Java Database Connectivity JDBC JDBC is an API, containing classes and

interfaces in the java programming language, to execute SQL sentences over an external database.

Offers a standard interface for connecting to any database available.

Helps obtaining the data more easily and comfortable in a client-server environments through the Internet/Intranet.

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What does JDBC do?

Allow a connection to a database server. Allow sending SQL sentences. Process the results of these Sentences. The classes that allow this, are in the

package java.sql (it has to be imported)

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Loading the Driver It is necessary to load a class with the driver

of the Database (This is provided by the DBMS vendor)

Example:Class c =

Class.forName(“com.informix.jdbc.IfxDriver"); Class.forName(“com.novell.sql.LDAPDriver”); Class.forName("com.mysql.jdbc.Driver");

This depends on the database that will be used

Then an instance of the class must be created.c.newInstance();

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Establishing a Connection

In the examples the login and password will be discarded

A Connection Object represents a connection to a database

The class “DriverManager” tries to find a driver to connect to the database represented in the URL.

Connection con = DriverManager.getConnection (url,"login", "password");

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Connection

URL Examples:jdbc:mysql://localhost/test (for examples)jdbc:oracle://oraserverjdbc:odbc:mydatabasejdbc:informix-sqli://hostname:port/

dbname:INFORMIXSERVER=serverName (for CGE databases)

jdbc:ldap://server;baseDN=baseDN;useCleartext=true

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...Connection

URL JDBC Syntaxis:jdbc:<subprotocol>:<subname><subprotocol> is the name of the driver or

mechanism of the connection. Example: odbc.<subname> is the identification of the

database. The format changes depending of the driver. Example: //server:port/db and parameters

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Sending SQL sentences

JDBC allows to send any SQL sentence. although this is dependent to the database, the only risk is the incompatibility between different databases.

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...Sending SQL Sentences JDBC provides 3 classes:

“Statement”: This object is used to send simple SQL sentences. It is created by the method: createStatement.

“PreparedStatement”: This object is used for sentences that needs one or more parameter. The sentence is pre-compiled and saved for a future use.

“CallableStatement”: It is used to execute procedures saved in the database.

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The Statement class Statement stmt = con.createStatement(); In this moment the statement exists but

doesn't have a SQL sentence to execute. This can be sended using the methods:executeUpdate(String), used to create/modify

tables (there is no results), commonly used for the create,update,delete sentences.

executeQuery(String) to make queries, returns results in an object from the class ResulSet, tipically for a select statement.

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excuteUpdate Examples stmt.executeUpdate(“create table students (name varchar(32), id integer, address varchar(40), yearborn integer, ...);

stmt.executeUpdate(“insert into students (‘Valeria Valencia ’, 00998852678, ‘Calle bonita 223 Vitacura Stgo’, 1974, ...);

stmt.executeUpdate(“update students set direccion = ‘?????’ where name = ‘Valeria Valencia’ ”);

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excuteUpdate Examples

• CreateCoffees: Creates a Coffees table in the test database.

• CreateSuppliers: Creates a Suppliers table in the test Databes.

• DropCoffees: Delete both tables.• MyConnection: encapsulates the connection to the

database (to be used for other applications)• InsertCoffees: Inserts rows in the table.• InsertSupliers: Inserts rows in the table.• InsertCoffeesDatos: Inserts rows from data readed

from a file.• ListCoffes, ListSuppliers: Show the content (uses

executeQuery)

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executeUpdate return

• The executeUpdate method returns an integer value corresponding to the number of rows that the SQL sentences modified.

• If it is a creation of a table, the return value is 0• If it is an insert of a value in a row the value is 1.• The results is interesting when using the SQL

sentence with the instruction: “update table set field = <expression> where <condition>

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excuteQuery Examples stmt.executeQuery(“select * from students “ + “where yearofborn = 1974”);

The results of a query is received in an ResultSet object.

