Object-Oriented Enterprise Application Development Introduction to JDBC

Topics During this class we will examine: What JDBC is and isn't Installing JDBC The JDBC Object Model Transaction Control

JDBC Introduction

JDBC Fact and Fiction Contrary to popular belief, JDBC is not an acronym for anything. Nevertheless it is often interpreted as standing for Java Data Base Connectivity. JDBC is a Java API that is used to access relational databases.

JDBC Goals The goal of JDBC is to provide a consistent interface that an application can use to perform data access. This allows the data source to be changed without requiring significant re-work of existing code.

Common JDBC Tasks We can use JDBC to perform common database access tasks such as: Establish connections Send SQL requests Process SQL results Transaction control This class assumes a working knowledge of relational database concepts and SQL.

Architecture The web server acts as our front end. The application server uses JDBC to access a back-end data source. The back-end data source can be any kind of data source that supports JDBC. Data Source JDBC Application Server Web Server

Versions The current version of JDBC is 2.0. For this class we'll use version 1.0. There are very few changes between the revisions that impact the mechanics we'll be discussing. The biggest change is in the way that connections to the database are established.

JDBC & Java

Required Packages JDBC is part of the JDK available from Sun. The only package required to use JDBC is java.sql.*. You'll find, with few exceptions, that all of the JDBC elements we use are interfaces and not classes.

Common Interfaces We won't use all of the interfaces provided by JDBC in this class. We'll focus on the most critical: DriverManager Connection Statement PreparedStatement ResultSet

Tasks There is a consistent set of steps to be followed when writing an application that accesses a data source: Connect to the data source Manipulate the data source Disconnect from the data source While conceptually these are very simple tasks, care must be taken to do them well.

Database Connectivity

Database Connectivity Evolution The process of connecting to a database has undergone a gradual evolution: Native API ODBC JDBC All database connectivity takes place through the use of something called a driver.

Native API The most efficient connections use the native database API. This is the fastest approach but the least portable. If we move to a new database we need to modify our code base. Native API

ODBC ODBC was created to "wrap" each vendor's native API within a common interface. Code was written to use ODBC rather than the native API. This was less efficient but more portable. Native API ODBC

JDBC JDBC is similar to ODBC in that it wraps a vendor's native API. The JDBC object model is much simpler than ODBC or most native APIs. Because it's Java- based, it's portable. Native API JDBC

JDBC-ODBC Bridge For this class we'll use the JDBC-ODBC Bridge driver. This is the driver shipped with the JDK from Sun. It isn't very efficient, but it's free and easy to install. Native API ODBC JDBC

Configure ODBC The first step is to configure ODBC with an appropriate Data Source Name or DSN. The process for creating this DSN is outlined in the ODBC document available on the course web site. For this course, please use a DSN of se452 for all of your assignments.

Loading a JDBC Driver The next step is to load an appropriate JDBC driver. To do this, we force the JVM to load the driver using the forName() method of the Class class: Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver");

Opening a Connection (1 of 4) The next step is to open a connection to the database using the driver that was just loaded. This step must be completed before any other work can be performed against the database by our application. The connection to the database is held by an instance of the Connection interface.

Opening a Connection (2 of 4) We create a physical connection to the data source by using the getConnection() method of the DriverManager class: Connection conn = DriverManager. getConnection(String URL, String UID, String PWD);

Opening a Connection (3 of 4) The following code segment connects the application to an ODBC data source name called se452 with no user id or password: Connection conn = DriverManager. getConnection("jdbc:odbc:se452", "", "");

Opening a Connection (4 of 4) Creating database connections is an expensive process requiring significant database resources. We typically create connections as late in our processing as possible and close them as soon as we can to minimize our resource usage against the database.

Sample Code – Connect (1 of 2) 1.import java.sql.*; 2.public class Connect { 3. Connection conn = null; 4. public static void main(String [] args) { 5. Connect myConnect = new Connect(); 6. myConnect.doConnect(); 7. }

Sample Code – Connect (2 of 2) 8. public void doConnect() { 9. try { 10. Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver"); 11. conn = DriverManager.getConnection( "jdbc:odbc:se452", "", "" ); 12. } 13. catch (ClassNotFoundException clfe) { 14. } 15. catch (SQLException sqle) { 16. } 17. } 18.}

Error Detection Notice that the loading of the JDBC driver and the creation of the connection are performed within a try…catch block. The Class.forName() method can generate a ClassNotFoundException. All JDBC operations can result in a SQLException.

