1. 1 JDBC "Java Database Connectivity"

2. 2 Getting Started Guide: http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/g etstart/GettingStartedTOC.fm.html java.sql Package API: http://java.sun.com/j2se/1.5.0/docs/api/java/sql/ package-summary.html Useful JDBC Links

3. 3 Introduction to JDBC JDBC allows for convenient database access from Java applications Data is transferred from relations to objects and vice-versa –databases optimized for searching/indexing –objects optimized for engineering/flexibility

4. 4 Why Access a Database from within a Program? As we saw last week, some queries can’t be computed in SQL. PL/pgSQL includes more programming tools than SQL However, sometimes using PL/pgSQL will not be suitable: –If we require object-oriented programming –If accessing the database is a small part of a large Java application –Etc. Why not keep all the data in Java objects? “Separation of concerns”: DBMSes concentrate on data storage and access; programs concentrate on algorithms, networking, etc.

5. 5 Packages to Import In order to connect to a database from java, import the following packages: –java.sql.*; (usually enough) –javax.sql.* (for advanced features, such as scrollable result sets)

6. 6 access Add the following line to your.classpath file (located in your home directory): setenv CLASSPATH ${CLASSPATH}:/usr/share/java/postgresql.jar And then open a new shell

7. 7 Six Steps Load the driver Establish the Connection Create a Statement object Execute a query Process the result Close the connection

8. JDBC Architecture (1) Application DriverManager is provided by Java Software as part of the Java 2 Platform. Drivers are provided by DBMS vendors. Driver Manager DBMS

9. JDBC Architecture (2) The application creates a driver instance and registers it with the DriverManager. The DriverManager tells the driver to connect to the DB The DriverManager keeps track of registered driver instances and their connections to DB’s. The Driver “talks” to a particular DB through the connection

10. 10Connecting 1.Initializing a driver and registering it with the DriverManager: Class.forName(“org.postgresql.Driver"); 2.Getting a connection: DriverManager.getConnection(URL) The URL is: “jdbc:postgresql://dbserver/public”,”YOUR-LOGIN”,null

11. 11 For example Class.forName(“org.postgresql.Driver"); DriverManager.getConnection (“jdbc:postgresql://dbserver/public”,”gidi”,null);

12. 12 Interacting with the DB Once you have established a connection, your would like to interact with the DB Interaction are done by sending Statements and PreparedStatements to the DB These are used for 2 things (using different methods): 1.Querying the DB (executeQuery) 2.Changing the DB (executeUpdate)

13. Statement Statement createStatement() –returns a new Statement object Used to send SQL commands to the DB Created via the connection object

14. 14 Statement query methods 1. stmt.executeQuery(String query): for queries that return a single ResultSet object (typically select) 2. stmt.executeUpdate(String query): for INSERT, UPDATE, DELETE, and SQL DDL directives

15. 15 Compilation When executing an SQL statement via JDBC, it is not checked for errors until it is run (Not checked during compilation)

16. 16 executeQuery String queryStr = "SELECT * FROM Sailors " + "WHERE Name = 'joe smith'"; Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(queryStr); The executeQuery method returns a ResultSet object representing the query result. No semi- colon(;)

17. 17 executeUpdate String deleteStr = “DELETE FROM Sailors " + "WHERE sid = 15"; Statement stmt = con.createStatement(); int delnum = stmt.executeUpdate(deleteStr); executeUpdate returns the number of rows modified No semi- colon(;)

18. 18 PreparedStatement motivation Suppose we would like to run the query SELECT * FROM Emp where name=‘moshe’; But we would like to run this for all employees (separately), not only ‘moshe’… Could we create a variable instead of ‘moshe’ which would get a different name every time??..

19. 19 PreparedStatement PreparedStatement prepareStatement(String) –returns a new PreparedStatement object

20. 20 Prepared Statements Prepared Statements are used for queries that are executed many times with possibly different contents. A PreparedStatement object includes the query and is prepared for execution (precompiled). Question marks can be inserted as variables. -setString(i, value) -setInt(i, value) The i-th question mark is set to the given value.

