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Persistenta datelor

Persistenta datelor. Multe aplicatii doresc sa asigure persistenta datelor Metode/Tehnologii: Fisiere Serializare Baze de date Probleme: Decuplarea componentelor care fac accesul la date de celelalte parti ale aplicatiei Abstractizarea accesului la date

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Persistenta datelor

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  1. Persistenta datelor • Multe aplicatii doresc sa asigure persistenta datelor • Metode/Tehnologii: • Fisiere • Serializare • Baze de date • Probleme: • Decuplarea componentelor care fac accesul la date de celelalte parti ale aplicatiei • Abstractizarea accesului la date • Transformarea datelor intre OO domain model si un model de persistenta non-OO (BD relationale, fisiere diferite structuri)

  2. Continutul cursului • Tehnologii • XML • JDBC • Data Acess Patterns • Data Transfer Object • Data Access Object

  3. XML • XML este un standard pentru descrierea structurii documentelor • eXtensible Markup Language • Format text pentru descrierea datelor => simplu de utilizat • Standardizat => exista API-uri care pot fi utilizate pentru a simplifica parsing-ul (sintaxa reprezentarii datelor) • Applicatiile care utilizeaza XML trebuie sa stabileasca doar partea semantica a reprezentarii datelor

  4. XML Tags • Tags • Reprezinta Metainformatii incluse in text • Similare ca forma cu cu HTML tags • Diferenta intre HTML tags si XML tags: HTML tags contin informatii de reprezentare a datelor (ex: <B>), in timp ce XML tags contin informatii de structurare si semantica a datelor • XML tags sunt case-sensitive • Pot contine text sau alte tag-uri • Fiecare trebuie sa aiba un tag de sfarsit: • <tag> </tag> • O pereche de tag-uri fara continut se poate scrie ca si <tag /> • Tag Attributes • Defineste perechi name-value in interiorul unui tag • <dot x=“72” y=“13” />

  5. Caractere speciale • Unele caractere sunt utilizate in tag-uri si descrieri si pentru reprezentarea lor explicita se folosesc secvente: • Exemple: • < se codifica &lt; • > se codifica &gt; • & se codifica &amp; • “ se codifica &quot; • ‘ se codifica &apos;

  6. Organizarea documentelor XML • Un document XML incepe cu o declaratie: <?xml version='1.0' encoding='utf-8'?> • Forma arborescenta: • Exista exact un element radacina • Alte elemente sunt incuibate • Prin element se intelege o secventa cuprinsa intre 2 tag-uri pereche <person> <firstname>Ion</firstname> <lastname>Popescu</lastname> <age>30</age> <ssn>2711130345678</ssn> </person>

  7. Reprezentarea datelor • Dorim sa reprezentam coordonatele unor puncte in plan XML

  8. Stiluri de reprezentare a datelor: Tag-uri sau atribute ? • Dorim sa reprezentam coordonatele unor puncte in plan • Exista 2 metode de structurare si reprezentare: • Utilizand atribute: <dot x=“25" y=“33"> • Utilizand tag-uri incuibate: <dot> <x>25</x> <y>33</y> </dot> • Ce metoda de reprezentare este de preferat? • Atribute: daca datele sunt scurte: • <dot x='65' y=‘23' /> • Tag-uri: daca datele sunt lungi: • <description>Acest program este foarte util tuturor</description> • Tag-uri: daca numarul de atribute ce caracterizeaza un element nu este fix: <polygon> <point> .. </point> <point>..</point> <point>..</point> </polygon>

  9. Exemplu 1: document XML – datele reprezentate in atribute • Dots – un set de puncte in plan, caracterizate de coordonatele (x,y) • Root node: “dots” • Child nodes: “dot”, cu atributele x si y dots.xml <?xml version="1.0" encoding="UTF-8" ?> <dots> <dot x="32" y="100" /> <dot x="17" y="14" /> <dot x="18" y="58" > </dot> </dots>

  10. Exemplu 2: document XML – datele reprezentate in tag-uri incuibate • Points – un set de puncte in plan, caracterizate de coordonatele (x,y) • Root node: “points” • Child nodes: “point”, fiecare avand child nodes “x” si “y” Un document XML trebuie sa fie: Well-formed si Valid points.xml <?xml version="1.0" encoding="UTF-8" ?> -<points> <point> <x>12</x> <y>24</y> </point> <point> <x>22</x> <y>11</y> </point> </points>

