JDBC – Java Database Connectivity CS 236607, Spring 2008/9

Today’s Menu JDBC Architecture Using JDBC Timeout ResultSet Object Null Values Prepared Statements

JDBC (Java Database Connectiveity) is an API (Application Programming Interface) That is, a collection of classes and interfaces JDBC is used for accessing databases from Java applications Information is transferred from relations to objects and vice-versa

JDBC Architecture

JDBC Architecture Java code calls JDBC library JDBC loads a driver The driver talks to a particular DBMS An application can work with several DBMS by using corresponding drivers

“Movies” Relation movieName producer releaseDate Movie1 Producer1 1.1.2000 Movie2 Producer2 1.1.2001 Movie3 Producer3 3.4.2003

7 Steps for Using JDBC Load the driver Define the connection URL Establish the connection Create a Statement object Execute a query using the Statement Process the result Close the connection

Loading the Driver Class.forName loads the given class dynamically Class.forName(“com.mysql.jdbc.Driver ”); Class.forName loads the given class dynamically When the driver is loaded, it automatically creates an instance of itself registers this instance within DriverManager Another way: Driver driver = new com.mysql.jdbc.Driver(); DriverManager.registerDriver(driver); MySql JDBC driver can be downloaded from here.

Define the connection URL Every database is identified by a URL Given a URL, DriverManager looks for the driver that can talk to the corresponding database DriverManager tries all registered drivers,until a suitable one is found

An Example // A driver for imaginary1 Class.forName("ORG.img.imgSQL1.imaginary1Driver"); // A driver for imaginary2 Driver driver = new ORG.img.imgSQL2.imaginary2Driver(); DriverManager.registerDriver(driver); //A driver for PostgreSQL Class.forName("org.postgresql.Driver");

Establish the connection Connection con = DriverManager.getConnection("jdbc:imaginaryDB1://localhost:3306/");

4. Create a Statement object We use Statement objects in order to Query the DB Update the DB(insert, update, create, drop, …)

5. Execute a query using the Statement executeQuery returns a ResultSet object representing the query result (discussed later…)

Manipulating DB with Statement String deleteStr = “delete from movies where movieName=‘Movie1’ ”; Statement stmt = con.createStatement(); int rowsDeleted = stmt.executeUpdate(deleteStr); executeUpdate is for data manipulation: insert, delete, update, create table, etc. executeUpdate returns the number of rows modified (or 0 for DDL commands)

6. Process the result We will discuss ResultSet in a while…

7. Close the connection Close Connections, Statements, and Result Sets con.close(); stmt.close(); rs.close(); ‘finally’ block is a good place…

ResultSet ResultSet objects provide access to the tables generated as results of executing Statement queries. Only one ResultSet per Statement can be open at a given time! The table rows are retrieved in sequence: A ResultSet maintains a cursor pointing to its current row. next() moves the cursor to the next row

ResultSet Methods boolean next() Activates the next row First call to next() activates the first row Returns false if there are no more rows Not all of the next calls actually involve the DB void close() Disposes of the ResultSet Allows to re-use the Statement that created it Automatically called by most Statement methods Type getType(int columnIndex) Returns the given field as the given type Indices start at 1 and not 0! Add the column name as a comment if it is known! Type getType(String columnName) Same, but uses name of field int findColumn(String columnName) Looks up column index given column name Type = int || double || long || boolean || byte || time || date …

Timeout Use setQueryTimeOut(int seconds) of Statement class to set a timeout for the driver to wait for a query to be completed. If the operation is not completed in the given time, an SQLException is thrown What is it good for?

Mapping Java Types to SQL Types

Null Values In SQL, NULL means the field is empty Not the same as 0 or “”! In JDBC, you must explicitly ask if the last read field was null ResultSet.wasNull(column) For example, getInt(column) will return 0 if the value is either 0 or NULL!

Database Time Times in SQL are notoriously non-standard Java defines three classes to help java.sql.Date year, month, day java.sql.Time hours, minutes, seconds java.sql.Timestamp year, month, day, hours, minutes, seconds, nanoseconds Usually use this one

Exceptions An SQLException is actually a list of exceptions

Prepared Statements The PreparedStatement object contains not just an SQL statement, but an SQL statement that has been precompiled. This means that when the PreparedStatement is executed, the DBMS can just run the PreparedStatement SQL statement without having to compile it first. Most often used for SQL statements that take parameters.

Creating a PreparedStatement Object As with Statement objects, you create PreparedStatement objects with a Connection method. The following code create a PreparedStatement object that takes two input parameters:

Supplying Values for PreparedStatement Parameters You need to supply values to be used in place of the question mark placeholders (if there are any) before you can execute a PreparedStatement object. You do this by calling one of the setXXX methods defined in the PreparedStatement class.

Example updateSales.setInt(1, 75); the following line of code sets the first question mark placeholder to a Java int with a value of 75: updateSales.setInt(1, 75); The next example sets the second placeholder parameter to the string " Colombian": updateSales.setString(2, "Colombian");

Another Example

Callable Statements Execute a call to a database stored procedure. We will not go into details

Resources used for this presentation http://www.cs.huji.ac.il/~dbi/recitations/JDBC-PSQL-c.pdf http://java.sun.com/docs/books/tutorial/jdbc/ http://www.java-samples.com/showtutorial.php?tutorialid=202