JDBC - Java Database Connectivity

The objectives of this chapter are:

To describe the architecture of JDBC

Driver Types

JDBC Configuration

To outline the classes in the java.sql package

Executing SQL Statements

sTo understand the use of JDBC

JDBC provides Java applications with access to most database systems via SQL

The architecture and API closely resemble Microsoft's ODBC

JDBC 1.0 was originally introduced into Java 1.1JDBC 2.0 was added to Java 1.2

JDBC is based on SQL-92

JDBC classes are contained within the java.sql package

There are few classesThere are several interfaces

What is JDBC?

3

JDBC ArchitectureJDBC Architecture

Java Application JDBC

Oracle

DB2

MySQL

Oracle

Driver

DB2

Driver

MySQL

Driver

Network

We will

use this one…

JDBC Architecture (cont.)JDBC Architecture (cont.)

Application JDBC Driver

• Java code calls JDBC library

• JDBC loads a driver

• Driver talks to a particular database

• An application can work with several databases by using all

corresponding drivers

• Ideal: can change database engines without changing any

application code (not always in practice)

Before APIs like JDBC and ODBC, database connectivity was tedious

Each database vendor provided a function library for accessing their databaseThe connectivity library was proprietary.If the database vendor changed for the application, the data access portions had to be rewrittenIf the application was poorly structured, rewriting its data access might involve rewriting the majority of the applicationThe costs incurred generally meant that application developers were stuck with a particular database product for a given application

Database Connectivity History

With JDBC, the application programmer uses the JDBC API

The developer never uses any proprietary APIs

• Any proprietary APIs are implemented by a JDBC driver• There are 4 types of JDBC Drivers

JDBC Architecture

Java Application

JDBC API

JDBC DriverManager

JDBC Driver JDBC Driver

There are 4 types of JDBC DriversType 1 - JDBC-ODBC BridgeType 2 - JDBC-Native BridgeType 3 - JDBC-Net BridgeType 4 - Direct JDBC Driver

Type 1 only runs on platforms where ODBC is available

ODBC must be configured separately

Type 2 Drivers map between a proprietary Database API and the JDBC API

Type 3 Drivers are used with middleware products

Type 4 Drivers are written in JavaIn most cases, type 4 drivers are preferred

JDBC Drivers Types

JDBC Classes

DriverManagerManages JDBC DriversUsed to Obtain a connection to a Database

• TypesDefines constants which identify SQL types

DateUsed to Map between java.util.Date and the SQL DATE type

• TimeUsed to Map between java.util.Date and the SQL TIME type

TimeStampUsed to Map between java.util.Date and the SQL TIMESTAMP type

JDBC Interfaces

DriverAll JDBC Drivers must implement the Driver interface. Used to obtain a connection to a specific database type

• ConnectionRepresents a connection to a specific databaseUsed for creating statementsUsed for managing database transactionsUsed for accessing stored proceduresUsed for creating callable statements

StatementUsed for executing SQL statements against the database

JDBC Interfaces

ResultSetRepresents the result of an SQL statementProvides methods for navigating through the resulting data

• PreparedStatementSimilar to a stored procedureAn SQL statement (which can contain parameters) is compiled and stored in the database

CallableStatementUsed for executing stored procedures

DatabaseMetaDataProvides access to a database's system catalogue

ResultSetMetaDataProvides information about the data contained within a ResultSet

Using JDBC

To execute a statement against a database, the following flow is observed

Load the driver (Only performed once)Obtain a Connection to the database (Save for later use)Obtain a Statement object from the ConnectionUse the Statement object to execute SQL. Updates, inserts and deletes return Boolean. Selects return a ResultSetNavigate ResultSet, using data as requiredClose ResultSetClose Statement

Database URLs

• When connecting to a database, you must specify the

data source and you may need to specify additional

parameters.For example, network protocol drivers may

need a port, and ODBC drivers may need various

attributes.

