Java E-Commerce Martin Cooke,

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1 Java E-Commerce Martin Cooke, Java Database Connectivity (JDBC) Plan Java database connectivity API (JDBC) Examples Advanced features JNDI JDBC 13/02/2004 Java E-Commerce Martin Cooke, Design goals ODBC JDBC architecture Build on the simplicity and portability of SQL Simple to use for non-database programmers queries return Java objects access problems return as exceptions Hide specific database details from programmer Generalise to any tabular data source Learn from (and avoid the mistakes of) ODBC Open database connectivity Open = so long as you operate within Windows :-) C API which doesn t translate well into Java Overly complex design from programmers perspective Complete failure to transit outside Windows environment MySQL Oracle Postgres 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

2 Java E-Commerce Martin Cooke, JDBC architecture JDBC architecture How-to JDBC defines a set of interfaces Vendors implement these for their database engine Known as a JDBC driver MySQL JDBC Postgres Oracle JDBC defines a set of interfaces Vendors implement these for their database engine Known as a JDBC driver Easy to switch vendors without changing a line of code (simple an entry in a configuration file choosing a different driver) Can develop on a toy system like MS Access then migrate to industrialstrength later with ease App 1 App 2 JDBC MySQL Oracle Postgres Acquire a (free) relational database engine postgresql MySQL msql instantdb Acquire a (free) JDBC driver for the chosen engine See list at industry.java.sun.com/products/jdbc/drivers 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, MySQL Some factoids Developed from 1995 by Michael Widenius in Sweden Described as one of the hottest grass roots software projects since Linux Ran on estimated 500,000 servers in 1999 and has grown significantly since Available for most platforms Industrial-strength: fast and stable Extensively documented Available in DCS Accessing your data in 4 steps 1: load the driver Connection con = null; Class.forName("org.gjt.mm.mysql.Driver").newInstance(); con = DriverManager.getConnection ("jdbc:mysql://hazel/studs", mpc",""); Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery("select name FROM student"); while (rs.next()) System.out.println(rs.getString(1)); catch (SQLException e) { System.out.println("SQLException: " + e.getmessage()); catch (Exception e) { System.err.println("Unable to load driver."); e.printstacktrace(); 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

3 Java E-Commerce Martin Cooke, : get a connection 3: execute a query 4: do something with the results Connection con = null; Class.forName("org.gjt.mm.mysql.Driver").newInstance(); con = DriverManager.getConnection ("jdbc:mysql://hazel/studs", mpc",""); Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery("select name FROM student"); while (rs.next()) System.out.println(rs.getString(1)); catch (SQLException e) { System.out.println("SQLException: " + e.getmessage()); catch (Exception e) { System.err.println("Unable to load driver."); e.printstacktrace(); Connection con = null; Class.forName("org.gjt.mm.mysql.Driver").newInstance(); con = DriverManager.getConnection ("jdbc:mysql://hazel/studs", mpc",""); Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery("select name FROM student"); while (rs.next()) System.out.println(rs.getString(1)); catch (SQLException e) { System.out.println("SQLException: " + e.getmessage()); catch (Exception e) { System.err.println("Unable to load driver."); e.printstacktrace(); Connection con = null; Class.forName("org.gjt.mm.mysql.Driver").newInstance(); con = DriverManager.getConnection ("jdbc:mysql://hazel/studs", mpc",""); Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery("select name FROM student"); while (rs.next()) System.out.println(rs.getString(1)); catch (SQLException e) { System.out.println("SQLException: " + e.getmessage()); catch (Exception e) { System.err.println("Unable to load driver."); e.printstacktrace(); 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Tidying up a precious resource Key JDBC classes More on statements rs.close(); stmt.close(); finally { if (con!= null) con.close(); DriverManager Connection Statement ResultSet Used, amongst other things, to obtain connection Logical connection used to send SQL to the database SQL statement Data returned by a query For queries public ResultSet executequery(string sql) For updates public int executeupdate(string sql) (returns number of rows affected) When you don t know which public boolean execute(string sql) 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