21. 21 PreparedStatement.executeQuery() String queryStr = "SELECT * FROM Sailors " + "WHERE Name = ? and Rating < ?”; PreparedStatement pstmt = con.prepareStatement(queryStr); pstmt.setString(1, “Joe”); pstmt.setInt(2, 8); ResultSet rs = pstmt.executeQuery(); 1 st question mark Value to insert

22. 22 PreparedStatement.executeUpdate() String deleteStr = “DELETE FROM Boats " + "WHERE Name = ? and Color = ?”; PreparedStatement pstmt = con.prepareStatement(deleteStr); pstmt.setString(1, “Fluffy”); pstmt.setString(2, "red"); int delnum = pstmt.executeUpdate();

23. 23 Will this work? PreparedStatement pstmt = con.prepareStatement(“select * from ?”); pstmt.setString(1, "Sailors"); No! We may put ? only instead of values

24. 24 Why use preparedStatement? In most cases, you can use a regular statement and just change the string you send to executeQuery each time You have to be careful

25. 25 Why use preparedStatement? Suppose google worked without PreparedStatements, they would implement search queries as something like: Statement s; s.executeQuery(‘select URL,Title from internet where content like ‘%”+searchString+”%’”); What would happen if a hacker searched for: bla bla’ UNION select company as URL, CreditCardNumber AS title from advertisingClients where company like ‘ Example taken from dbi course

26. ResultSet (1) A ResultSet is an object which contains the result of a query - a “table”. At most one ResultSet per Statement can be open at the same time(!!). A ResultSet maintains a cursor pointing to its current row of data. The 'next' method moves the cursor to the next row As of JDBC 2.0, scrollable ResultSets are available, which also include ‘previous’, ’first’, ‘last’, etc..

27. ResultSet (2) resultSet methods work on the current row. The cursor is positioned before the first row upon creation.

28. ResultSet (3) Statement stmt= con.createStatement(); ResultSet rs = stmt.executeQuery ( "SELECT * FROM Table1"); while (rs.next()) { //something… }

29. 29 ResultSet methods Getting the value in some column (for the current row): –getString(int columnNum); –getString(String columnName); –getInt(int columnNum); –getInt(String columnName); –Etc… To check if NULL was returned, you have to use wasNull() on the ResultSet after getting the value. String s = rs.getString(“column1");

30. 30 Example revisited Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1"); // retrieve and print the values for the current row while (rs.next()) { int i = rs.getInt("a"); String s = rs.getString("b"); float f = rs.getFloat("c"); System.out.println("ROW = " + i + " " + s + " " + f); }

31. 31 ResultSetMetaData ResultSetMetaData rsmd = rs.getMetaData(); int numcols = rsmd.getColumnCount(); for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rsmd.getColumnLabel(i)); } An object created by the ResultSet which holds information about its columns

32. 32 Printing Query Output: Result Set (2) while (rs.next()) { for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rs.getString(i)); } System.out.println(""); } getString() is allowed to access all simple JDBC types

33. 33 Cleaning Up After Yourself Remember to close the Connections, Statements, PreparedStatements and ResultSets con.close(); stmt.close(); pstmt.close(); rs.close();

34. 34 Dealing With Exceptions catch (SQLException e) { //human readable message about the exception System.out.println(e.getMessage()); //String describing the reason of the exception System.out.println(e.getSQLState()); //driver-dependent code for the exception System.out.println(e.getErrorCode()); }

35. 35 Mapping SQL and Java Types SQL and Java data types are not identical There are significant variations between the SQL types supported by different database products JDBC defines a set of generic SQL type identifiers in the class java.sql.Types The driver is responsible for mapping between the DB SQL types and JDBC SQL types

36. 36 Transactions in JDBC

37. 37 Transactions Transaction = 2 or more statements which must all succeed (or all fail) together If one fails, the system must reverse all previous actions Aim: don’t leave DB in inconsistent state halfway through a transaction COMMIT = complete transaction ROLLBACK = abort

38. 38 Example Suppose we want to transfer money from bank account 13 to account 72: PreparedStatement pstmt = con.prepareStatement(“UPDATE BankAccount SET amount = amount + ? WHERE accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); What happens if this update fails?

39. 39 Transaction Management The connection has a state called AutoCommit mode if AutoCommit is true, then every statement is automatically committed if AutoCommit is false, then every statement is added to an ongoing transaction Default: true

40. 40 AutoCommit If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback() con.setAutoCommit(boolean val)

41. 41 Example con.setAutoCommit(false); try { PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ? where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); con.commit(); }catch (SQLException e) { con.rollback(); }