Java Database Connectivity Chapter 6: JDBC Objects...The Statement Object - 2 2. executeUpdate()...

Java Database Connectivity Chapter 6: JDBC Objects

Slides material compiled from The Complete Reference J2EE

By Jim Keogh

The Concept of JDBC

• Many industrial-strength DBMS available in the market – Oracle, DB2, Sybase, MySQL

• Each DBMS defines its own low-level way to interact with programs to access data stored in its databases

• JDBC driver is a translator that converts low level proprietary DBMS messages to low-level messages understood by the JDBC API, and vice versa

• Thus JDBC drivers and the JDBC API allow Java developers to write high level code that accesses any DBMS

Rakhi Saxena (Internet Technologies) 2

JDBC: Java Database Connectivity • JDBC is a standard Java API for handling

database related activities

• Note: The JDBC driver for different database is different. But, as an end-user, we don’t have to bother about their implementation.


JDBC Architecture


JDBC Driver Types – Type 1

• Type 1 JDBC/ODBC Bridge

– uses bridge technology to connect a Java client to a third-party API such as Open DataBase Connectivity (ODBC)

– eg - Sun's JDBC-ODBC bridge



• Type 2 Java/Native Code Driver

– This type of driver wraps a native API with Java classes

– eg : Oracle Call Interface (OCI) driver



• Type 3 JDBC Driver

– This type of driver communicates using a network protocol to a middle tier server

– middle tier in turn communicates to the database



• Type 4 JDBC Driver

– These convert JDBC requests to database-specific network protocols, so that Java programs can connect directly to a database

– written entirely in Java


JDBC Packages

• Two packages named java.sql and javax.sql

– Java.sql

• contains core data objects of JDBC API

• Provides basics for connecting to the DBMS and interacting with data stored in the DBMS

– Javax.sql :

• Extends java.sql

• contains Java data objects that interact with Java Naming and Directory Interface (JNDI) and that manage connection pooling, etc.


Brief Overview of the JDBC process

• Process divided into five routines

– Load the driver

– Connect to the DBMS

– Create and Execute a Statement object

– Process data returned by the DBMS

– Terminate the connection with the DBMS


Steps to use JDBC in Java Program

Phase Task Relevant java.sql classes

Initialisation Load Driver Create connection

DriverManager Connection

Processing Generate SQL statements Process result data

Statement ResultSet

Termination Terminate connection Release data structures

Connection Statement


Loading the JDBC driver

• JDBC drivers must be loaded before the Java application connects to the DBMS

• The Class.forName() is used to load the JDBC driver

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


Connect to the DBMS

• Use DriverManager.getConnection() method – DriverManager is highest class in Java.sql

hierarchy; responsible for managing driver related information

– method is passed the URL of the database and user ID and password required by the database

– URL is the string object that contains the driver name that is being accessed

– method returns Connection interface that is used throughout the process to reference the database


Connect to the DBMS • Parameters of DriverManager.getConnection()

method

– (String url, String userID, String password);

– URL format : <protocol>:<subprotocol>:<dsn-name>

• Conncetion con =

DriverManager.getConnection(“jdbc:odbc:cus

tomer”, “root”, “password”);

• Connection con =

DriverManager.getConnection(

"jdbc:mysql://localhost/TEST","root",

"password"); Rakhi Saxena (Internet Technologies) 14

Create Statement object

• Create Statement object which will be used to execute the query

Statement stmt=con.createStatement();


Execute the query

• Execute and process the query which returns the ResultSet object

• ResultSet object is assigned the results received from the DBMS after the query is processed

ResultSet rs = stmt.executeQuery("select * from Employees");

• Employees is the table name


Process the results - 1

• next() method of ResultSet is used to iterate throughout the result set

while(rs.next()) {

System.out.println(

rs.getInt(1)+" "+

rs.getString(2) )+" "+

rs.getString(3)+" "+

rs.getString(4));

}


Process the results - 2

• ResultSet also contains several getXxx( ) methods to read the value from particular column of current row

• Eg - getString(“name”) will read the value from column ‘name’ in the form of string

String name;

int age;

do

{

name = rs.getString(“name”);

age = rs.getInt(“age”);

System.out.println(name+“--”+age);

} while(rs.next());


Terminate the Connection

• Use close() method of Connection object once Java program has finished accessing the DBMS

• Method throws as exception if problem is encountered

con.close();


Exception Handling- 1

• Class.forName() method throws a ClassNotFoundException if an error occurs while loading the JDBC driver

try

{


} catch(ClassNotFoundException e) {

System.err.println(“Unable to load the driver” + e);

System.exit(1);

