1. 5 Copyright © 2005, Oracle. All rights reserved. Accessing the Database with Servlets

2. 5-2 Copyright © 2005, Oracle. All rights reserved. Objectives After completing this lesson, you should be able to do the following: Load and register a JDBC driver Connect to an Oracle database by using data sources Navigate in a ResultSet Use PreparedStatement Create a pool of connections

3. 5-3 Copyright © 2005, Oracle. All rights reserved. Review of JDBC JDBC is a standard interface for connecting to relational databases from Java. The JDBC classes and interfaces are in the java.sql package.

4. 5-4 Copyright © 2005, Oracle. All rights reserved. Querying in JDBC Connect Close Query import java.sql.*; DriverManager.registerDriver(…) Connection conn = DriverManager.getConnection( "jdbc:oracle:thin … Statement stmt = conn.createStatement (); ResultSet rset = stmt.executeQuery ( "select * from EMPLOYEES"); while (rset.next ()) System.out.println( rset.getString (2)); rset.close(); stmt.close(); conn.close(); Process results

5. 5-5 Copyright © 2005, Oracle. All rights reserved. JDBC and Servlets There are three ways to use JDBC in a servlet: –Register the JDBC driver within the servlet by hard coding the driver name in either the servlet or in a properties file. –Use the JDBC driver from the data-sources.xml file that is provided with Oracle Application Server 10g. –Use a properties file to store connection details. In all cases, optimize the connection: –Initialize the database connection in the servlets init() method (or retrieve from a pool). –Close the database connection in the destroy() method (or return to a pool).

6. 5-6 Copyright © 2005, Oracle. All rights reserved. Synchronizing Shared Resources Reuse PreparedStatement objects. Sharing Statement objects may not be thread safe. Use a synchronized block. PreparedStatement ps = … … synchronized (ps) { ps.clearParameters(); ps.setInt(1,3); ps.setDouble(2, 3.14); ps.executeUpdate(); }

7. 5-7 Copyright © 2005, Oracle. All rights reserved. Transaction Handling Initialize Connection in the servlets init() method. Problems with transactions: –The Connection object in the servlets init() method is shared. –The commit() method depends on the Connection object. Solutions for transactions: –Create a new Connection object. –Use the synchronized keyword. –Use the SingleThreadModel interface. –Use session tracking.

8. 5-8 Copyright © 2005, Oracle. All rights reserved.

9. 5-9 Copyright © 2005, Oracle. All rights reserved. Connection Pooling Pooled Connection objects are used and released by servlet instances. A connection pool performs the following tasks: Preallocates database connections Manages available connections Allocates new connections Closes connections that are no longer in use

10. 5-10 Copyright © 2005, Oracle. All rights reserved. Data Sources Data sources provide logical mappings of databases: –Developer uses the logical representation of a database. –Deployer maps to the physical data sources. J2EE applications use published DataSource objects by: –Looking up the published name via Java Naming and Directory Interface (JNDI). –Using JDBC Connection methods to connect to the database. Data sources are published in the JNDI tree. Data sources come in different varieties.

11. 5-11 Copyright © 2005, Oracle. All rights reserved. Data Source Definition Global data sources are defined in the OC4J_HOME/config/data-sources.xml file. –You specify each data source by using an XML tag. –Attributes specify values for the data source. Application-specific data sources: Use the tag in the application.xml file. OC4J_HOME/applications myapp application.xml data-sources.xml Points to

12. 5-12 Copyright © 2005, Oracle. All rights reserved. data-sources.xml : Example <data-source class="com.evermind.sql.DriverManagerDataSource" connection-driver="oracle.jdbc.driver.OracleDriver" name="OracleDS" location="jdbc/OracleCoreDS" xa-location="jdbc/xa/OracleDS" ejb-location="jdbc/OracleDS" min-connections="5" max-connections="25" username="oe" password="oe" url="jdbc:oracle:thin:@ : : " />

13. 5-13 Copyright © 2005, Oracle. All rights reserved. Using Data Sources To use a defined data source in a servlet: 1.Use the lookup method of the javax.naming.Context class to retrieve the named data source class. 2.Create the connection. try { Context ic = new InitialContext(); DataSource ds = (DataSource)ic.lookup("jdbc/OracleDS"); Connection conn = ds.getConnection(); } catch (SQLException se) { … } catch (NamingException ne) {… }

14. 5-14 Copyright © 2005, Oracle. All rights reserved. Summary In this lesson, you should have learned how to: Create a servlet to connect to the database by using JDBC Load and register a JDBC driver Connect to an Oracle database by using data sources Navigate in a ResultSet Use PreparedStatement Improve database performance by using connection pooling

15. 5-15 Copyright © 2005, Oracle. All rights reserved. Practice 5-1: Overview This practice covers the following topics: Connecting to the database by using JDBC Retrieving database information and formatting it for output in a servlet

16. 5-16 Copyright © 2005, Oracle. All rights reserved.

17. 5-17 Copyright © 2005, Oracle. All rights reserved.

18. 5-18 Copyright © 2005, Oracle. All rights reserved.