I want to improve my knowledge about creating new database.Then I study the details about JDBC - Database Connections.There are different type of database connections.So I study one by one and improve my knowledge. Among these I understand how to create database connection according to the company.I improve my current knowledge by using internet and helps of the company developers.
JDBC - Database Connections
The programming involved to
establish a JDBC connection is fairly simple. Here are these simple four steps
−
·Import JDBC Packages: Add import statements to your Java program to import
required classes in your Java code.
·Register JDBC Driver: This step causes the JVM to load the desired
driver implementation into memory so it can fulfill your JDBC requests.
·Database URL Formulation: This is to create a properly formatted
address that points to the database to which you wish to connect.
·Create Connection Object: Finally, code a call to the DriverManager object'sgetConnection( ) method to establish actual database
connection.
Import JDBC Packages
The Import statements tell the Java compiler where to
find the classes you reference in your code and are placed at the very
beginning of your source code.
To use the standard JDBC package,
which allows you to select, insert, update, and delete data in SQL tables, add
the following imports to your source code −
import java.sql.* ; // for standard JDBC programs
import java.math.* ; // for BigDecimal and BigInteger support
Register JDBC Driver
You must register the driver in
your program before you use it. Registering the driver is the process by which
the Oracle driver's class file is loaded into the memory, so it can be utilized
as an implementation of the JDBC interfaces.
Approach I -
Class.forName()
The most common approach to
register a driver is to use Java's Class.forName() method, to dynamically load the driver's
class file into memory, which automatically registers it. This method is
preferable because it allows you to make the driver registration configurable
and portable.
The following example uses
Class.forName( ) to register the Oracle driver −
try {
Class.forName("oracle.jdbc.driver.OracleDriver");
}
catch(ClassNotFoundException ex) {
System.out.println("Error: unable to load driver class!");
System.exit(1);
}
You can use getInstance() method to work around noncompliant JVMs, but
then you'll have to code for two extra Exceptions as follows −
try {
Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();
}
catch(ClassNotFoundException ex) {
System.out.println("Error: unable to load driver class!");
System.exit(1);
catch(IllegalAccessException ex) {
System.out.println("Error: access problem while loading!");
System.exit(2);
catch(InstantiationException ex) {
System.out.println("Error: unable to instantiate driver!");
System.exit(3);
}
Approach II -
DriverManager.registerDriver()
The second approach you can use
to register a driver, is to use the staticDriverManager.registerDriver() method.
You should use the registerDriver() method if you are using a non-JDK compliant
JVM, such as the one provided by Microsoft.
The following example uses
registerDriver() to register the Oracle driver −
try {
Driver myDriver = new oracle.jdbc.driver.OracleDriver();
DriverManager.registerDriver( myDriver );
}
catch(ClassNotFoundException ex) {
System.out.println("Error: unable to load driver class!");
System.exit(1);
}
Database URL
Formulation
After you've loaded the driver,
you can establish a connection using theDriverManager.getConnection() method. For easy reference, let me list the
three overloaded DriverManager.getConnection() methods −
·
getConnection(String url)
·
getConnection(String url, Properties prop)
·
getConnection(String url, String user, String password)
Here each form requires a
database URL. A
database URL is an address that points to your database.
Formulating a database URL is
where most of the problems associated with establishing a connection occurs.
Following table lists down the
popular JDBC driver names and database URL.
|
RDBMS
|
JDBC
driver name
|
URL
format
|
|
MySQL
|
com.mysql.jdbc.Driver
|
jdbc:mysql://hostname/ databaseName
|
|
ORACLE
|
oracle.jdbc.driver.OracleDriver
|
jdbc:oracle:thin:@hostname:port Number:databaseName
|
|
DB2
|
COM.ibm.db2.jdbc.net.DB2Driver
|
jdbc:db2:hostname:port Number/databaseName
|
|
Sybase
|
com.sybase.jdbc.SybDriver
|
jdbc:sybase:Tds:hostname: port Number/databaseName
|
All the highlighted part in URL
format is static and you need to change only the remaining part as per your
database setup.
Create Connection
Object
We have listed down three forms
of DriverManager.getConnection() method to create a connection object.
Using a Database URL
with a username and password
The most commonly used form of
getConnection() requires you to pass a database URL, ausername, and a password:
Assuming you are using Oracle's thin driver, you'll specify a
host:port:databaseName value for the database portion of the URL.