4 Java E-Commerce Martin Cooke, More on result sets More on result sets Provides access to database queries one row at a time Moves through sequentially, once only ResultSet object is a reference, since actual result set could be very large Methods for getting common data types getdate() getint() getstring() Can use getobject() if you don t know the type JDBC 2.0 provides scrollable result sets (later) Other features of JDBC 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Other features of JDBC Meta data Prepared statements Transactions Scrollable result sets Storing binary data Meta data Suppose we don t know how many columns will be returned by a query; or what the columns are called Use ResultSetMetaData class ResultSetMetaData rsmd = rs.getmetadata() rsmd.getcolumncount(i) rsmd.getcolumnlabel(i) Prepared statements Like an ordinary Statement in that it represents an SQL statement Is precompiled by database for faster execution Can be modified after compilation PreparedStatement pstmt = con.preparestatement( SELECT name FROM student WHERE id=? ); pstmt.clearparameters(); pstmt.setint(1,somevalue); ResultSet rs = pstmt.executequery(); 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

5 Java E-Commerce Martin Cooke, Prepared statements contd Transactions Transaction issues Useful when need to run same (parameterised) statement many times Also useful when there are lots of parameters to fill in, to avoid huge and messy SQL strings Also useful because of the quotes problem: INSERT INTO student VALUES (null, Basil d Oliveira ) Either remember to escape quotes with \ or Use prepared statement Classic example: transferring money between accounts: Debit account 1, credit account 2 What if system fails between the two? Less problematic in some applications reading incorrect data Dirty reads non-repeatable reads Phantom reads transaction 1 can see uncommitted changes from transaction 2 1. transaction 1 reads a row 2. transaction 2 alters the row 3. transaction rereads row, getting different values Changes in another transaction cause new rows to match your WHERE clause 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Transaction isolation Transactions in JDBC MySQL support for transactions Transaction isolation defines the visibility of one transaction on another JDBC specifies 5 levels of isolation weakest: TRANSACTION_NONE (no support for transactions) stongest: TRANSACTION_SERIALIZABLE (full isolation) Typically, tradeoff between isolation and speed Full isolation is costly and often OTT 13/02/2004 Java E-Commerce Martin Cooke, con.setautocommit(false); // defaults to true s = con.createstatement(); s.executeupdate( INSERT ); s.close(); s = con.createstatement(); s.executeupdate( INSERT ); con.commit(); s.close(); catch (SQLException e) { if (con!= null) { con.rollback(); catch (SQLException e) { 13/02/2004 Java E-Commerce Martin Cooke, Yes, since version Done via BerkeleyDB Simply add table type at table create time: CREATE TABLE student ( id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, name VARCHAR(4), SSID INT DEFAULT 0 ) TYPE=BDB; 13/02/2004 Java E-Commerce Martin Cooke,

6 Java E-Commerce Martin Cooke, Scrollable result sets Storage/retrieval of large/binary data Binary and character large objects Added in JDBC 2.0 Allows in-place modification of result sets and more control of the result set cursor Can be inefficient for some database drivers Large text strings (eg entire books), up to several gigs Binary objects (eg GIF images, sound files) LONGVARCHAR BINARY VARBINARY LONGVARBINARY getasciistream() getbinarystream() Large text strings (eg entire books), up to several gigs Binary objects (eg GIF images, sound files) LONGVARCHAR CLOB BINARY VARBINARY LONGVARBINARY BLOB getasciistream() getclob() getbinarystream() getblob() Can be source of problems due to drivers Sometimes have small maximum sizes Underlying object is often read all at once New in JDBC 2.0 Return streams, so data read at your leisure Not so simple to store 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Connection bottlenecks Solutions Connection pooling Creating a connection is a very expensive operation (several seconds, typically) The open connection, talk to database, close connection model is simply infeasible for typical e-commerce applications Singleton: open a single connection when webapp inits and use it for all DB operations + guaranteed to be thread safe, but consequently doesn t avoid performance bottleneck + easy and fast for small sites - doesn t exploit database support for simultaneous connections 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