}


Exception Handling - 2 • DriverManager.getConnection() method throws a

SQLException if the driver rejects access to the DBMS

try

{


Connection con = DriverManager.getConnection ("jdbc:mysql://localhost/TEST","root","password");

} catch(ClassNotFoundException e) {

System.err.println(“Unable to load the driver”+ e);

System.exit(1);

} catch(SQLException e) {

System.err.println(“Cannot connect to the database”+ e);

System.exit(1);

}


Complete Program import java.sql.*;

class JDBCDemo{

public static void main(String args[]){

try{


Connection con =

DriverManager.getConnection("jdbc:mysql://localhost/TEST",

"root","password");

//here TEST is the database name,

//root is the username and root is the password

Statement stmt=con.createStatement();

ResultSet rs=stmt.executeQuery("select * from Employees");

while(rs.next())

System.out.println(rs.getInt(1)+" "+rs.getString(2) +"

"+rs.getString(3)+rs.getString(4));

con.close();

}catch(Exception e) { System.out.println(e);}

}

} Rakhi Saxena (Internet Technologies) 22

Statement objects

• Once the connection to the database is opened, Java application creates and sends a query to access data contained in the database.

• Three type of statement objects:

– Statement Object: Executes a query immediately

– PreparedStatement Object: Executes a compiled query

– CallableStatement Object: Executes a stored procedure


The Statement Object - 1

• Statement object is used whenever a Java program needs to immediately execute a query without first having query compiled

• Three different methods available 1. executeQuery()

– This method returns the ResultSet object that contains rows, columns and metadata that represent data requested by the query.

– Generally, this method is used to execute only the ‘SELECT’ query of SQL

– Method Signature:

ResultSet executeQuery(String query);



2. executeUpdate()

– This method is used to execute the queries that contain INSERT, DELETE and UPDATE statements

– Method returns integer indicating the number of rows that were updated by the query

– Signature: int executeUpdate(String query);

For example:

int rows = st.executeUpdate("DELETE

FROM EMPLOYEES WHERE STATUS=0");



3. execute()

– This method is used to execute SQL statement which may return multiple results

– Signature :

public boolean execute(String sql)

For example:

if(st.execute()) rs = st.getResultSet();

– Signatures of other methods: • public ResultSet getResultSet()

• public int getUpdateCount()

• public boolean getMoreResults()


PreparedStatement object -1

• SQL query must be compiled before the DBMS processes it

• Compiling done after Statement object’s execution method is called

• Compiling a query is an overhead that is acceptable if the query is called once

• Compiling can become an expensive overhead if the query is executed several times by the same program during the same session



• PreparedStatement object can be used to precompile and execute SQL query

• Query is created similar to other queries

• However, a question mark is given on the place for the value that is inserted into the query after it is compiled

• It is the value that changes each time the query is executed

• For example

“select * from nation where population > ?”



• This type of the query is passed as the parameter to the prepareStatement( ) method of the Connection object which then returns the PreparedStatement object

• For example

“select * from nation where population > ?”

PreparedStatement ps = prepareStatement(query);

• Once the PreparedStatement object is obtained, the setXxx( ) methods is used to replace question mark with the value passed to setXxx() method


PreparedStatement object -4 • Each setXxx() method specifies the data type of value that is being

passed

• For example

ps.setInt(1, 100000);

– First parameter (int--identifies position of the question mark placeholder )

– Second Parameter (value that replaces the question mark)

• Next, use appropriate execute method depending upon type of the query without any parameters

ResultSet rs = ps.executeQuery();

• This will generate the ResultSet object as the execution of the query

• PreparedStatement contains all three execute methods but without any parameters

ResultSet executeQuery( ) int executeUpdate( ); boolean

execute( )


PreparedStatement object -5 • The setXxx( ) methods:

void setBoolean(int index, boolean value);

void setByte(int index, byte value);

void setDate(int index, Date value);

void setDouble(int index, double value);

void setFloat(int index, float value);

void setInt(int index, int value);

void setLong(int index, long value);

void setObject(int index, Object value);

void setShort(int index, short value);

void setString(int index, String value);


Example: Prepared Statement import java.sql.*;

class JDBCPreparedStatement{

public static void main(String args[]){

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

Connection con=DriverManager.getConnection(

"jdbc:mysql://localhost/TEST","root","password");

PreparedStatement ps = con.prepareStatement("select * from

Employees where Age > ?");

ps.setInt(1,18); //set question marks place holder

ResultSet rs = ps.executeQuery(); //execute

System.out.println("Employees having age > 5 are:");

while(rs.next()) {

System.out.println(rs.getInt(1)+" "+ rs.getString(2)+" "+

rs.getString(3)+" "+ rs.getString(4));

}

con.close();

