2. Persistent data
¤ When data have to be stored persistently, i.e., so as to
restore them when needed, a database can be used
¤ A database is a structured collection of data, that are
typically organized to model relevant aspects of reality,
e.g., availability of rooms in a hotel
¤ A database management system (DBMS) is a set of
programs that enables you to store, modify and extract
information from a database
3. Relational databases
¤ A Relational Database Management System (RDMBS)
organizes data into tables
¤ Each table is organized into rows and columns
¤ Particular rows and columns in a table can be related to
one or more rows and columns in another table
¤ Data can be read, updated, appended and deleted
using the SQL language
¤ SQL: Structured Query Language, application-specific
declarative language used for handling a relational
database
4. Database connection flow
¤ Database interaction flow
¤ Connect to the database
¤ Send a SQL statement
¤ Read the results
¤ Close connection
Get connection
SQL query
Query result
5. Interfaces
¤ Web applications can connect to databases, but each
of them has its own interface that can be used in order to
manipulate data
¤ ODBC
¤ Open Database Connectivity. Interface written in C
¤ Problem: programs written in C are not as portable as the
ones written in Java
¤ JDBC:
¤ Java Database Connectivity. Interface written in Java
¤ It can be seen as a porting of ODBC in Java
6. JDBC
¤ JDBC is a SQL-level API that allows to execute SQL
statements and retrieve the results
Application
The driver
converts JDBC
method calls into
ODBC function
calls.
7. Package java.sql (1)
¤ java.sql.Connection
¤ Connection (session) with a specific database
¤ SQL statements are executed and results are returned within the
context of a connection
¤ java.sql.Statement
¤ Used for executing a static SQL statement and returning the
results it produces
¤ Only one ResultSet object per statement can be opened at
the same time. If two ResultSet are read at the same time,
they have been generated by different Statement objects
¤ All execution methods in the Statement interface implicitly close
a statement's current ResultSet object if an opened one exists
8. Package java.sql (2)
¤ java.sql.ResultSet
¤ A table of data representing a database result set, which
is usually generated by executing a statement that
queries the database
¤ java.sql.DriverManager
¤ The basic service for managing a set of JDBC drivers
¤ When initialized, it will attempt to load the driver classes
referenced in the "jdbc.drivers" system property
¤ When the method getConnection is called, it will attempt to
locate a suitable driver from amongst those loaded at
initialization and those loaded explicitly
9. Example: DBPhoneLookup (1)
import java.io.*;!
import java.sql.*;!
!
import javax.servlet.*;!
import javax.servlet.http.*;!
!
public class DBPhoneLookup extends HttpServlet {!
!
public void doGet(HttpServletRequest request,
HttpServletResponse response)!
! !throws ServletException, IOException {!
Connection connection = null;!
! !!
response.setContentType("text/plain");!
PrintWriter out = response.getWriter();!
! !!
Package for JDBC + SQL
10. Example: DBPhoneLookup (2)
try {!
Class.forName("com.mysql.jdbc.Driver");!
!
connection = DriverManager.getConnection!
("jdbc:mysql://localhost:3306/test", "root", "");!
!
Statement statement = connection.createStatement();!
ResultSet resultSet = statement.executeQuery("SELECT * !
FROM employees");!
! ! !!
out.println("Phone numbers:");!
while (resultSet.next())!
out.println("- " + resultSet.getString("name") + ":
" + resultSet.getString("phone"));!
}!
Driver used for connecting to a MySQL database
Retrieve a
connection
Database address
Username Password
SQL query
Navigate result set
Retrieve an attribute from
the current tuple
12. More syntax
¤ When the attributes names and types are unknown,
ResultSetMetadata gives us a way of reading them
¤ resultSet.getMetadata(): reads the result set metadata
¤ metadata.getColumnCount(): returns the number of columns
in the result set
¤ metadata.getColumnLabel(i): returns the name of the i-th
column in the result set
¤ Result set data retrieval
¤ resultSet.getObject(): extract a non-String value from the
result set
¤ resultSet.getString(): extract a String value from the result
set
13. Modify data
¤ Execute a generic query:
¤ boolean state = statement.execute(query);
¤ When used for updating: the state is false since a result set is not
retrieved (the updated tuples count is retrieved)
¤ Execute an update:
¤ int count = statement.executeUpdate(updateQuery);
¤ Example: “DELETE FROM products WHERE id = 7”
¤ Retrieve results:
¤ resultSet = statement.getResultSet();
¤ count = statement.getUpdateCount();
14. Prepared statements
¤ A PreparedStatement is a Statement objects that is
precompiled by the database for faster execution
¤ Example: PreparedStatement statement =
connection.prepareStatement(“INSERT INTO ORDERS(ID,
CLIENTID, TOTAL) VALUES (?,?,?)”);
¤ The placeholders are substituted later:
statement.clearParameters();
statement.setInt(1,2);
statement.setInt(2,4);
statement.setDouble(3, 53.43);
statement.executeUpdate();
15. Reuse database connections
¤ Since creating and deleting database connections is
very expensive, a good idea is to reuse them
¤ How?