If you have a host at TCP/IP
address 192.0.0.1 with a host name of amrood, and your Oracle listener is
configured to listen on port 1521, and your database name is EMP, then complete
database URL would be −
jdbc:oracle:thin:@amrood:1521:EMP
Now you have to call
getConnection() method with appropriate username and password to get a Connection object as follows −
String URL = "jdbc:oracle:thin:@amrood:1521:EMP";
String USER = "username";
String PASS = "password"
Connection conn = DriverManager.getConnection(URL, USER, PASS);
Using Only a Database
URL
A second form of the
DriverManager.getConnection( ) method requires only a database URL −
DriverManager.getConnection(String url);
However, in this case, the
database URL includes the username and password and has the following general
form −
jdbc:oracle:driver:username/password@database
So, the above connection can be
created as follows −
String URL = "jdbc:oracle:thin:username/password@amrood:1521:EMP";
Connection conn = DriverManager.getConnection(URL);
Using a Database URL
and a Properties Object
A third form of the
DriverManager.getConnection( ) method requires a database URL and a Properties
object −
DriverManager.getConnection(String url, Properties info);
A Properties object holds a set
of keyword-value pairs. It is used to pass driver properties to the driver
during a call to the getConnection() method.
To make the same connection made
by the previous examples, use the following code −
import java.util.*;
String URL = "jdbc:oracle:thin:@amrood:1521:EMP";
Properties info = new Properties( );
info.put( "user", "username" );
info.put( "password", "password" );
Connection conn = DriverManager.getConnection(URL, info);
Closing JDBC
Connections
At the end of your JDBC program,
it is required explicitly to close all the connections to the database to end
each database session. However, if you forget, Java's garbage collector will
close the connection when it cleans up stale objects.
Relying on the garbage
collection, especially in database programming, is a very poor programming
practice. You should make a habit of always closing the connection with the
close() method associated with connection object.
To ensure that a connection is closed,
you could provide a 'finally' block in your code. Afinally block always executes, regardless of an
exception occurs or not.
To close the above opened
connection, you should call close() method as follows −
conn.close();
Explicitly closing a connection
conserves DBMS resources, which will make your database administrator happy.
JDBC - Statements, PreparedStatement
and CallableStatement
Once a connection is obtained we
can interact with the database. The JDBC Statement, CallableStatement, and PreparedStatement interfaces define the methods and properties
that enable you to send SQL or PL/SQL commands and receive data from your
database.
They also define methods that
help bridge data type differences between Java and SQL data types used in a database.
The following table provides a
summary of each interface's purpose to decide on the interface to use.
|
Interfaces
|
Recommended
Use
|
|
Statement
|
Use the for
general-purpose access to your database. Useful when you are using static SQL
statements at runtime. The Statement interface cannot accept parameters.
|
|
PreparedStatement
|
Use the when
you plan to use the SQL statements many times. The PreparedStatement
interface accepts input parameters at runtime.
|
|
CallableStatement
|
Use the when
you want to access the database stored procedures. The CallableStatement
interface can also accept runtime input parameters.
|
The Statement Objects
Creating Statement Object
Before you can use a Statement
object to execute a SQL statement, you need to create one using the Connection
object's createStatement( ) method, as in the following example
Statement stmt = null;
try {
stmt = conn.createStatement( );
. . .
}
catch (SQLException e) {
. . .
}
finally {
. . .
}
Once you've created a Statement
object, you can then use it to execute an SQL statement with one of its three
execute methods.
· boolean execute (String SQL): Returns
a boolean value of true if a ResultSet object can be retrieved; otherwise, it
returns false. Use this method to execute SQL DDL statements or when you need
to use truly dynamic SQL.
· int executeUpdate (String SQL): Returns
the number of rows affected by the execution of the SQL statement. Use this
method to execute SQL statements for which you expect to get a number of rows
affected - for example, an INSERT, UPDATE, or DELETE statement.
· ResultSet executeQuery (String SQL): Returns
a ResultSet object. Use this method when you expect to get a result set, as you
would with a SELECT statement.
Closing Statement
Object
Just as you close a Connection
object to save database resources, for the same reason you should also close
the Statement object.
A simple call to the close()
method will do the job. If you close the Connection object first, it will close
the Statement object as well. However, you should always explicitly close the
Statement object to ensure proper cleanup.
Statement stmt = null;
try {
stmt = conn.createStatement( );
. . .
}
catch (SQLException e) {
. . .
}
finally {
stmt.close();
}
The PreparedStatement
Objects
The PreparedStatement interface extends the Statement interface,
which gives you added functionality with a couple of advantages over a generic
Statement object.
This statement gives you the
flexibility of supplying arguments dynamically.
