a parser specified in terms of specified in terms of interfaces interfaces (i.e., (i.e., collections of methods)collections of methods)
My classificationMy classification of SAX interfaces: of SAX interfaces:– Application-to-parser interfacesApplication-to-parser interfaces
» to use the parserto use the parser
– Parser-to-application (or call-back) interfacesParser-to-application (or call-back) interfaces» to act on various parsing eventsto act on various parsing events
– Auxiliary interfacesAuxiliary interfaces» to manipulate parser-provided informationto manipulate parser-provided information
Implemented by Implemented by parserparser (or a SAX driver): (or a SAX driver):
– XMLReaderXMLReader» methods to invoke the parser, and to register methods to invoke the parser, and to register
objects that implement call-back interfacesobjects that implement call-back interfaces
– XMLFilterXMLFilter (extends (extends XMLReaderXMLReader))» interface to connect interface to connect XMLReaderXMLReaders in a row as a s in a row as a
sequence of filterssequence of filters» obtains events from an obtains events from an XMLReaderXMLReader and passes and passes
them further (possibly modified)them further (possibly modified)
SDPL 2011 3: XML APIs and SAX 15
Call-Back InterfacesCall-Back Interfaces
Implemented by Implemented by applicationapplication to act on parse events to act on parse events(A (A DefaultHandlerDefaultHandler quietly ignores most of them) quietly ignores most of them)– ContentHandlerContentHandler
» methods to process document parsing eventsmethods to process document parsing events– DTDHandlerDTDHandler
» methods to receive notification of unparsed external methods to receive notification of unparsed external entities and notations declared in the DTDentities and notations declared in the DTD
– ErrorHandlerErrorHandler» methods for handling parsing errors and warningsmethods for handling parsing errors and warnings
– EntityResolverEntityResolver» methods for customised processing of external methods for customised processing of external
AttributesAttributes– methods to access a list of attributes, e.g:methods to access a list of attributes, e.g:
int int getLength()getLength()String String getValue(String attrName)getValue(String attrName)
LocatorLocator– methods for locating the origin of parse events: methods for locating the origin of parse events: getSystemID()getSystemID(), , getPublicID()getPublicID(), , getLineNumber()getLineNumber(), , getColumnNumber()getColumnNumber()
– for example, to report application-specific for example, to report application-specific semantic errors semantic errors
SDPL 2011 3: XML APIs and SAX 17
The The ContentHandlerContentHandler Interface Interface
Information of general document events. (See API Information of general document events. (See API documentation for a complete list):documentation for a complete list):
setDocumentLocator(Locator locator)setDocumentLocator(Locator locator) – to receive a locator for the origin of SAX document eventsto receive a locator for the origin of SAX document events
startDocument()startDocument();; endDocument()endDocument() – notify the beginning/end of a document. notify the beginning/end of a document.
endElementendElement for for htmlhtml would give would give– nsURI =nsURI = http://www.w3.org/TR/xhtml1/strict http://www.w3.org/TR/xhtml1/strict
(as default namespace for element names without a (as default namespace for element names without a prefix),prefix), localname =localname = html html, , rawName =rawName = html html
SDPL 2011 3: XML APIs and SAX 20
<!DOCTYPE A [<!ELEMENT A (B)> <!DOCTYPE A [<!ELEMENT A (B)> <!ELEMENT B (#PCDATA)> ]> <!ELEMENT B (#PCDATA)> ]>
characters(char ch[], characters(char ch[], int start, int length)int start, int length)
– notification of character data notification of character data ignorableWhitespace(char ch[], ignorableWhitespace(char ch[],
int start, int length)int start, int length)– notification of ignorable whitespace in element content notification of ignorable whitespace in element content
Ignorable whitespaceIgnorable whitespace
Text contentText content
SDPL 2011 3: XML APIs and SAX 21
SAX Processing Example (1/9)SAX Processing Example (1/9)
InputInput: XML representation of a personnel database:: XML representation of a personnel database:
Event-based processing strategy:Event-based processing strategy:– at the start of at the start of personperson, record the , record the idnumidnum (e.g.,(e.g., 12341234))
– record starts and ends of record starts and ends of lastlast and and firstfirst to store to store their contents (e.g., "their contents (e.g., "KilpeläinenKilpeläinen" and "" and "PekkaPekka")")
– at the end of a at the end of a personperson, output the collected data, output the collected data
SDPL 2011 3: XML APIs and SAX 23
SAX Processing Example (3/9)SAX Processing Example (3/9)
if (rawName.equals("person")) if (rawName.equals("person")) IdNum = atts.IdNum = atts.getValuegetValue("idnum");("idnum");
if (rawName.equals("first")) if (rawName.equals("first")) InFirst = true;InFirst = true;
if (rawName.equals("last")) if (rawName.equals("last")) InLast = true;InLast = true;
} // startElement} // startElement
SDPL 2011 3: XML APIs and SAX 26
SAX Processing Example (6/9)SAX Processing Example (6/9)
Call-back methods continue:Call-back methods continue:– Record the text content of elements Record the text content of elements firstfirst and and lastlast::
public void public void characterscharacters ( (char buf[], int start, int length) {char buf[], int start, int length) {
if (InFirst) FirstName = if (InFirst) FirstName = new String(buf, start, length);new String(buf, start, length);
if (InLast) LastName = if (InLast) LastName = new String(buf, start, length); new String(buf, start, length);
} // characters } // characters
SDPL 2011 3: XML APIs and SAX 27
SAX Processing Example (7/9)SAX Processing Example (7/9)
At the end of At the end of personperson, output the collected data:, output the collected data:
public void public void endElementendElement(String nsURI,(String nsURI, String localName, String qName) { String localName, String qName) {
if (qName.equals("person")) { if (qName.equals("person")) {
SAX Processing Example (9/9)SAX Processing Example (9/9)
MainMain method continues: method continues:// Instantiate and pass a new // Instantiate and pass a new // ContentHandler to xmlReader:// ContentHandler to xmlReader:
ContentHandlerContentHandler handler = new SAXDBApp(); handler = new SAXDBApp(); xmlReader.xmlReader.setContentHandlersetContentHandler(handler);(handler); for (int i = 0; i < args.length; i++) {for (int i = 0; i < args.length; i++) { xmlReader.xmlReader.parseparse(args[i]);(args[i]); }}
System.exit(1); }System.exit(1); }} // main} // main
SDPL 2011 3: XML APIs and SAX 30
SAX: SummarySAX: Summary
A low-level parser-interface for XML A low-level parser-interface for XML documentsdocuments
Reports document parsing events Reports document parsing events through method call-backsthrough method call-backs– > efficient: does not create in-memory > efficient: does not create in-memory
representation of the documentrepresentation of the document– > used often on servers and on resource-> used often on servers and on resource-
limited devices (palm-tops), and to process limited devices (palm-tops), and to process LARGE documentsLARGE documents
– No limit on document size!No limit on document size!