CS4273: Distributed System Technologies and Programming I

Lecture 8: Java Database Connection (JDBC)

2

Java DataBase Connections (JDBC)

JDBC driver• A JDBC driver is located in cslab’s dir:

/usr/local/jt6/lib/mysql-connector-java-5.1.7-bin.jar

• You can either:copy the zipped file to your local directory and unzip it:> jar xvf mysql-connector-java-5.1.7-bin.jaror make a link from directory “com” in your local dir to the system JDBC driver.

• A brief manual of MySQL JDBC in CSlab can be found at:http://personal.cs.cityu.edu.hk/jdemo/

3

JDBC Structure

Java applications

JDBC driver manager

JDBC/ODBC Bridge

Vendor’s JDBC driver

ODBC driver

database

database

JDBC API

JDBC driver API

4

2-Tier’s Structure

applet

Web server site

JDBC (proxy): 4040

DBMS system

5

3-Tiers Structure

applet

Web server site

HTTP server

your CGI server

DBMS system

6

Demonstration of using MySQL in Cslab

Access MySQL interactive interface:

mysql -hhostname -uusername -ppasswd

e.g.

% mysql -hjserv -ujdemo -papple1

(jserv is the host name of DBMS, jdemo the user name and apple1 the password.)

> use db_jdemo; // set database to db_jdemo

> select * from COFFEES;

……

7

Demonstration of using MySQL in Cslab (Cont.)

More SQL statements

> show tables; // list all the table

> help;

> help contents;

> select * from myCOFFEES;

> drop table myCOFFEES;

> exit; // quit the mysql interface

8

Java applications and DBMS using JDBC

Note: no proxy is needed in this case.

Java Application

JDBC DBMS

9

JDBC: Java applications and MySQL

import java.sql.*;public class CreateCoffees {public static void main(String args[]) { String url =“jdbc:mysql://jserv.cs.cityu.edu.hk:3306/

db_jdemo”; Connection con; Statement stmt; String createString = "create table

myCOFFEES " +"(COF_NAME varchar(32), " + "SUP_ID int, " + "PRICE float, " +"SALES int, " + "TOTAL int)";

try { Class.forName("com.mysql.jdbc.Driver");

con = DriverManager.getConnection(url, "jdemo", "apple1");

stmt = con.createStatement();stmt.executeUpdate(createString);stmt.close(); con.close();

} catch(SQLException e) {System.err.println(e.getMessage()); }

}}

10

JDBC: Java applications and MySQL (select)

import java.sql.*;public class SelectCoffees {

public static void main(String args[]) {String url =

"jdbc:mysql://jserv.cs.cityu.edu.hk:3306 /db_jdemo”;

Connection con; Statement stmt;String query = "select COF_NAME, PRICE

from COFFEES";try { Class.forName("com.mysql.jdbc.Driver");con = DriverManager.getConnection(url, "jdemo", "apple1");stmt = con.createStatement();ResultSet rs = stmt.executeQuery(query);

while (rs.next()) { // loop on each row of "rs"String s =

rs.getString("COF_NAME");float f = rs.getFloat("PRICE");System.out.println(s + " " + f);

}stmt.close(); con.close();} catch(SQLException e) {System.err.println(e.getMessage()); }

}}

11

Steps of JDBC Connection

1. Load driver managerClass.forName("Driver’s name"), e.g., Class.forName("com.mysql.jdbc.Driver");Class.forName("com.sybase.jdbc.SybDriver");Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

Note: name components separated by ‘.’ are a dir path name, relative to the current directory.

2. Make a connection to DBMS systemDatabase URL

• A database URL specifies where the database is. The general syntax:jdbc: subprotocol_name: other_stuff

• The format of other_stuff depends on the subprotocol used, e.g., String url="jdbc:mysql://jserv.cs.cityu.edu.hk:3306/db_50704380"String url = “jdbc:odbc://whitehouse.gov:5000/Cat”String url = "jdbc:sybase:Tds:ntr10:4100";Database Connection

• Connection con = DriverManager.getConnection(url, usr, pswd);e.g., con = DriverManager.getConnection(url, "jdemo", "apple1");

12

Steps of JDBC Connection (Cont.)

3. Create a statement object• create a Statement object out of Connection object for every SQL

statement:Statement stmt = con.createStatement();

4. Execute an SQL statement• Two types of SQL statements: queries and updates

ExecuteQueryResultSet rs = stmt.executeQuery (“select * from books”);or:String query = “select * from books”;ResultSet rs = stmt.executeQuery (query);

ExecuteUpdatestmt.executeUpdate(createString);(there is no return needed for update statements)

13

Steps of JDBC Connection (Cont.)

5. Get results from DBMS

• The execution of an sql query returns a set of rows. Each row consists of several attributes (fields):

ResultSet rs = stmt.executeQuery (query);

Access Rows of a ResultSet

• The basic loop for analyzing a result set uses format:

while ( rs.next()) {

// rs.next pointers to the next row

analyze a row of the result set;

}

Access Attributes of a row• Get the value of an attribute by methods

of format:Xxx getXxx (int clmn_num) orXxx getXxx (String clmn_name)

e.g., String cofName = rs.getString(1) or String cofName = rs.getString(“COF_NAME”);

• There are many other getXxx methods:int supId = rs.getInt (“SUP_ID”);float price = rs.getDouble (“PRICE”);…….

• For other types, please refer to JDBC manual.

