This document provides an overview of using Java Database Connectivity (JDBC) to connect to and interact with relational databases from Java servlets. It describes relational databases and how they organize data into tables. It explains the basic JDBC connection flow of getting a connection, executing SQL statements, and reading results. It also covers key JDBC interfaces like Connection, Statement, and ResultSet. The document provides an example servlet for looking up phone numbers from a database and discusses best practices like reusing connections and using connection pooling.

1. Java Servlets
JDBC and database connectivity
Eleonora Ciceri
ciceri@elet.polimi.it

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

11. Example: DBPhoneLookup (3)
catch (ClassNotFoundException e) {!
out.println("Couldn't load database driver: " + e.getMessage());!
}!
!
catch (SQLException e) {!
out.println("SQLexception caught: " + e.getMessage());!
}!
! !!
finally {!
if (connection != null) {!
try {!
connection.close();!
}!
catch (SQLException ignored) {!
}!
}!
}!
}!
}
Always close the
connection!!

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

24. Connection pooling: example
¤ init()
ServletContext context = config.getServletContext();
String loginUser = context.getInitParameter("dbUser");
String loginPasswd = context.getInitParameter
("dbPassword");
String driver = context.getInitParameter("dbDriver");
String loginUrl = context.getInitParameter("dbUrl");
dataSource = setupDataSource(driver, loginUrl, loginUser,
loginPasswd);
¤ doGet()
connection = dataSource.getConnection();
Examples: DBPhonePool+ DBPhoneSingle

25. References

26. References
¤ Java Servlet Programming, Jason Hunter and William
Crawford, O’Reilly