7 Java E-Commerce Martin Cooke, Solutions Solutions Connection pool how-to: 1 Singleton: open a single connection when webapp inits and use it for all DB operations Connection pool: open a pool of connections on init and dish them out as needed + guaranteed to be thread safe, but consequently doesn t avoid performance bottleneck + easy and fast for small sites - doesn t exploit database support for simultaneous connections + supported in JDBC easy + fast + scales to very large sites Singleton: open a single connection when webapp inits and use it for all DB operations Connection pool: open a pool of connections on init and dish them out as needed Session connections: allocate a connection for each active user (session) + guaranteed to be thread safe, but consequently doesn t avoid performance bottleneck + easy and fast for small sites - doesn t exploit database support for simultaneous connections + supported in JDBC easy + fast + scales to very large sites + very fast + quite easy to implement - not good for sites with very high traffic Use 3rd party connection pool class com.javaexchange.dbconnectionbroker mybroker = new DbConnectionBroker (driver,dburl,user,pwd, mincs,maxcs,logfile,maxconnecttime); conn = mybroker.getconnection(); mybroker.freeconnection(conn); Not necessarily portable since doesn t follow API 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Connection pool how-to: 2 DataSource DataSource Use JDBC 2.0 Optional Package (ie find a driver that supports it) Feb 2002 version of mm driver Several advantages other than pooling Abstracts most connection details Supports distributed transactions Transactions which span more than one datasource eg buy book, update book db, credit card db, Not norm yet, but likely to be soon because of vendor-independence engendered by JDBC Context ctx = new InitialContext(); DataSource ds = (DataSource) ctx.lookup( jdbc/mydb ); A context is a JNDI concept (see next slide) Use context to lookup a DataSource Context ctx = new InitialContext(); DataSource ds = (DataSource) ctx.lookup( jdbc/mydb ); Connection con = ds.getconnection(user,pwd); // use connection con.close() A context is a JNDI concept (see next slide) Use context to lookup a DataSource Get connection (from pool) Use it Close it (returns it to the pool) 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

8 Java E-Commerce Martin Cooke, JNDI JNDI JNDI Java Naming and Directory Interface Enterprise-level API used to provide uniform access to services: File systems Databases Distributed objects (later) A naming service matches things such as printers, files, file servers, to names A naming service matches things such as printers, files, file servers, to names A directory service adds attributes to the names (eg colour printer, database driver class) Source: Source: 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, Why JNDI? Uniform access to existing & diverse naming services: COS (Common Object Services) Naming: The naming service for CORBA applications; DNS (Domain Name System): The Internet's naming service; maps people-friendly names (such as into computer-friendly IP (Internet Protocol) addresses in dotted-quad notation ( ). LDAP (Lightweight Directory Access Protocol): Developed by the University of Michigan; as its name implies, it is a lightweight version of DAP (Directory Access Protocol), which in turn is part of X.500, a standard for network directory services. NIS (Network Information System) and NIS+: Network naming services developed by Sun Microsystems. Both allow users to access files and applications on any host with a single ID and password. Source: JNDI benefits Virtual: uses simple URLs and hierarchical structure Dynamic: can configure services at runtime Enables finding by logical names rather than by knowing the paths DataSource ds = (DataSource) ctx.lookup( jdbc/mydb ); JNDI how-to Objects (such as DataSource) have to be bound to the JNDI service before they can be accessed by clients Usually done with a GUI tool; one-off process Can be done programmatically: Context ctx = new InitialContext(); ds = new MysqlDataSource(); ds.setservername("localhost"); ds.setdatabasename("test"); ctx.bind( jdbc/test", ds); ctx.close(); Don t forget to import java.naming.* 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,

9 Java E-Commerce Martin Cooke, Book Online documents Resources Reese (2000) JDBC and Java (2 nd ed), O Reilly, JNDI articles 13/02/2004 Java E-Commerce Martin Cooke, /02/2004 Java E-Commerce Martin Cooke,