jdbc in servlets
DESCRIPTION
TRANSCRIPT
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
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
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
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
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.
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
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
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
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
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!!
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
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();
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();
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
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
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
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();
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
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>, ...]}
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
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
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
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
References
References
¤ Java Servlet Programming, Jason Hunter and William Crawford, O’Reilly