Download catch

Tutorial:
Advanced Java Programming
and Database connection
Eran Toch
Methodologies in the Development
of Information Systems
November 2003
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
Agenda
• Exceptions and Error Handling
– What is it and why do we need it?
– The try, catch, finally procedure
• Database Access with JDBC
– Installations
– Connecting and querying the database
– Complete example
• References
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
2
Exceptions - Introduction
• Definition: An exception is an event that
occurs during the execution of a program that
disrupts the normal flow of instructions.
• What’s wrong with using the return value for
error handling?
– Advantage 1: Separating Error Handling Code from
"Regular" Code
– Advantage 2: Propagating Errors Up the Call Stack
– Advantage 3: Grouping Error Types and Error
Differentiation
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
3
Exceptions – Advantage 1
Separating Error Handling Code from "Regular" Code:
errorCodeType readFile {
initialize errorCode = 0;
open the file;
if (theFileIsOpen) {
determine the length of the
file;
if (gotTheFileLength) {
allocate that much memory;
if (gotEnoughMemory) {
read the file into
memory;
if (readFailed) {
errorCode = -1;
}
} else {
errorCode = -2;
}
. . .
readFile {
try {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
} catch (fileOpenFailed) {
doSomething;
} catch (sizeDeterminationFailed) {
doSomething;
} catch (memoryAllocationFailed) {
doSomething;
} catch (readFailed) {
doSomething;
} catch (fileCloseFailed) {
doSomething;
}
}
Without Exception
With Exception
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
4
The try Block
• try block:
try {
System.out.println("Entering try statement");
out = new PrintWriter(new FileWriter("OutFile.txt"));
}
for (int i = 0; i < size; i++)
out.println("Value at: " + i + " = " + victor.elementAt(i));
• A try statement must be accompanied by at
least one catch block or one finally block.
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
5
The catch Block
• catch block handles the exception:
try {
. . .
} catch (ArrayIndexOutOfBoundsException e) {
System.err.println("Caught
ArrayIndexOutOfBoundsException: " +
e.getMessage());
} catch (IOException e) {
System.err.println("Caught IOException: "
+ e.getMessage());
}
• Multiple catch blocks can be placed, each
handling a different type of exception
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
6
Catching Multiple Exceptions
• Java exceptions are Throwable objects
Throwable
Exception
MyException
MySpecificException2
MySpecificException1
• Catching MyException will catch both the
subclasses. Catching Exception will catch all
types of Exceptions
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
7
Catching Multiple Exceptions – cont’d
• Example: The following catch block, will catch
all types of exceptions:
} catch (Exception e) {
System.err.println("Exception caught: " + e.getMessage());
}
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
8
The finally Block
• We can never be sure that either the try block
or the finally block will be fully executed.
• finally block code will always be executed:
finally {
if (out != null) {
System.out.println("Closing PrintWriter");
out.close();
} else {
System.out.println("PrintWriter not open");
}
}
• Used frequently for cleanup processes.
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
9
Putting it All Together
public void writeList() {
PrintWriter out = null;
try {
System.out.println("Entering try statement");
out = new PrintWriter(
new FileWriter("OutFile.txt"));
for (int i = 0; i < size; i++)
out.println("Value at: " + i + " = " + victor.elementAt(i));
} catch (ArrayIndexOutOfBoundsException e) {
System.err.println("Caught ArrayIndexOutOfBoundsException: " +
e.getMessage());
} catch (IOException e) {
System.err.println("Caught IOException: " + e.getMessage());
} finally {
if (out != null) {
System.out.println("Closing PrintWriter");
out.close();
} else {
System.out.println("PrintWriter not open");
}
}
}
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
10
throw Statement
• All Java methods use the throw statement to
throw an exception
• The method must declare that it might throw
something, by using the throws statement
public Object pop() throws EmptyStackException {
Object obj;
if (size == 0)
throw new EmptyStackException(“exception text”);
obj = objectAt(size - 1);
setObjectAt(size - 1, null);
size--;
return obj;
}
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
11
Exceptions and JavaDoc
• Exception can be documented by Javadoc
using the @exception statement
/**
* regular javadoc text…
* @throwsExceptionIf the Driver was not found.
* @throwsSQLExceptionIf the the <code>DriverManager.getConnection
* </code> method returned an error.
*/
public void createConnection()throws SQLException, Exception{
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
12
Agenda
• Exceptions and Error Handling
– What is it and why do we need it?
– The try, catch, finally procedure
• Database Access with JDBC
– Installations
– Connecting and querying the database
– Complete example
• References
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
13
Database Connection - Overview
• Four stages:
–
–
–
–
Install and configure the database
Download and configure the JDBC
Create a connection to the database
Access the database
• In this tutorial, examples will be based on
MySQL. The reference section include a link
to instructions for MS Access.