• General Syntax :

- jdbc:subprotocol name:other stuff

• EX : jdbc:localhost:DB_NAME

Loading/Registering a Driver

• The first step to develop a JDBC application is to load

and register the required driver using the driver

manager .

• You can load and register a driver :

1. Programmatically :

. Using the forName() method

. Using the registerDriver () method

2. Manually:

. By setting system property

Using the forName () Method

• The forName() method is available in the

java.lang.Class class.

• The forName() method loads the JDBC driver and

registers the driver with the driver manager.

• The syntax to load a JDBC driver to access a

database is :

Class.forName (“driver_name”);

• You can load the JDBC-ODBC Bridge driver using the

following method call:

Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”

);

Using the registerDriver () method

• You can create an instance of the Driver class to load a JDBC

driver.

• The syntax to declare an instance of the Driver class is :

Driver d = new <driver-name>;

• You can use the following statement to crate an instance of

the Driver class:

Driver d = new

sun.jdbc.odbc.JdbcOdbcDriver ();

• Once you have created the Driver object ,call the

registerDriver () method to register it with the DriverManager.

• You can register the JDBC-ODBC Bridge driver using the

following method call to registerDriver () method:

DriverManager.registerDriver (d) ;

Setting System Property

• Driver can also be loaded by setting system property for JDBC

drivers.

• You add the driver name to the jdbc.drivers system property

to load a JDBC driver.

• You use the –D command line option to set the system property

on the command line.

• The command to set the system property is :

java –Djdbc.drivers=sun.jdbc.odbc.JdbcOdbcDriver

Demo

• In the preceding command , jdbc.drivers is the property name

and sun.jdbc.odbc.JdbcOdbcDriver is the value that you

need to set for the property.

• After you load a driver , you need to establish the connection

with a database.

Even a good API can have problemsLoading drivers fits into this category

The DriverManager is a singleton

Each JDBC Driver is also a singleton

When a JDBC Driver class is loaded, it must create an instance of itself and register that instance with the JDBC DriverManager

How does one load a "class" into the Virtual machine?Use the static method Class.forName()

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

Loading Drivers

Connecting to a Database

Once a Driver is loaded, a connection can be made to the database

The connection is defined by URLThe URL has the following form: jdbc:driver:databasename

• Examples:jdbc:odbc:MyOdbcDatabasejdbc:postgres:WebsiteDatabasejdbc:oracle:CustomerInfo

A connection is obtained in the following manner:

Connection aConnection = DriverManager.getConnection("jdbc:odbc:myDatabase");

• Overloaded versions of the getConnection method allow the specification of a username and password for authentication with the database.

Executing SQL Statments

• Managing Connections

• Statements

• Resultset

• SQL Exceptions

• Populating Database

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Managing / Using a Connection

The Connection interface defines many methods for managing and using a connection to the database

public Statement createStatement()public PreparedStatement prepareStatement(String sql)public void setAutoCommit(boolean)public void commit()public void rollback()public void close()

• The most commonly used method is createStatement()

When an SQL statement is to be issued against the database, a Statement object must be created through the Connection

Statements

The Statement interface defines two methods for executing SQL against the database

public ResultSet executeQuery(String sql)public int executeUpdate(String sql)

• executeQuery returns a ResultSet• All rows and columns which match the query are contained

within the ResultSet• The developer navigates through the ResultSet and uses

the data as required.

• executeUpdate returns the number of rows changed by the update statement

This is used for insert statements, update statements and delete statements

Result Set

The ResultSet interface defines many navigation methods

public boolean first()public boolean last()public boolean next()public boolean previous()

The ResultSet interface also defines data access methods

public int getInt(int columnNumber) -- Note: Columns are numberedpublic int getInt(String columnName) -- from 1 (not 0)public long getLong(int columnNumber)public long getLong(String columnName)public String getString(int columnNumber)public String getString(String columnName)