  11. Documente XML: Well-formed • "well-formed": un document corect din punctul de vedere al regulilor sintactice generale XML • are exact un element radacina • fiecare element are un tag de sfarsit • elementele sunt incuibate corect • valorile atributelor sunt intre ghilimele

  12. Documente XML: ValideXML Schema • “Valid”: un document care respecta anumite reguli impuse structurii • Metode de specificare formala a structurii unui document (unei clase de documente) XML: • XML DTD (Data Type Definition): • XML Schema (XSD): un limbaj ce impune constrangeri asupra structurii documentelor XML • Pentru o clasa de documente XML, se pot impune reguli privitoare la: • Ce tag-uri sunt permise, in ce ordine pot sa apara, de cate ori, ce atribute pot sa aiba, de ce tipuri, etc. • Parserele XML cu validare verifica respectarea constrangerilor impuse de o schema specificata • XML Schema Tutorial: http://www.w3schools.com/schema/default.asp

  13. Exemplu 1 XML Schema • Pentru reprezentarea unui set de puncte in plan, se stabilesc urmatoarele reguli: • Elementul radacina este dots <xs:element name="dots"> • Acesta poate contine un numar oarecare de elemente de tip dot • Este un tip complex ptr ca contine alte elemente <xs:complexType> • Contine o secventa de alte elemente <xs:sequence> • Fiecare element dot are 2 atribute, x si y, cu valori intregi <xs:attribute name="x" type="xs:integer" />

  14. Exemplu 1 XML Schema dots.xsd <?xml version="1.0"?> <xs:schema xmlns:xs=http://www.w3.org/2001/XMLSchema> <xs:element name="dots"> <xs:complexType> <xs:sequence> <xs:element name="dot" maxOccurs="unbounded"> <xs:complexType> <xs:attribute name="x" type="xs:integer" use="required"/> <xs:attribute name="y" type="xs:integer" use="required"/> </xs:complexType> </xs:element> </xs:sequence> </xs:complexType> </xs:element> </xs:schema>

  15. Exemplu: document XML cu schema dots.xml <?xml version="1.0" encoding="UTF-8" ?> <dots xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="dots.xsd">> <dot x="32" y="100" /> <dot x="17" y="14" /> <dot x="18" y="58" > </dot> </dots>

  16. Exemplu 2 XML Schema • Pentru reprezentarea unui set de puncte in plan, se stabilesc urmatoarele reguli: • Elementul radacina este points <xs:element name=“points"> • Acesta poate contine un numar oarecare de elemente de tip point • Este un tip complex ptr ca contine alte elemente <xs:complexType> • Contine o secventa de alte elemente point <xs:sequence> • Fiecare element point este de tip complex, fiind o secventa de 2 elemente x si y <xs:element name="x" type="xs:integer" /> • Elementele x si y sunt elemente simple (contine numai text, nu contine alte elemente sau atribute)

  17. Exemplu 2 XML Schema points.xsd <?xml version="1.0" encoding="utf-8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="points"> <xs:complexType> <xs:sequence> <xs:element maxOccurs="unbounded" name="point"> <xs:complexType> <xs:sequence> <xs:element name="x" type="xs:integer" /> <xs:element name="y" type="xs:integer" /> </xs:sequence> </xs:complexType> </xs:element> </xs:sequence> </xs:complexType> </xs:element> </xs:schema>

  18. Suport pentru editarea XML • Fisierele XML pot fi editate cu orice editor de texte (Notepad, etc) • Este recomandabila utilizarea unui XML Editor care poate ajuta la validarea unui document XML in conformitate cu o schema data si poate impune respectarea structurii permise inca din timpul editarii • XMLSpy • Mediile Integrate de Dezvoltare (IDE) curente contin diverse facilitati pentru lucrul cu XML : • NetBeans • .NET Studio • Eclipse

  19. Suport pentru prelucrarea XML in Java • JAXP (Java API for XML Processing) • http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JAXPIntro.html • Suporta procesarea datelor XML de catre aplicatii scrise in Java • Suport pentru XML parsing: 2 standarde diferite: • SAX (Simple API for XML Parsing): in timpul operatiei de parsing se genereaza evenimente care anunta elementele identificate, este sarcina aplicatiei sa trateze acele evenimente furnizand metodele de callback (pentru a-si construi o structura de date) • DOM (Document Object Model): operatia de parsing construieste in memorie o reprezentare arborescenta a datelor din XML • Suport pentru transformarea documentelor XML: • XSLT (Extensible Stylesheet Language Transformation).

