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IT 46 - OOP

JDBC

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Environments, Connections, Queries

The database is, in many DB-accesslanguages, an environment.

Database servers maintain some numberof connections, so app servers can askqueries or perform modifications.

The app server issues statements:queries and modifications, usually.

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Diagram to Remember

Environment

Connection

Statement

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JDBC

Java Database Connectivity(JDBC) is alibrary similar to SQL/CLI, but with Java

as the host language.

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Making a Connection

import java.sql.*;

Class.forName(com.mysql.jdbc.Driver);

Connection myCon =

DriverManager.getConnection();

The JDBC classes

The driverfor mySql;others exist

URL of the databaseyour name, and passwordgo here.

Loaded byforName

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Statements

JDBC provides two classes:

1. Statement = an object that can accept a

string that is a SQL statement and canexecute such a string.

2. PreparedStatement = an object that hasan associated SQL statement ready toexecute.

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Creating Statements

The Connection class has methods to createStatements and PreparedStatements.

Statement stat1 = myCon.createStatement();PreparedStatement stat2 =

myCon.createStatement(

SELECT beer, price FROM Sells +WHERE bar = Joe s Bar

); createStatementwith no argument returnsa Statement; with one argument it returns

a PreparedStatement.

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Executing SQL Statements

JDBC distinguishes queries frommodifications, which it calls updates.

Statement and PreparedStatement eachhave methods executeQueryandexecuteUpdate.

For Statements: one argument: the query ormodification to be executed.

For PreparedStatements: no argument.

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Example: Update

stat1 is a Statement.

We can use it to insert a tuple as:

stat1.executeUpdate(

INSERT INTO Sells +

VALUES(Brass Rail,Bud,3.00)

);

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Example: Query

stat2 is a PreparedStatement holdingthe query SELECT beer, price FROM

Sells WHERE bar = JoesBar .executeQueryreturns an object of class

ResultSet

The query:

ResultSet menu = stat2.executeQuery();

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Accessing the ResultSet

An object of type ResultSet issomething like a cursor.

Method next()advances the cursor tothe next tuple.

The first time next()is applied, it gets the

first tuple. If there are no more tuples, next()returns

the value false.

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Accessing Components of Tuples

When a ResultSet is referring to atuple, we can get the components of

that tuple by applying certain methodsto the ResultSet.

Method getX (i), where X is some

type, and i is the component number,returns the value of that component.

The value must have type X.

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Example: Accessing Components

Menu = ResultSet for query SELECT beer,price FROM Sells WHERE bar = Joe s Bar .

Access beer and price from each tuple by:

while ( menu.next() ) {

theBeer = Menu.getString(1);

thePrice = Menu.getFloat(2);/*something with theBeer and

thePrice*/

}

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import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.SQLException;

public class Connect {

public static void main(String args[]) {

Connection con = null;

try {

Class.forName("com.mysql.jdbc.Driver"); //.newInstance();

con = DriverManager.getConnection("jdbc:mysql:///test",

"root", "root");

if(!con.isClosed())

System.out.println("Successfully connected to " +

"MySQL server using TCP/IP...");

} catch(Exception e) {

System.err.println("Exception: " + e.getMessage());

} finally {try {

if(con != null)

con.close();

} catch(SQLException e) {}

}

}

} 14

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Critical steps in using

JDBCThere are five critical steps in using JDBC to manipulate adatabase:

Load and register the JDBC driver classes (programming

interface)for the database server that you intend to use. Get a Connectionobject that represents a connection to

the database server (analogous to logging onto theserver).

Get one or more Statementobjects for use inmanipulating the database.

Use the Statementobjects to manipulate the database.

Close the connection to the database.

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Register the JDBC driverfor MySQL

Class.forName("com.mysql.jdbc.Driver");

shows the statement that implements the first critical steplisted

earlier (load and register the JDBC driver classes). This statementregisters the MySQL driver classes with the Java program, makingit possible for this program to manipulate data on the MySQLserver. The DriverManagerwill try to load as many drivers as it can find and then for

any given connection request, it will ask each driver in turn to try to connect tothe target URL. ...

When a Driverclass is loaded, it should create an instance of itself and registerit with the DriverManager. This means that a user can load and register adriver by calling

Class.forName("foo.bah.Driver")"

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http://www.developer.com/java/data/article.php/3417381http://www.developer.com/java/data/article.php/3417381

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Get a connection to thedatabase

con = DriverManager.getConnection("jdbc:mysql:///test","root", "root");

gets a connection to the database at the specified URL (mysql on

localhost)for a user named rootwith a root password. As you are alreadyaware, this user is the default administrator having full privileges to doanything, including creating new databases and registering new users onthose databases.

The getConnection method is a static method ofthe DriverManager class. When getConnection is invoked,the DriverManagerwill attempt to locate a suitable driver from amongthose loaded at initialization and those loaded explicitly using the sameclassloader as the current applet or application.

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Get a Statement object

stmt = con.createStatement();

This code invokes thecreateStatementmethod of the Connectioninterface to get

an object of type Statement.

Recall that conis a reference to an object oftype Connection. A Connectionobject defines a connection (session)with aspecific database. SQL statements are executed and results are returned within

the context of a connection. According to Sun, a Statement object is:

"... used for executing a static SQL statement and returning the results it produces."

The results, if any, are returned in the form of a ResultSetobject.

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use of a Statement object to

manipulate the databaseExample:

stmt.executeUpdate( "CREATE DATABASE JunkDB");

invokes the executeUpdatemethod on the Statementobject tocreate the new database named JunkDB.

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Close the connection

con.close();

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A ResultSet object

A ResultSetobject provides access to anencapsulated table of data. The object

maintains a cursor pointing to its current rowof data. Initially the cursor is positionedbefore the first row. The nextmethodmoves the cursor to the next row (similar to

an iterator or an enumerator in Java).Once the results are encapsulated in

a ResultSetobject, the ResultSetinterfaceprovides several methods that can be used toextract the information from the object. 21

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Methods of a ResultSet object

The getmethods (such as getString)retrieve columnvalues for the current row. You can retrieve values usingeither the index number of the column or the name of the

column. I have read that using the column index is moreefficient but I can't give you a reference on that.

(Columns are numbered beginning with 1, not with 0.)

For the getmethods, the JDBC driver attempts to convert

the underlying data to the specified Java type and returnsa suitable Java value.

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The life of a ResultSet Object

A ResultSetobject is automaticallyclosed by the Statementobject that

generated it whenthat Statementobject is closed, re-executed, or used to retrieve the next

result from a sequence of multipleresults.

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Perform the query

rs = stmt.executeQuery("SELECT * " + "from myTable ORDER BY test_id");

invokes the executeQuerymethod to perform the queryon the database selecting all columns in all rows, andsorting the results in order by the values in the columnnamed test_id. The results are encapsulated in

the ResultSetobject referred to by the reference variablenamed rs.

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Display all of the results in

the ResultSet objectSystem.out.println("Display all results:");

while(rs.next()){

int theInt= rs.getInt("test_id");

String str = rs.getString("test_val");

System.out.println("\ttest_id= " + theInt + "\tstr = " + str);}

Iterate on the ResultSetobject one row at a time.

Invoke the getIntmethod to get and save the value in the column

named test_idfor each row. Invoke the getStringmethod to get and save the value in the column

named test_valfor each row.

Display the two values on a new output line on the screen.

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THANK YOU

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