}catch(Exception e){ System.out.println(e);}

}

} Rakhi Saxena (Internet Technologies) 32

CallableStatement Object

• CallableStatement - used to call stored procedures from JDBC application program

• Stored procedure - block of code identified by a unique name (can be written in PL/SQL, Transact-SQL, C, etc)

• CallableStatement object uses three types of parameters when calling a stored procedure – IN, OUT, INOUT – IN parameter contains the data that needs to be passed to the

stored procedure whose value is assigned using setXxx() method – OUT parameter contains the value returned by the stored

procedure, if any – INOUT parameter is a single parameter that is used to both pass

information and retrieve information from a stored procedure


ResultSet

• SQL statements that read data from a database query, return the data in a result set

• ResultSet object maintains a cursor (virtual pointer) that points to the current row in the result set

• Initially, cursor points to before the first row • next() method used to move the cursor to the one row

next from the current position – Returns true if row contains data, false otherwise – getXxx() method used to copy data from the row to a

collection, object, or variable – Columns appear in the ResultSet in the order in which

column names appear in the SELECT statement in the query


Scrollable ResultSet • Three ResultSet types:

• TYPE_FORWARD_ONLY (default type )

– ResultSet can only be navigated forward, move from row 1, to row 2, to row 3 etc

• TYPE_SCROLL_INSENSITIVE

– ResultSet can be navigated (scrolled) both forward and backwards

– Can also jump to a position relative to the current position, or jump to an absolute position

– The ResultSet is insensitive to changes in the underlying data source

– if a record in the ResultSet is changed in the database by another thread or process, it will not be reflected in already opened ResulsSet's of this type

• TYPE_SCROLL_SENSITIVE

– ResultSet can be navigated (scrolled) both forward and backwards

– can also jump to a position relative to the current position, or jump to an absolute position

– The ResultSet is sensitive to changes in the underlying data source

– if a record in the ResultSet is changed in the database by another thread or process, it will be reflected in already opened ResulsSet's of this type.


Navigation Methods METHOD DESCRIPTION

absolute() Moves the ResultSet to point at an absolute position. The position is a row number passed as parameter to the absolute() method.

afterLast() Moves the ResultSet to point after the last row in the ResultSet.

beforeFirst() Moves the ResultSet to point before the first row in the ResultSet.

first() Moves the ResultSet to point at the first row in the ResultSet.

last() Moves the ResultSet to point at the last row in the ResultSet.

next() Moves the ResultSet to point at the next row in the ResultSet.

previous() Moves the ResultSet to point at the previous row in the ResultSet.

relative() Moves the ResultSet to point to a position relative to its current position. The relative position is passed as a parameter to the relative method, and can be both positive and negative.


Other ResultSet interface methods

METHOD DESCRIPTION

getRow() Returns the row number of the current row - the row currently pointed to by the ResultSet.

getType() Returns the ResultSet type.

isAfterLast() Returns true if the ResultSet points after the last row. False if not.

isBeforeFirst() Returns true if the ResultSet points before the first row. False if not.

isFirst() Returns true if the ResultSet points at the first row. False if not.

METHOD DESCRIPTION

refreshRow() Refreshes the column values of that row with the latest values from the database.

NOTE: Not all JDBC Drivers are not Scrollable Rakhi Saxena (Internet Technologies) 37

Updatable ResultSet

• ResultSet concurrency determines whether the ResultSet can be updated, or only read

• Not all databases and JDBC drivers support that the ResultSet is updated

• ResultSet can have one of two concurrency levels

– CONCUR_READ_ONLY – means the ResultSet can only be read

– CONCUR_UPDATABLE - means that the ResultSet can be both read and updated


Updating a ResultSet • Can update the columns of each row in

the ResultSet by using updateXXX() methods result.updateString ("name" , "Alex");

result.updateInt ("age" , 55);

result.updateRow();

• Can also update a column using column index instead of column name result.updateString (1, "Alex");

result.updateInt (2, 55);

result.updateRow();

• It is when updateRow() is called that the database is updated with the values of the row. – If this method is not called, values updated in

the ResultSet are never sent to the database


Inserting Rows into a ResultSet • If ResultSet is updatable it is also possible to insert

rows into it result.moveToInsertRow();

result.updateString (1, "Alex");

result.updateInt (2, 55);

result.insertRow();

result.beforeFirst();

• The row pointed to after calling moveToInsertRow() is a special row, a buffer, which can be used to build up the row until all column values has been set on the row.

• Once the row is ready to be inserted into the ResultSet, the insertRow() method is called

• Important to move the ResultSet to a valid position after inserting the new row


Java Database Connectivity Chapter 6: JDBC Objects...The Statement Object - 2 2. executeUpdate()...

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