  20. SAX http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JAXPIntro4.html

  21. Citire XML cu SAX • Exemplu: XMLDotReader - citeste date din fisierul dots.xml • // standard imports for SAX • import java.io.*; • import java.util.*; • import javax.xml.parsers.*; • import org.xml.sax.*; • import org.xml.sax.helpers.*; • // Implementing a ContentHandler to handle the SAX events • public class XMLDotReader extends DefaultHandler { • … • }

  22. Construirea parserului SAX • // create an instance of the ContentHandler • DefaultHandler handler= new XMLDotReader(); • SAXParserFactory factory = SAXParserFactory.newInstance(); • try { • // use the default non-validating parser • SAXParser saxParser = factory.newSAXParser(); • saxParser.parse(new File(“dots.xml”), handler); • } catch (Exception ex) { • ex.printStackTrace(); • }

  23. Tipuri de evenimente • public void startDocument() throws SAXException ; • public void endDocument() throws SAXException ; • // Called at start of each element • public void startElement(String namespaceURI, String localName, • String qName, Attributes atts) throws SAXException ; • // Called at the end of each element • public void endElement(java.lang.String uri, java.lang.String localName, java.lang.String qName) throws SAXException; • // Called for characters between nodes. • public void characters(char buf[], int offset, int len) throws SAXException;

  24. Tratarea evenimentelor • public class XMLDotReader extends DefaultHandler { • … • public void startElement(String namespaceURI, String localName, • String qName, Attributes atts) throws SAXException { • System.out.println("start element:" + qName); • if (qName.equals("dot")) { • x = Integer.parseInt(atts.getValue("x")); • y = Integer.parseInt(atts.getValue("y")); • System.out.println(x + ", " + y); • } • } • }

  25. Cod Sursa Exemplu1 • Vezi pagina web: XMLDotsReader.java

  26. Rezultatele rularii startDocument start element:dots start element:dot dot: 32, 100 end element:dot start element:dot dot: 17, 14 end element:dot start element:dot dot: 18, 58 end element:dot end element:dots endDocument dots.xml <?xml version="1.0" encoding="UTF-8" ?> <dots> <dot x="32" y="100" /> <dot x="17" y="14" /> <dot x="18" y="58" > </dot> </dots>

  27. Rezultatele rularii • Daca se modifica dots.xml astfel incat sa nu mai fie well-formed (lipseste inchiderea la al 2-lea tag dot) startDocument start element:dots start element:dot dot: 32, 100 end element:dot start element:dot dot: 17, 14 start element:dot dot: 18, 58 end element:dot org.xml.sax.SAXParseException: The end-tag for element type "dot" must end with a '>' delimiter. at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAX ParseException(Unknown Source)

  28. Rezultatele rularii • Daca se modifica fisierul dots.xml astfel incat sa contina si alte nume de tag-uri (se inlocuieste al doilea tag dot cu dotu), nemaifiind conform cu schema startDocument start element:dots start element:dot dot: 32, 100 end element:dot start element:dotu end element:dotu start element:dot dot: 18, 58 end element:dot end element:dots endDocument

  29. Parser cu validare • public class ValidatingXMLDotReader extends DefaultHandler { • // changes to create a validating parser • Static final String JAXP_SCHEMA_LANGUAGE= "http://java.sun.com/xml/jaxp/properties/schemaLanguage"; • static final String W3C_XML_SCHEMA = "http://www.w3.org/2001/XMLSchema"; • SAXParserFactory factory = SAXParserFactory.newInstance(); • try { • factory.setValidating(true); • factory.setNamespaceAware(true); • SAXParser saxParser = factory.newSAXParser(); • saxParser.setProperty(JAXP_SCHEMA_LANGUAGE, W3C_XML_SCHEMA); • saxParser.parse(new File(“dots.xml”), handler); • } catch (Exception ex) { • ex.printStackTrace(); • }

  30. Parser cu validare – evenimentul error • public void error(SAXParseException e) • throws SAXParseException • { • throw e; • }

  31. Exceptii SAX – tratare in detaliu • try { • … • }catch (SAXParseException spe) { • // Error generated by the parser • System.out.println(“Parsing Error: line ”+spe.getLineNumber()+” , ”+spe.getMessage()); • } catch (SAXException sxe) { • // Error generated by application or parser initialization • } catch (ParserConfigurationException pce) { • // parser with specified options cann’t be built • }catch (IOException ioe) { • }catch (Throwable t) { • }