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
14
Database Install
• Download the MySQL database from:
http://www.mysql.com/downloads/
• Install it
• Create a specific database:
create database mytest;
• Create a user account:
grant all on mytest.* to eran identified by ‘1234’
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
15
JDBC Install
• Download Connector/J from:
http://www.mysql.com/downloads/api-jdbc.html
• Unzip it
• In order the library to be found, either:
– Copy the .jar file to:
$JAVA_HOME/jre/lib/ext
– Or, add a classpath to the JDBC:
C:\> set CLASSPATH=\path\to\mysql-connector-java-[version]bin.jar;%CLASSPATH%
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
16
Accessing Database
1.
2.
3.
4.
Load the driver
Creating a connection object
Create a statement object
Execute an SQL query and get results using
the ResultSet object
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
17
Example – Database Management
mysql> create database mytest;
Creating the DB
Query OK, 1 row affected (0.05 sec)
mysql> grant all on *.* to eran@localhost identified by '1234';
Creating user account
Query OK, 0 rows affected (0.14 sec)
mysql>create table phones (name varchar(255) not null unique key, phone
varchar(25) not null);
Creating the ‘phones’
mysql>describe phones;
table
+-------+--------------+------+-----+---------+-------+
| Field | Type
| Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
Is everything alright?
| name | varchar(255) |
| PRI |
|
|
Let’s see…
| phone | varchar(25) |
| |
|
|
+-------+--------------+------+-----+---------+-------+
mysql> insert into phones values ('Eran Toch', '+972-4-9831894');
Query OK, 1 row affected (0.11 sec)
Inserting some data
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
18
Example – Connection
import java.sql.*;
public class SQLConnect {
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
Importing java.sql.* that
contains all the classes we
need
Connection, Statement and
ResultSet are defined as
class variables
public SQLConnect(){}
public void createConnection(){
try{
Class.forName("com.mysql.jdbc.Driver").newInstance();
}
catch (Exception E){
Dynamically loading the specific
System.out.println(E);
JDBC driver. The runtime environment
}
must know where the library is located!
try{
conn =
DriverManager.getConnection("jdbc:mysql://localhost/mytest?user=test
master&password=1234");
}
Connecting to the database
catch (SQLException E){
using the url
System.out.println(E);
}
}
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
19
Example – Locating Libraries
• If the following error message occurs:
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
java.sql.SQLException: No suitable driver
• Then the driver was not found.
– For Eclipse, add it in the
project properties window
– For runtime, add it to the
classpath
Project properties window in
Eclipse
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
20
Example – Access and Query
Creating a statement
public String getPhones(){
String output = "";
Creating a ResultSet, based
try {
on a SQL statement
stmt = conn.createStatement();
rs = stmt.executeQuery("SELECT * FROM phones");
if (rs != null){
while (rs.next()){
Going through the
output += rs.getString("phone") + "\n"; ResultSet by using
}
rs.next(). Remember – you
}
need to call the next method
}
before you start reading from the
catch (Exception E){
ResultSet
System.out.println(E.getMessage());
}
Reading a field from the
ResultSet
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
21
Example – Cleaning off
finally {
if (rs != null) {
try {
rs.close();
}
catch (SQLException sqlEx) {}
rs = null;
}
if (stmt != null) {
try {
stmt.close();
}
catch (SQLException sqlEx) {}
stmt = null;
}
}
return output;
}
Cleaning off the connection,
in a different method (why?)
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
Cleaning off is best done in
the “finally” clause
Cleaning off ResultSet
Cleaning off Statement,
after the ResultSet
public void closeConnection(){
if (conn != null){
try {
conn.close();
}
catch (SQLException sqlEx){}
conn = null;
}
}
22
Example – Test Client
public class Test {
public static void main(String[] args) {
SQLConnect connect = new SQLConnect();
connect.createConnection();
String allPhones = connect.getPhones();
connect.closeConnection();
System.out.println("phones:");
System.out.println(allPhones);
}
}
Output
phones:
+972-4-9831894
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
23
Agenda
• Exceptions and Error Handling
– What is it and why do we need it?
– The try, catch, finally procedure
• Database Access with JDBC
– Installations
– Connecting and querying the database
– Complete example
• References
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
24
References
• Exception handling in the Java tutorial:
http://java.sun.com/docs/books/tutorial/essential/exceptions/index.html
• JDBC Tutorial:
http://java.sun.com/docs/books/tutorial/jdbc/
• MySQL Tutorial:
http://www.mysql.com/documentation/mysql/bychapter/
• MySQL JDBC Connector/J Tutorial:
http://www.mysql.com/documentation/connector-j/
• Using Microsoft Access with JDBC:
http://www.javaworld.com/javaworld/javaqa/2000-09/03-qa-0922access.html
Advanced Java Programming – Eran Toch
Methodologies in Information System Development
25