Closing a Connection (1 of 2) Database connections consume resources on both the client and the database server. We need to close the open connections in a to ensure that these resources are returned to the client or database in a timely manner. Do not wait for the garbage collector to free these resources for you.

Closing a Connection (2 of 2) We don't need to do anything to close the DriverManager. However, every connection that was opened using the getConnection() method must be closed using the close() method on the Connection interface: conn.close();

Sample Code – Connect (1 of 3) 1.import java.sql.*; 2.public class Connect { 3. Connection conn = null; 4. public static void main(String [] args) { 5. Connect myConnect = new Connect(); 6. myConnect.doConnect(); 7. // do stuff 8. myConnect.doDisconnect(); 9. }

Sample Code – Connect (2 of 3) 10. public void doConnect() { 11. try { 12. Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver" ); 13. conn = DriverManager.getConnection( "jdbc:odbc:se452", "", "" ); 14. } 15. catch (ClassNotFoundException clfe) { 16. } 17. catch (SQLException sqle) { 18. } 19. }

Sample Code – Connect (3 of 3) 20. public void doDisconnect() { 21. try { 22. conn.close(); 23. } 24. catch (SQLException sqle) { 25. } 26. } 27.}

Making the Code More Robust While the code I've show you is adequate as written, it isn't really robust or user-friendly. In this context a user is any other developer who is using the code you are writing. We're going to re-write the code so that it demonstrates better software engineering principles of maintainability and reusability.

Sample Code – Connect (1 of 3) 1.import java.sql.*; 2.public class Connect { 3. Connection conn = null; 4. public static void main(String [] args) { 5. Connect myConnect = new Connect(); 6. myConnect.doConnect(); 7. // do stuff 8. myConnect.doDisconnect(); 9. }

Sample Code – Connect (rev.) (1 of 3) 1.import java.sql.*; 2.public class Connect { 3. Connection conn = null; 4. public static void main(String [] args) { 5. Connect myConnect = new Connect(); 6. try { 7. myConnect.doConnect(); 8. // do stuff 9. } 10. finally { 11. myConnect.doDisconnect(); 12. } 13. }

Sample Code – Connect (2 of 3) 10. public void doConnect() { 11. try { 12. Class.forName( 13. "sun.jdbc.odbc.JdbcOdbcDriver" ); 14. conn = DriverManager.getConnection( 15. "jdbc:odbc:se452", "", "" ); 16. } 17. catch (ClassNotFoundException clfe) { 18. } 19. catch (SQLException sqle) { 20. } 21. }

Sample Code – Connect (rev.) (2 of 3) 14. public void doConnect() { 15. try { 16. Class.forName( 17. "sun.jdbc.odbc.JdbcOdbcDriver" ); 18. if (conn != null) { 19. this.doDisconnect(); 20. } 21. conn = DriverManager.getConnection( 22. "jdbc:odbc:se452", "", "" ); 23. } 24. catch (ClassNotFoundException clfe) { 25. } 26. catch (SQLException sqle) { 27. } 28. }

Sample Code – Connect (3 of 3) 22. public void doDisconnect() { 23. try { 24. conn.close(); 25. } 26. catch (SQLException sqle) { 27. } 28. } 29.}

Sample Code – Connect (rev.) (3 of 3) 29. public void doDisconnect() { 30. try { 31. if (conn != null) { 32. conn.close(); 33. conn = null; 34. } 35. } 36. catch (SQLException sqle) { 37. } 38. } 39.}

Error Detection In the sample code there are exceptions that are caught but nothing is done about it. Within those blocks I'd likely embed code to display the stack trace of the exception so an effective post-mortem could be done. If you can't handle an exception, you shouldn't catch it.