ResultSet rs = stmt.executeQuery(....); A ResultSet can be seen as an enumeration of

rows that represents the results. There are adequate methods to go through all the

elements of this enumeration and obtain the values of the fields

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The ResultSetResultSet rs = stmt.executeQuery(“select name where address like Santiago”);

while (rs.next()) { String s = rs.getString(“name”); int y = rs.getInt(“yearofborn”); System.out.println(s+” “+y);} ResultSet rs contains a collection of rows with the results

of the query instruction next advances a pointer which informs in what row are we now. At the beginning this is before the first row, so it is necessary to execute a next() method to point to the first row.

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The getXXX Instruction It can be used with two types of

parameters:getXXX(“Name of the field of the table”)

example: getString(“name”)getXXX(number of field of the table)

example: getString(1)

getXXX tries to read what is on the field and converts it to the type specified in the “XXX”

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The Instruction getXXX Sometimes, even if the SQL data type does not

exactly matches the XXX type a conversion is possible ( with getString & getObject almost everything can be retrieved)

In other cases the conversion is possible but with some loss of information (getByte for a numeric or longvarchar)

In other cases it is just impossible (use getLong for Time)

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Prepared Statements Everywhere a Statement has been used it is possible to

use a PreparedStatement in order to make queries more efficient

An instruction containing a PreparedStatement will be translated to a native SQL statement of the database

Another advanatage is that it allows the usage of parameters

PreparedStatement us = con.prepareSatatement(“update alumnos set comuna = ? where direccion like = ?);

us.setString(1,’Vitacura’) us.setString(2,’Hualtatas’);

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Prepared Statements: ExamplePreparedStatement updateSales;String updateString = "update COFFEES " + "set SALES = ? where COF_NAME like ?";updateSales = con.prepareStatement(updateString);int [] salesForWeek = {175, 150, 60, 155, 90};String [] coffees = {"Colombian", "French_Roast", "Espresso",

"Colombian_Decaf", "French_Roast_Decaf"};int len = coffees.length;for(int i = 0; i < len; i++) { updateSales.setInt(1, salesForWeek[i]); updateSales.setString(2, coffees[i]); updateSales.executeUpdate(); }

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Transactions

A transaction consists in one or more sentences that have been executed and then confirmed (commit) or deleted (rolled back)

Auto-commit is pre-set. if Auto-commit is deactivated, the methods

commit or rollback have to be used in a explicit way.

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Transactions

To use Transactions, the auto-commit must be disabled.con.setAutoCommit(false)PreparedStatement ps = .........ps.executeUpdate() ....ps.excecuteUpdate() ...con.commit();

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Stored Procedures Is a group of SQL sentences that are grouped

logically in a unit to do a specific work. Exists in most of the DBMS, but they are

dependent to this (The way of how these procedures are written or executed isn't so standard)

Generally receives parameters. They are “written” with an Update and executed

with a Query.

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An ExampleTo create a Stored procedureString crearSP = “create prodcedure SHOW_SUPPLIERS”+ “ as ”+

“select SUPPLIERS.SUP_NAME, COFFEES.COF_NAME”+ “from SUPPLIERS, COFFEES ”+ “where SUPPLIERS.SUP_ID = COFFEES.SUP_ID”

Statement stmt = con.CreateStatement();stmt.executeQuery(createSP);

to call the Stored ProcedureCallableStatement cs;cs = con.prepareCall(“{call SHOW_SUPPLIERS}”);ResultSet rs = cs.executeQuery();

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Using metadata The metadata is the information about the structure of a

databaste or a ResultSet It is obtained with the method getMetaData()

stmt = con.createStatement();ResultSet rs = stmt.executeQuery(query);ResultSetMetaData rsmd = rs.getMetaData();int numberOfColumns = rsmd.getColumnCount();int rowCount = 1;while (rs.next()) {

System.out.println("Row " + rowCount + ": ");for (int i = 1; i <= numberOfColumns; i++) {

System.out.print(" Column " + i + ": ");System.out.println(rs.getString(i));

}System.out.println("");rowCount++;

}

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Using metadata Example to know the tables of a database.

DatabaseMetaData dbmd = con.getMetaData();String dbmsName = dbmd.getDatabaseProductName();ResultSet rs = dbmd.getTableTypes();System.out.print("The following types of tables are ");System.out.println("available in " + dbmsName + ": ");while (rs.next()) {

String tableType = rs.getString("TABLE_TYPE");System.out.println(" " + tableType);

}