Java Reference
In-Depth Information
The class
PersistentDB
implements the DBManager interface. It accesses a
SQL database by means of the JDBC technology (see Sidebar 14.2). There is
only one attribute,
connection
, that contains the connection string. The
program that instantiates the DB manager is responsible for loading the
appropriate driver.
The class has a set of methods,
getItem()
,
getEmployee()
and
getCustomer()
,
that retrieve the information from a database in the form of an object. In
addition there are the methods that operate on the transaction and a method
to store a new acquisition.
The methods
startTransaction()
and
buy()
are synchronized because they
need exclusive access to the database in order to assign a unique ID to the
new row.
package
MarketDB;
import
java.sql.*;
import
javax.sql.*;
import
java.util.Vector;
import
java.util.List;
public class
PersistentDB
implements
DBManager {
private
String connection;
// creates the db manager storing the connection string
public
PersistentDB(String connection){
this
.connection
#
connection;}
// utility method to simplify the execution of queries
ResultSet select(String query) throws SQLException {
Connection conn
#
DriverManager
.getConnection(connection);
Statement stmt
#
conn.createStatement(
ResultSet.TYPE_FORWARD_ONLY,
ResultSet.CONCUR_READ_ONLY);
return
stmt.executeQuery(query);
}
// creates an Item object from the table ITEM
public
Item getItem(
long
code) {
try
{
ResultSet rs
#
select("SELECT * FROM ITEM WHERE ID
#
"
!
code);
if
(rs.next())
return
new Item(rs);
}
catch
(SQLException e){ }
return null
;
}
// creates an Employee object from the table EMPLOYEE
public
Employee getEmployee(
long
code) { ... }
// creates a Customer object from the table CUSTOMER
public
Customer getCustomer(
long
code) { ... }
// starts a customer transaction