  32. Cod Sursa Exemplu2 • Vezi pagina web: ValidatingXMLDotsReader.java

  33. Rezultatele rularii • Se ruleaza ValidatingXMLDotReader pe varianta modifica fisierul dots.xml care contine si alte nume de tag-uri (se inlocuieste al doilea tag dot cu dotu), nemaifiind conform cu schema startDocument start element:dots start element:dot 32, 100 end element:dot ** Parsing error, line 5, uri file:/C:/Documents%20and%20Settings/user/Desktop/x ml-marti/dots.xml cvc-complex-type.2.4.a: Invalid content was found starting with element 'dotu '. One of '{dot}' is expected. org.xml.sax.SAXParseException: cvc-complex-type.2.4.a: Invalid content was found starting with element 'dotu'. One of '{dot}' is expected. at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAX ParseException(Unknown Source) at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.error(Unk nown Source)

  34. DOM http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JAXPIntro5.html

  35. Element Node • Un element/nod corespunde unei sectiuni cuprinsa intre <tag>… </tag> • Un nod poate contine alte noduri child • Un nod poate avea atribute cu valori • Exista un nod radacina a documentului

  36. Citirea unui XML Document in DOM • // Standard imports for XML • import javax.xml.parsers.*; • import org.xml.sax.*; • import org.xml.sax.helpers.*; • import org.w3c.dom.*; • ....

  37. Construirea parserului XML DOM • DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); • try { • DocumentBuilder db = dbf.newDocumentBuilder(); • // Gets default non-validating parser • // Parse the XML to build the whole doc tree • Document doc = db.parse(new File(“dots.xml”)); • } catch (SAXParseException spe) { // Error handling code the same as with SAX • } catch (SAXException sxe) { • } catch (ParserConfigurationException pce) { • } catch (IOException ioe) { • } catch (Throwable t) { • }

  38. Traversarea DOM • // Get root node of document • Element root = doc.getDocumentElement(); • // Get list of children of given tag name • NodeList list = root.getElementsByTagName(“dot"); • // Number of children in list • int len = list.getLength(); • // Get nth child • Element elem = (Element) list.item(n); • // Get an attribute out of a element • // (returns "" if there is no such attribute) • String s = elem.getAttribute(“x");

  39. Modificarea DOM in memorie • // Create a new node (still needs to be added) • Element elem = document.createElement(“dot"); • // Append a child node to an existing node • node.appendChild(elem); • // Set an attribute/value binding in a node. • elem.setAttribute(“x”, “12”);

  40. XSLT http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JAXPIntro6.html

  41. Scrierea DOM din memorie in XML • import javax.xml.transform.*; • import javax.xml.transform.dom.*; • import javax.xml.transform.stream.*; • // Document doc exists already in memory • try { • TransformerFactory tranFact = TransformerFactory.newInstance( ); • Transformer tran = tranFact.newTransformer( ); • DOMSource DSource = new DOMSource(doc); • StreamResult SResult = new StreamResult(new FileOutputStream(“copie.xml”)); • tran.transform(DSource, SResult); • } catch (TransformerConfigurationException tce) { • } catch (TransformerException te) { • }

  42. Cod Sursa Exemplu3 • Vezi pagina web: XMLDotsDOM.java

  43. Concluzii: XML: Avantaje si dezavantaje • Format standard • Similar fisierelor text – editabil manual, citibil direct • Big and Slow: • Reprezentarea datelor in format text ocupa foarte mult spatiu ! • Adecvat pentru reprezentarea datelor din fisiere de configurare, sau ca format de transport al datelor • NU este adecvat pentru reprezentarea continutului unei ”baze de date” de mari dimensiuni

  44. Concluzii: XML: SAX sau DOM sau ... • SAX: • mai rapid, consuma mai putina memorie • Aplicatia trebuie sa isi construiasca singura modelul de date • DOM: • construieste intai intregul model in memorie • Modelul este mai usor de transformat si salvat din nou ca XML • … Sau utilizand tool-uri care automatizeaza XML data binding: • XML data binding: procesul de reprezentare a informatiilor dintr-un document XML ca obiect in memorie • Tool-uri/API-uri care automatizeaza procesul de XML data binding: creaza automat mapari intre elementele dintr-o schema XML si campurile unei clase • Exemplu: JAXB

  45. JAXB • The Java Architecture for XML Binding (JAXB) • “provides a fast and convenient way to bind between XML schemas and Java representations” http://java.sun.com/javaee/5/docs/tutorial/doc/bnazf.html

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