There are MANY more methods. Check the API documentation for a complete list

SQL Types/Java Types Mapping

SQL Type Java Type

CHAR StringVARCHAR StringLONGVARCHAR StringNUMERIC java.Math.BigDecimalDECIMAL java.Math.BigDecimalBIT booleanTINYINT intSMALLINT intINTEGER intBIGINT longREAL floatFLOAT doubleDOUBLE doubleBINARY byte[]VARBINARY byte[]DATE java.sql.DateTIME java.sql.TimeTIMESTAMP java.sql.Timestamp

Connection aConnection;try {

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");}catch(ClassNotFoundException x){

System.out.println("Cannot find driver class. Check CLASSPATH");return;

}try{

aConnection = DriverManager.getConnection("jdbc:odbc:MyDatabase","Username", "Password");

}catch(SQLException x){

System.out.println("Exception connecting to database:" + x);return;

}

Example Code:

try{Statement aStmt = aConnection.createStatement();StringBuffer sb = new StringBuffer("SELECT Employee_id,Employee_Name");sb.append(" FROM Employee WHERE EmployeeId>100");ResultSet rs = aStmt.executeQuery(sb.toString());while(rs.next()){

int employeeId = rs.getInt(1);String employeeName = rs.getString(2);System.out.println("Id:" + employeeId + "\nName:" +

employeeName);}rs.close();aStmt.close();

}catch(SQLException x){

System.out.println("Exception while executing query:" + x);}

Example Code (continued):

PreparedStatement interface

• The PreparedStatement objects accepts the runtime

parameters.

• The PreparedStatement interface is derived from the

Statement interface and is available in the java.sql

package.

• The PreparedStatement objects are compiled and

prepared only once by JDBC.

• The future invocation of the PreparedStatement object

does not recompile the SQL statements.

• This helps in reducing the load on the database server

and thus improving the performance of the application .

• The PreparedStatement interface inherits the following methods to execute SQL statement s from the Statement interface:

1. ResultSet executeQuery () : - Executes a SELECT statement and returns the result in a

ResultSet object.1. int executeUpdate():

Executes an SQL statement, INSERT , UPDATE , or DELETE and returns the count of the rows

affected.1. boolean execute () : Executes an SQL statement and

returns a boolean value.

stat = con.prepareStatement(“SELECT * FROM authors WHERE au_id = ? ”) ;

The SQL statement can contain ‘ ? ‘ symbol as placeholder that can be replaced by input parameters at runtime.

Before executing the SQL statements specified in the PreparedStatement object , you must set the value of each ‘ ? ‘ parameter.

stat.setString (1, “123”);

ResultSet rs = stat.executeQuery ();

• The PreparedStatement interface provides various methods to set the value of the placeholders for the specific data types.

1. void setByte (int index , byte val) : Sets the java byte type value for the parameter corresponding to index passed as a parameter.

2. void setBytes (int index , byte[] val) : Sets the Java byte type array for the parameter corresponding to index passed as a parameter.

3. void setBoolean (int index , boolean val) : Sets the Java boolean type value for the parameter corresponding to index passed as a parameter.

4. void setDouble (int index, double val) : Sets the Java double type value value for the parameter corresponding to the index passed as a parameter.

5. void setInt (int index , int val) : Sets the Java int type value for the parameter corresponding to index passed as a parameter.

6. void setLong(int index , long val) : Sets the Java long type value for the parameter corresponding to index passed as a parameter.

7. void setFloat (int index , float val) : Sets the Java float type value for the parameter corresponding to index passed as a parameter.

8. void setShort (int index , short val) : Sets the Java short type value for the parameter corresponding to index passed as a parameter.

9. void setString(int index , String val) : Sets the Java String type value for the parameter corresponding to index passed as a parameter.

Retrieving Rows

String str = “SELECT * FROM titles WHERE au_id = ? ” ;

PreparedStatement ps = con.prepareStatement(str) ;

ps.setString(1 , “101”);

ResultSet rs = ps.executeQuery ();

Inserting Rows

String str = “ INSERT INTO authors (au_id , au_fname , au_lname) VALUES (? , ?