Creating PreparedStatement
Object
PreparedStatement pstmt = null;
try {
String SQL = "Update Employees SET age = ? WHERE id = ?";
pstmt = conn.prepareStatement(SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
. . .
}
All parameters in JDBC are
represented by the ? symbol, which is known as the parameter
marker. You must supply values for every parameter before executing the SQL
statement.
The setXXX() methods bind values to the parameters, where XXX represents the Java data type of the value
you wish to bind to the input parameter. If you forget to supply the values,
you will receive an SQLException.
Each parameter marker is referred
by its ordinal position. The first marker represents position 1, the next
position 2, and so forth. This method differs from that of Java array indices,
which starts at 0.
All of the Statement object's methods for interacting with the database (a)
execute(), (b) executeQuery(), and (c) executeUpdate() also work with the
PreparedStatement object. However, the methods are modified to use SQL
statements that can input the parameters.
Closing
PreparedStatement Object
Just as you close a Statement
object, for the same reason you should also close the PreparedStatement object.
A simple call to the close()
method will do the job. If you close the Connection object first, it will close
the PreparedStatement object as well. However, you should always explicitly
close the PreparedStatement object to ensure proper cleanup.
PreparedStatement pstmt = null;
try {
String SQL = "Update Employees SET age = ? WHERE id = ?";
pstmt = conn.prepareStatement(SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
pstmt.close();
}
The CallableStatement
Objects
Just as a Connection object
creates the Statement and PreparedStatement objects, it also creates the
CallableStatement object, which would be used to execute a call to a database
stored procedure.
Creating
CallableStatement Object
Suppose, you need to execute the
following Oracle stored procedure −
CREATE OR REPLACE PROCEDURE getEmpName
(EMP_ID IN NUMBER, EMP_FIRST OUT VARCHAR) AS
BEGIN
SELECT first INTO EMP_FIRST
FROM Employees
WHERE ID = EMP_ID;
END;
NOTE: Above stored procedure has been written for
Oracle, but we are working with MySQL database so, let us write same stored
procedure for MySQL as follows to create it in EMP database −
DELIMITER $$
DROP PROCEDURE IF EXISTS `EMP`.`getEmpName` $$
CREATE PROCEDURE `EMP`.`getEmpName`
(IN EMP_ID INT, OUT EMP_FIRST VARCHAR(255))
BEGIN
SELECT first INTO EMP_FIRST
FROM Employees
WHERE ID = EMP_ID;
END $$
DELIMITER ;
Three types of parameters exist:
IN, OUT, and INOUT. The PreparedStatement object only uses the IN parameter.
The CallableStatement object can use all the three.
Here are the definitions of each
−
|
Parameter
|
Description
|
|
IN
|
A parameter
whose value is unknown when the SQL statement is created. You bind values to
IN parameters with the setXXX() methods.
|
|
OUT
|
A parameter
whose value is supplied by the SQL statement it returns. You retrieve values
from theOUT parameters with the getXXX() methods.
|
|
INOUT
|
A parameter
that provides both input and output values. You bind variables with the
setXXX() methods and retrieve values with the getXXX() methods.
|
The following code snippet shows
how to employ the Connection.prepareCall() method to instantiate a CallableStatement object based on the preceding stored
procedure −
CallableStatement cstmt = null;
try {
String SQL = "{call getEmpName (?, ?)}";
cstmt = conn.prepareCall (SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
. . .
}
The String variable SQL,
represents the stored procedure, with parameter placeholders.
Using the CallableStatement
objects is much like using the PreparedStatement objects. You must bind values
to all the parameters before executing the statement, or you will receive an
SQLException.
If you have IN parameters, just
follow the same rules and techniques that apply to a PreparedStatement object;
use the setXXX() method that corresponds to the Java data type you are binding.
When you use OUT and INOUT
parameters you must employ an additional CallableStatement method,
registerOutParameter(). The registerOutParameter() method binds the JDBC data
type, to the data type that the stored procedure is expected to return.
Once you call your stored
procedure, you retrieve the value from the OUT parameter with the appropriate
getXXX() method. This method casts the retrieved value of SQL type to a Java
data type.
Closing
CallableStatement Object
Just as you close other Statement
object, for the same reason you should also close the CallableStatement object.
A simple call to the close()
method will do the job. If you close the Connection object first, it will close
the CallableStatement object as well. However, you should always explicitly
close the CallableStatement object to ensure proper cleanup.
CallableStatement cstmt = null;
try {
String SQL = "{call getEmpName (?, ?)}";
cstmt = conn.prepareCall (SQL);
. . .
}
catch (SQLException e) {
. . .
}
finally {
cstmt.close();
}
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