14

JDBC Connection between Applet and DBMS2-Tier Structure

1. Install a JDBC proxy at the web server site (“personal.cs.cityu”).

2. An applet makes a JDBC connection to DBMS via the proxy, bcs applets can only connect to its home server.

3. The result type of a sql query is ResultSet (process them in the same way as discussed before).

applet

Web server site

JDBC (proxy): 4040

DBMS system

15

An Example of 2 Tier Structure(Applet – DBMS)

public class myapplet extends Applet implements Runnable {

private Vector queryResults;

public synchronized void start() {

if (worker == null) {

message = "Connecting to database";

worker = new Thread(this); worker.start(); }

}

public void run() { String url = "jdbc:mysql:

//personal.cs.cityu.edu.hk:4040/db_jdemo "; String query = "select COF_NAME, PRICE

from COFFEES"; try {

Class.forName("com.mysql.jdbc.Driver");Vector results = new Vector();

con= DriverManager.getConnection(url, "jdemo", "apple1");

stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(query); while (rs.next()) {// process rs row by row String s = rs.getString("COF_NAME"); float f = rs.getFloat("PRICE"); String text = s + " " + f; results.addElement(text); } stmt.close(); con.close(); setResults(results);

repaint(); } catch(SQLException e) {;} }

16

Disadvantages of the 2-Tier Structure

1. Fat clients (applets).

2. Little flexibility on the security control to the DBMS accesses.

17

3-Tiers Structure

applet

Web server site

HTTP server

your CGI server

DBMS system

18

Applet design in 3-tier (interact with CGI)

1. make socket connection to HTTP server and start cgi-svr by POST method.2. receive String lines from cgi-svr, one for each row of data, and put them into a Vector.

Vector results = new Vector();while ((line = in.readLine())!=null)

results.addElement(line);3. paint() extracts data rows from the vector by using an Enumerration object.

Enumeration enum = queryResults.elements();a) check if it’s the end of the object:

enum.hasMoreElements();b) get the next element from the object:

String text = (String) enum.nextElement();4. for each line of data, break it into attributes

(separated by “|”) by using an StringTokenizer object.StringTokenizer st = new StringTokenizer(text, "|");String cof_name = st.nextToken();String cof_price = st.nextToken();

applet

Web server site

HTTP server

CGI server

19

Applet in 3-tier JDBC

public class myapplet extends Applet implements Runnable {

Thread worker; Vector queryResults; public synchronized void start() { ......... worker.start(); } public void run() { Vector results = CallCgiSvr(); if (results != null) setResults(results); } Vector CallCgiSvr() { Vector results = new Vector(); String sdata = "START_QUERY";

s = new Socket(“personal.cs.cityu.edu.hk",80);in = new DataInputStream(s.getInputStream());out = new PrintStream(s.getOutputStream());out.println("POST /3tier/shellcgi.cgi HTTP/1.0\r");out.println("Content-type: plain/text\r");out.println("Content-length: "+ sdata.length()+ "\r\r");out.println(sdata+"\r");

while (! in.readLine.equals("START_DATA"));while ((line = in.readLine()) != null)

if (line.length() > 0)results.addElement(line);

in.close(); out.close(); return(results);

}public synchronized void paint(Graphics g) {

g.drawString("Prices of coffee per pound: ", 5, 10);int x = 5, y = 30;Enumeration enum = queryResults.elements();while (enum.hasMoreElements()) {

String text = (String)enum.nextElement();StringTokenizer st = new StringTokenizer(text,

"|");String cof_name = st.nextToken();String cof_price = st.nextToken();g.drawString(cof_name, x, y);g.drawString(cof_price, x+140, y);y = y + 15; }

}

20

CGI Program in 3-Tier JDBC

1. cgi-svr acts as a gateway between applet and DBMS. No more need of a proxy at the web site.

2. cgi-svr gets requests from client via stdin, connects to DBMS by JDBC, executes queries and receives results:a) get query results of type ResultSet:

ResultSet rs = stmt.executeQuery (query);b) analyze each row and make it a line. Attributes of a line are separated by “|”.

String s = rs.getString("COF_NAME");float f = rs.getFloat("PRICE");String text = s + "|" + f;

c) append the lines into a StringBuffer. StringBuffer results = new StringBuffer();results.append (text + "\n");

d) convert StringBuffer to a single string and send it to applet via stdout.String line = results.toString();outs.println(line); // outs is the stdout stream

applet HTTP server

CGI server

DBM

21

CGI program in 3-Tier JDBC

class cgisvr { public static void main(String[] args) { String request, line; try {

DataInputStream ins = new DataInputStream(System.in); PrintStream outs = new PrintStream(System.out);

while ((request = ins.readLine()) != null) {if (request.equals("START_QUERY")) {line = ReqSql ();outs.println(START_DATA); // a start tokenouts.println(line); }} // send reply to appletouts.close();

} catch (Exception e) { System.out.println("Error"+e);}}

22

CGI program in 3-Tier JDBC (Cont.)

static String ReqSql() {

String url = "jdbc:mysql: //jserv.cs.edu.hk:3306/db_jdemo ";

String query = "select COF_NAME, PRICE from COFFEES";

StringBuffer results = new StringBuffer();

Class.forName("com.mysql.jdbc.Driver");

con= DriverManager.

getConnection(url, "jdemo", "apple1");

Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery(query);

while (rs.next()) {

String s = rs.getString("COF_NAME");float f = rs.getFloat("PRICE");String text = s + "|" + f;results.append(text + "\n");

} stmt.close(); con.close(); return(results.toString()); }

23

Shell Script starting the Java Program

#!/bin/sh

echo Content-type: text/plain

Echo

#the java command must use full path!

/usr/local/jdk/bin/java cgisvr