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JDBC

JDBC. Eseguire una query String query = "SELECT * FROM COFFEES"; Statement stmt = con.createStatement (); ResultSet rs = stmt.executeQuery ( query ); while ( rs.next ()) { String s = rs.getString ("COF_NAME"); float n = rs.getFloat ("PRICE");

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JDBC

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  1. JDBC

  2. Eseguire una query Stringquery = "SELECT * FROM COFFEES"; Statement stmt = con.createStatement(); ResultSetrs = stmt.executeQuery(query); while (rs.next()) { String s = rs.getString("COF_NAME"); float n = rs.getFloat("PRICE"); System.out.println(s + " " + n); } rs.close(); stmt.close();

  3. rs.next() sempre… query=“SELECT count(*) AS C”; rs=stmt.executeQuery(query); if (rs.next()) int m = rs.getString("C"); else System.out.println(“ahhhhhhh”); }

  4. Ogni operazione sui dati va incluso in un blocco try catch per gestire eventuali eccezioni try { … } catch(SQLException se) { //fai qualcosa } catch(Exception e) { //fai qualcos’altro }

  5. Eseguire un update

  6. Transazioni in JDBC • Scelta della modalità delle transazioni: un metodo definito nell'interfaccia Connection: setAutoCommit(booleanautoCommit) • con.setAutoCommit(true) • (default) "autocommit": ogni operazione è una transazione • con.setAutoCommit(false) • gestione delle transazioni da programma con.commit() con.rollback() • non c'è start transaction

  7. Transazioni in JDBC try { con.setAutoCommit(false); Statement st=con.createStatement(); st.executeUpdate(“…”); st.executeUpdate(“…”); … con.commit(); } catch (Exception ex) { try { con.rollback();} catch (SQLException sqx) }

  8. CONNECTION POOL import java.sql.*; public class MyConnectionPool { // array di connessioni al database Connection con[]; // array delle disponibilità delle connessioni boolean busy[]; // registra chi sta tenendo occupata la connessione String who[]; // numero di connessioni attive int numCon; // incremento dimensione del pool per accogliere nuove richieste int inc;

  9. // parametri di accesso al database String dbUrl; String dbUsername; String dbPswd; String driverString;/** Costruttore */ public MyConnectionPool ( String dbUrl, String dbUsername, String dbPswd, int numCon, int inc, String driverString ) throws Exception { this.dbUrl = dbUrl; this.dbUsername = dbUsername; this.dbPswd = dbPswd; this.numCon = numCon; this.inc = inc; this.driverString = driverString; newConnections(); }

  10. /** * newConnections */privatesynchronizedvoidnewConnections() throwsException {// alloca gli array globali (connessioni e info) con = new Connection[numCon];busy = new boolean[numCon];who = new String[numCon];Class.forName(driverString); for (int i = 0; i < numCon; i++) {con[i] = DriverManager.getConnection(dbUrl,dbUsername,dbPswd);busy[i] = false;who[i] = ""; } }

  11. /** * extendConnections */ public synchronizedvoidextendConnections() throwsException {// copia dei vecchi vettori Connection con2[] = con;boolean busy2[] = busy;String who2[] = who;// creazione dei nuovi vettori estesi con = new Connection[numCon+inc];busy = new boolean[numCon+inc];who = new String[numCon+inc];

  12. //ciclo per trasferire le vecchie connessioni nei nuovi array for (int i = 0; i < numCon; i++) {con[i] = con2[i];busy[i] = busy2[i];who[i] = who2[i]; } //ciclo per creare le nuove connessioni da aggiungere alle precedenti for (int i = numCon; i < numCon+inc; i++) {con[i] = DriverManager.getConnection(dbUrl,dbUsername,dbPswd);busy[i] = false;who[i] = ""; }numCon += inc; }

  13. /** getConnection - assegna una connessione all’utente who */ public synchronized Connection getConnection(Stringwho) throwsException {intindFree = findFreeConnection();if (indFree<0) { // se arriva qui, il pool è saturo: lo estende e // richiama ancora findFreeConn…()extendConnections();indFree= findFreeConnection(); if(indFree < 0 ) returnnull; // se arriva qui non ci sono proprio più risorse } // salvo catastrofi verrà sempre eseguito questo codice busy[indFree] = true;who[indFree] = who;return con[indFree]; }

  14. /** getConnection */ public synchronized Connection getConnection() throwsException {returngetConnection(“noName”); }/** * releaseConnection - la connessione viene solo “liberata” */ public synchronizedvoidreleaseConnection(Connection c) { for (int i = 0; i < numCon; i++) {if (con[i] == c) {who[i] = "";busy[i] = false; } } }

  15. /** findFreeConnection - scandisce il vettore e restituisce l’indice */protectedintfindFreeConnection() { for(int i = 0; i < numCon; i++)if ( ! busy[i] ) return i;return -1; }/** printStatusConnection */ public StringprintStatusConnection() { Stringresult = ""; for (int i = 0; i < numCon; i++) result += "Conn. " + i + ": " + busy[i] + " used by: " + who[i]; returnresult; } } // chiude la classe

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