¤ Create them in the init() method
¤ Another “good practice”?
¤ Store the database connection parameter in the context.
Pro: sharing across all servlets, not written in the code
16. Reuse database connections (2)
public void init(ServletConfig config) throws ServletException {!
try {!
Class.forName("com.mysql.jdbc.Driver");!
!
ServletContext context = config.getServletContext();!
String url = context.getInitParameter("dbUrl");!
String user = context.getInitParameter("dbUser");!
String password = context.getInitParameter("dbPassword");!
connection = DriverManager.getConnection(url, user, password); ! !!
}!
catch (ClassNotFoundException e) {!
! throw new UnavailableException("Couldn't load database driver");!
}!
catch (SQLException e) {!
! throw new UnavailableException("Couldn't get db connection");!
} ! !
}
Retrieve parameters
from the context
Examples: DBPhoneLookupReuse + HtmlSQLResult
17. Transactions
¤ A transaction comprises a unit of work performed within a
DBMS against a database
¤ Example: a series of SQL queries for updating data in a
database when a user performs an order
¤ A database transaction must be:
¤ Atomic: all occur, or nothing occurs
¤ Consistent: it does not violate any integrity constraint during the
execution
¤ Isolated: defines how and when the changes made by the
transaction become visible to other operations
¤ Durable: transactions that have committed will survive
permanently
18. Transactions vs. JDBC
¤ Transactions are managed by using the Connection
object
¤ Use the transactions:
¤ connection.setAutoCommit(false);
¤ Commit the result:
¤ connection.commit();
¤ Rollback when an error occurs:
¤ connection.rollback();
19. Transactions vs. JDBC: example
synchronized (connection) {!
connection.setAutoCommit(false);!
!
PreparedStatement statement = connection.prepareStatement("INSERT
INTO employees (NAME, PHONE, DepID) VALUES (?, ?, ?)");!
statement.setString(1, (String) request.getParameter("name"));!
statement.setString(2, (String) request.getParameter("phone"));!
statement.setInt(3, 2);!
statement.executeUpdate();!
!
Statement statement2 = connection.createStatement();!
statement2.executeUpdate("UPDATE departments SET numemployees =
(numemployees + 1) WHERE id = 2"); ! ! !
} ! ! ! ! !
!
connection.commit(); !
Activate transactions
Execute the
first query
Execute the
second query
rollback(): called whenever an Exception is captured
Examples: DBPhoneTransaction
20. CallableStatement (1)
¤ The interface used to execute SQL stored procedures
¤ The provided syntax allows stored procedures to be called in
a standard way for all RDBMSs.
¤ Call: connection.prepareCall(java.lang.String)
¤ The syntax requires one result parameter (optional,
indicated as OUT) and several IN/OUT parameters.
Parameters are referred using numbers, starting from 1
¤ {?= call <procedure-name>[<arg1>,<arg2>, ...]}
¤ {call <procedure-name>[<arg1>,<arg2>, ...]}
21. CallableStatement (2)
¤ Handling parameters:
¤ IN: set using the set methods of PreparedStatement
¤ OUT: set the type before the call, retrieve the values
after the execution using the get methods
¤ Result: multiple ResultSet objects are handled using
operations inherited from Statement
22. javax.sql.*
¤ The package provides the API for server side data source
access and processing
¤ It integrates java.sql.*
¤ The package provides for the following:
¤ The DataSource interface as an alternative to the
DriverManager for establishing a connection with a data
source
¤ Connection pooling
¤ Distributed transaction
23. Connection pooling
¤ This procedure improves performance, avoiding to
create new connections
¤ A connection is used and reused
¤ The number of new connections that need to be created is
cut down
¤ Connection pooling is transparent and done
automatically in the middle tier of a J2EE configuration
