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JDBC – Java Database Connectivity

CS 236369, Spring 2010

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Today’s Menu

JDBC Architecture Using JDBC Timeout ResultSet Object Null Values Prepared Statements

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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

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JDBC Architecture

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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

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“Movies” Relation

movieName producer releaseDate

Movie1 Producer1 1.1.2000

Movie2 Producer2 1.1.2001

Movie3 Producer3 3.4.2003

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7 Steps for Using JDBC

1. Load the driver2. Define the connection URL3. Establish the connection4. Create a Statement object5. Execute a query using the Statement6. Process the result7. Close the connection

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1. Loading the Driver

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.

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2. 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

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An Example

// A driver for imaginary1Class.forName("ORG.img.imgSQL1.imaginary1Driver");// A driver for imaginary2Driver driver = new ORG.img.imgSQL2.imaginary2Driver();DriverManager.registerDriver(driver);//A driver for PostgreSQLClass.forName("org.postgresql.Driver");

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3. Establish the connection

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

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4. Create a Statement object

We use Statement objects in order to Query the DB Update the DB(insert, update, create, drop, …)

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5. Execute a query using the Statement

executeQuery returns a ResultSet object representing the query result (discussed later…)

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Manipulating DB with Statement

executeUpdate is for data manipulation: insert, delete, update, create table, etc.

executeUpdate returns the number of rows modified (or 0 for DDL commands)

String deleteStr = “delete from movies where movieName=‘Movie1’ ”;

Statement stmt = con.createStatement();int rowsDeleted = stmt.executeUpdate(deleteStr);

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6. Process the result

We will discuss ResultSet in a while…

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7. Close the connection

Close Connections, Statements, and Result Sets con.close(); stmt.close(); rs.close();

‘finally’ block is a good place…

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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

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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 …

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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?

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Mapping Java Types to SQL Types

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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!

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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

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Exceptions

An SQLException is actually a list of exceptions

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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.

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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:

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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.

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Example

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");

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Another Example

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Callable Statements

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

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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

Resources used for this presentation