Retrieving Data

Select Statements One we have a connection to the data source, we can begin to issues queries and process the results. This requires three (3) new interfaces: Statement PreparedStatement ResultSet

Tasks To issue read requests to the data source we will perform the following tasks: Create the statement Execute the statement Process the results

Creating the Statement A Statement object is used to send SQL queries to the database. It's created using a Connection object: Statement stmt = conn.createStatement();

Executing the Statement (1 of 2) Creating a Statement object doesn't itself execute queries against the database. To do this, we pass a SQL statement to the database using the executeQuery() method on that Statement object: String SQL = "select * from STATE"; stmt.executeQuery( SQL );

Executing the Statement (2 of 2) The call to the executeQuery() method returns a ResultSet object containing the results of the query: String SQL = "select * from STATE"; ResultSet rs = stmt.executeQuery( SQL );

Processing the Result Processing a ResultSet is similar to processing a sequential file in that we process each individual row until we hit the end of the ResultSet object. This loop is accomplished using the next() method of the ResultSet : while ( rs.next() ) { … }

Anatomy of a ResultSet A ResultSet is nothing more than a two-dimensional table. There is a "pointer" showing the current row. Each call to next() moves this pointer to the next row. Row n … Row 2 Row 1 Start of ResultSet

Columns (1 of 2) In addition to rows, each ResultSet contains one column for each column specified by the underlying select statement. Each column can be accessed by either its name or relative position within the ResultSet.

Columns (2 of 2) To retrieve a column's value for the current row in the ResultSet we use one of many column accessor methods. Each accessor method is overloaded. One variation accepts a string that corresponds to the column's name. One variation accepts an integer that corresponds to the column's relative position.

Columns by Name We can now construct code to print each element within the ResultSet using the column name: while ( rs.next() ) { String code = rs.getString( "STATE_I" ); String name = rs.getString( "STATE_M" ); }

Columns by Number We can now construct code to print each element within the ResultSet using the column number: while ( rs.next() ) { String code = rs.getString(1); String name = rs.getString(2); }

Sample Code – Result (1 of 4) 1.import java.sql.*; 2.public class Connect { 3. Connection conn = null; 4. public static void main(String [] args) { 5. Connect myConnect = new Connect(); 6. try { 7. myConnect.doConnect(); 8. myConnect.doQuery(); 9. } 10. finally { 11. myConnect.doDisconnect(); 12. } 13. }

Sample Code – Result (2 of 4) 14. public void doConnect() { 15. // as before 28. } 29. public void doDisconnect() { 30. // as before 38. }

Sample Code – Result (3 of 4) 39. public void doQuery() { 40. Statement stmt = null; 41. ResultSet rs = null; 42. try { 43. if (conn != null) { 44. stmt = conn.createStatement(); 45. rs = stmt.executeQuery( "select * from STATE" );

Sample Code – Result (4 of 4) 46. while ( rs.next() ) { 47. System.out.println( "Code: " + rs.getString( "STATE_I" ); 48. System.out.println( "Name: " + rs.getString( "STATE_M" ); 49. } 50. } 51. } 52. catch (SQLException sqle) { 53. } 54. } 55.}

Closing a Statement As with a Connection, always close a Statement object. This ensures that the appropriate cleanup is performed and the resources held by that Statement are released: stmt.close();

Closing a ResultSet As with Connection and Statement objects, always close ResultSet objects. This ensures that the appropriate cleanup is performed and the resources held by that ResultSet are released: rs.close();

Making the Code More Robust We're going to re-write the doQuery() method so that it demonstrates better software engineering principles by closing all of the appropriate objects created during its lifetime. We'll accomplish this by using a try…catch…finally block.

Sample Code – Result (rev.) (1 of 2) 39. public void doQuery() { 40. Statement stmt = null; 41. ResultSet rs = null; 42. try { 43. // all code as before 51. } 52. catch (SQLException sqle1) { 53. } 54. finally { 55. if ( rs != null ) { 56. try { 57. rs.close(); 58. }

Sample Code – Result (rev.) (2 of 2) 59. catch (SQLException sqle2) { 60. } 61. } 62. if ( stmt != null) { 63. try { 64. stmt.close(); 65. } 66. catch (SQLException sqle3) { 67. } 68. } 69. } 70. } 71.}

Commonly Executed Statements

Statements Revisited Executing a SQL statement against the data source involves many stages: Validate the syntax Validate permissions Construct a query plan Execute the query Build the result

PreparedStatements For commonly executed queries, or queries that can be parameterized, we can use a PreparedStatement object in place of a Statement object. A PreparedStatement can be executed many times against the database without incurring the overhead involved with syntax checking and other validations.

Creating the PreparedStatement A PreparedStatement object is created in much the same way as a Statement. Instead of createStatement(), we use the prepareStatement() method: String SQL = "select * from STATE"; PreparedStatement stmt = conn.prepareStatement( SQL );

Executing the PreparedStatement Creating a PreparedStatement object doesn't execute it against the database. As with the Statement object we can execute the PreparedStatement and capture its ResultSet : ResultSet rs = stmt.executeQuery();

Parameters (1 of 3) Sometimes a given SQL statement only changes in the values that we pass to the database for processing. For instance we might execute a query for states with names beginning with 'S' and then later for states with names beginning with 'T'. PreparedStatements allow this kind of query using parameters.

