Framing. Accessing DBMS from your code. A possible way of writing applications accessing DBMS. ODBC: Open Data Base Connectivity

Transcription

1 JDBC

2 Framing Accessing DBMS from your code A possible way of writing applications accessing DBMS You ll see other ways via Web technologies ODBC: Open Data Base Connectivity C/C++ API JDBC: Java Data Base Connectivity Java API Both define interfaces, i.e. Set of methods you can use to do SQL queries (both DDL and DML) 2

3 JDBC Architecture 3

4 Advantages Separate your code from the native language of the DBMS You can use JDBC with any DBMS that provides a driver (or for which a bridge is available, see later) Not necessarily a relational DBMS But JDBC always accepts SQL-like statements! You can re-use your code with any DBMS Provided you have the relative driver 4

5 Lab setup PostgreSQL provides native Java drivers Postgresql JDBC library must be included in NetBeans projects project s properties libraries run and/or compile add postgresql JDBC driver 8

6 Typical actions within the code Load the DBMS driver Create a Connection with the DBMS Any number of times Submit SQL statements Retrieve results Process data/errors Commit or rollback Close the connection 9

7 Code spots Using (at least) one java package to interact with the JDBC driver import java.sql.*; in your import list javax.sql: advanced feature (we will not cover them) Load the driver Class.forName( org.postgresql.driver ); Catch a ClassNotFound exception!!! Connect to the DBMS String url= jdbc:postrgesql://delmastrof.iit.cnr.it/u01db ; Connection con = DriverManager.getConnection(url, u01, m-s-t ); user password 10

8 (A bit more) General format of the URL string Connect to the DBMS String url= jdbc:postrgesql://delmastrof.iit.cnr.it/u01db ; Connection con = DriverManager.getConnection(url, u01, m-s-t ); 11

9 Submit SQL statements DML Statement st = con.createstatement(); ResultSet rs = st.executequery( SELECT * FROM net WHERE gw= ); Or, you can use PreparedStatements: DDL PreparedStatement pst = con.preparestatement( S E L E C T * F R O M n e t W H E R E a d m i n =? ) ; pst.clearparameters(); pst.setstring(1, gianni ); ResultSet rs = pst.executequery(); Statement st = con.createstatement( INSERT INTO net VALUES ( andrea, , /24 ) ); int numrows = st.executeupdate(); 12

10 Getting results back DDL statements return the number of rows modified Statement st = con.createstatement( INSERT INTO net VALUES ( andrea, , /24 ) ); int numrows = st.executeupdate(); DML statements return a ResultSet (an iterator) ResultSet rs = st.executequery(query); while(rs.next()){ String ad = rs.getstring( admin ); // or, String ad = rs.getstring(1); if admin is the first // field of the table } Move in the ResultSet with methods previous(), absolute(int), relative(int), first(), last() 13

11 ResultSet and Statements A ResultSet is associated with the previous statement Don t issue another query via the same statement before getting back all results: WRONG ResultSet rs = st.executequery(query); while(rs.next()){ S t r i n g a d = r s. g e t S t r i n g ( a d m i n ) ; ResultSet rs1 = st.executequery(query1); } CORRECT ResultSet rs = st.executequery(query); Statement st1 = con.createstatement(); while(rs.next()){ S t r i n g a d = r s. g e t S t r i n g ( a d m i n ) ; ResultSet rs1 = st1.executequery(query1); } 14

12 Data types Most are already defined by Java integer, String, Other are defined in the java.sql package java.sql.date, java.sql.time, java.sql.timestamp, Other within the driver package Org.postgresql.geometric.PGcircle, Org.postgresql.geometric.PGline, See index.html 15

13 Transactions By default, each statement is a transaction To alter this, Connection con = ; con.setautocommit(false); // con.getautocommit() to retrieve the current commit mode con.commit(); to close a transaction explicitly 16

14 Exceptions Methods throw SQLException getsqlstate() return a SQL state identifier geterrorcode() return a vendor-specific error code So, typically try{ Statement st = con.createstatement(); ResultSet rs = st.executequery(query); while(rs.next()){ // process results } } catch SQLException(e){ // process exception here } 17

15 Getting metadata Use ResultSetMetaData type to get metadata about a ResultSet Information such as the name and type of fields Have a look at host.java In general, use the Java API specs for documentation 18

16 Secure connections String url = jdbc:postrgesql://delmastrof.iit.cnr.it/u01db? user=u01&password=m-s-t+20+12&ssl=true ; Connection con = DriverManager.getConnection(url); 19

17 Secure connections String url = jdbc:postrgesql://delmastrof.iit.cnr.it/u01db? user=u01&password=m-s-t+20+12&ssl=true& sslfactory=org.postgresql.ssl.nonvalidatingfactory ; Connection con = DriverManager.getConnection(url); String url = "jdbc:postgresql://delmastrof.iit.cnr.it/u01db ; Properties props = new Properties(); props.setproperty("user", u01"); props.setproperty("password", m-s-t+20+12"); props.setproperty("ssl","true"); props.setproperty("sslfactory", "org.postgresql.ssl.nonvalidatingfactory"); // no cert. validation Connection conn = DriverManager.getConnection(url, props); See

18 Useful examples provafile.java Managing I/O from/to file provaio.java Managing input from keyboard provainet.java Managing network data types 21

19 NetBeans working directory header of any relative path in your source code 22