, ? ) ” ;

PreparedStatement ps = con.prepareStatement(str);

ps.setString (1, “101”); ps.setString (2, “ABC”); ps.setString (3 , “XYZ”);

int rt = ps.executeUpdate() ;

Updating & Deleting Row String str = “ UPDATE authors SET state = ? WHERE city = ? “ ;

PreparedStatement ps = con.prepareStatement(str);

ps.setString (1, “MH”); ps.setString (2, “Pune”); int rt = ps.executeUpdate() ;

String str = “ DELETE FROM authors WHERE au_fname = ? “ ;

PreparedStatement ps = con.prepareStatement(str);

ps.setString (1, “PPPP”); int rt = ps.executeUpdate() ;

Scrollable ResultSet• Statement createStatement( int resultSetType, int resultSetConcurrency)

• resultSetType:

• ResultSet.TYPE_FORWARD_ONLY

• -default; same as in JDBC 1.0

• -allows only forward movement of the cursor

• -when rset.next() returns false, the data is no longer available and the result set is closed.

• ResultSet.TYPE_SCROLL_INSENSITIVE

• -backwards, forwards, random cursor movement.

• -changes made in the database are not seen in the result set object in Java memory.

• ResultSetTYPE_SCROLL_SENSITIVE

• -backwards, forwards, random cursor movement.

• -changes made in the database are seen in the result set object in Java memory.

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Scrollable ResultSet (cont’d)Scrollable ResultSet (cont’d)

• resultSetConcurrency:• ResultSet.CONCUR_READ_ONLY

• This is the default (and same as in JDBC 1.0) and allows only data to be read from the database.

• ResultSet.CONCUR_UPDATABLE

• This option allows for the Java program to make changes to the database based on new methods and positioning ability of the cursor.

• Example:

• Statement stmt = conn.createStatement( ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);

• ResultSetrset= stmt.executeQuery( “SHOW TABLES”);

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Scrollable ResultSet (cont’d)Scrollable ResultSet (cont’d)

public boolean absolute(int row) throws SQLException

• -If the given row number is positive, this method moves the cursor to the given row number (with the first row numbered 1).

• -If the row number is negative, the cursor moves to a relative position from the last row.

• -If the row number is 0, an SQLException will be raised.

public boolean relative(int row) throws SQLException

• This method call moves the cursor a relative number of rows, either positive or negative.

• An attempt to move beyond the last row (or before the first row) in the result set positions the cursor after the last row (or before the first row).

public boolean first() throws SQLException

public boolean last() throws SQLException

public boolean previous() throws SQLException

public boolean next() throws SQLException 36

Scrollable ResultSet (cont’d)Scrollable ResultSet (cont’d)

public void beforeFirst() throws SQLException

public void afterLast() throws SQLException

public boolean isFirst() throws SQLException

public boolean isLast() throws SQLException

public boolean isAfterLast() throws SQLException

public boolean isBeforeFirst() throws SQLException

public int getRow() throws SQLException

• getRow() method retrieves the current row number: The first row is number 1, the second number 2, and so on.

37

38

Transactions and JDBCTransactions and JDBC

• Transaction: more than one statement that must all

succeed (or all fail) together

- e.g., updating several tables due to customer purchase

• If one fails, the system must reverse all previous actions

• Also can’t leave DB in inconsistent state halfway

through a transaction

• COMMIT = complete transaction

• ROLLBACK = cancel all actions

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ExampleExample

• 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?

40

Transaction ManagementTransaction Management

• Transactions are not explicitly opened and closed

• 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

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AutoCommitAutoCommit

• If you set AutoCommit to false, you must explicitly commit or

rollback the transaction using Connection.commit() and Connection.rollback()

• Note: DDL statements (e.g., creating/deleting tables) in a

transaction may be ignored or may cause a commit to occur

- The behavior is DBMS dependent

setAutoCommit(boolean val)