Parameters (2 of 3) To indicate that a given value is a parameter, we simple embed a ' ? ' in the SQL statement where the argument will eventually be provided. You can only use parameters for values in where and having clauses. You cannot dynamically alter the columns, tables, grouping, or sort order of the query.

Parameters (3 of 3) To construct a query that will change based on the value of the state's code we can construct a parameterized SQL statement: String SQL = "select * from STATE " + "where STATE_I like ?"; PreparedStatement stmt = conn.prepareStatement( SQL );

Executing Parameterized PreparedStatements Before we can execute a parameterized PreparedStatement, we first have to provide values for the parameters: stmt.setString( 1, "I%" ); ResultSet rs = stmt.executeQuery();

Sample Code – Prepared (1 of 2) 39. public void doQuery( String arg ) { 40. PreparedStatement stmt = null; 41. ResultSet rs = null; 42. try { 43. String SQL = "select * from STATE"+ " where STATE_I like ?"; 44. stmt = conn.prepareStatement(SQL); 45. stmt.setString( 1, arg ); 46. rs = stmt.executeQuery(); 47. }

Sample Code – Prepared (2 of 2) 48. catch (SQLException sqle1) { 49. } 50. finally { 51. // all code here as before 52. } 53. }

Writing to the Database

Other Queries So far we've only looked at select statements. We can also use the Statement and PreparedStatements interfaces to write to the database using insert, update, and delete statements.

Insert Statement String SQL = "insert into STATE " + "values ('OH', 'Ohio')"; Statement stmt = conn.createStatement(); int count = stmt.executeUpdate( SQL );

Insert PreparedStatement String SQL = "insert into STATE " + "values (?, ?)"; PreparedStatement stmt = conn.prepareStatement( SQL ); stmt.setString( 1, 'OH' ); stmt.setString( 2, 'Ohio' ); int count = stmt.executeUpdate();

Update Statement String SQL = "update STATE " + "set STATE_M = 'Skippy' " + "where STATE_I = 'IL'"; Statement stmt = conn.createStatement(); int count = stmt.executeUpdate( SQL );

Update PreparedStatement String SQL = "update STATE " + "set STATE_M = ? " + "where STATE_I = ?"; PreparedStatement stmt = conn.prepareStatement( SQL ); stmt.setString( 1, 'Skippy' ); stmt.setString( 2, 'IL' ); int count = stmt.executeUpdate();

Delete Statement String SQL = "delete from STATE " + "where STATE_I = 'IL'"; Statement stmt = conn.createStatement(); int count = stmt.executeUpdate( SQL );

Delete PreparedStatement String SQL = "delete from STATE " + "where STATE_I = ?"; PreparedStatement stmt = conn.prepareStatement( SQL ); stmt.setString( 1, 'IL' ); int count = stmt.executeUpdate();

Transaction Control

Transactions Defined Transactions are used to ensure that multiple SQL insert, update, and delete statements are completed as a single unit. Either all of the statements in the unit complete successfully, or none of the changes are applied.

Starting a Transaction By default most databases are configured such that each individual SQL statement executes within its own transaction. To alert the database that multiple statements will be a part of the transaction we need to set the auto-commit property of the Connection object: conn.setAutoCommit( false );

Ending a Transaction (1 of 2) To end a transaction you must instruct the Connection object to perform only one (1) of two (2) actions: Commit: The changes made to the database are saved. Rollback: The changes made to the database are discarded. Once a transaction has ended, a new one is begun automatically.

Ending a Transaction (2 of 2) To commit or rollback a transaction, simply invoke the appropriate method on the Connection object: conn.commit() conn.rollback(); You should always commit or rollback a transaction as soon as possible to release the resources it is using.

Review During this class we have discussed: What JDBC is and isn't Installing JDBC The JDBC Object Model Transaction Control

Resources JDBC Database Access with Java Graham Hamilton, Rick Cattell, Maydene Fisher, Addison-Wesley, ISBN: Core Servlets and JavaServer Pages Marty Hall, Prentice-Hall, Inc., pp ISBN:

Coming Attractions Next week we'll look at JDBC 2.0.