advanced java programming – eran toch methodologies in information system development tutorial:...

Advanced Java Programming – Eran TochMethodologies in Information System Development

Tutorial: Tutorial: Advanced Java Programming Advanced Java Programming

and Database connection and Database connection

Eran Toch

Methodologies in the Development of Information Systems

November 2003


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AgendaAgenda

• 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


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Exceptions - IntroductionExceptions - 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


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Exceptions – Advantage 1Exceptions – 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


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The The trytry Block Block

• try block:

• A try statement must be accompanied by at least one catch block or one finally 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));


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The The catchcatch Block Block

• catch block handles the exception:

• Multiple catch blocks can be placed, each handling a different type of exception

try {. . . } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getMessage());} catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage());}


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Catching Multiple ExceptionsCatching Multiple Exceptions

• Java exceptions are Throwable objects

• Catching MyException will catch both the subclasses. Catching Exception will catch all types of Exceptions

Throwable

Exception

MyException

MySpecificException1MySpecificException2


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Catching Multiple Exceptions – cont’dCatching 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());}


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The The finallyfinally Block 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:

• Used frequently for cleanup processes.

finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); } }


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Putting it All TogetherPutting it All Togetherpublic 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"); } }}


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throwthrow Statement 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;}


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Exceptions and JavaDocExceptions 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{


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AgendaAgenda



• References


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Database Connection - OverviewDatabase 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.


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Database InstallDatabase 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’


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JDBC InstallJDBC 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%


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Accessing DatabaseAccessing Database

1. Load the driver

2. Creating a connection object3. Create a statement object4. Execute an SQL query and get results

using the ResultSet object


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Example – Database ManagementExample – Database Management

mysql> create database mytest;

Query OK, 1 row affected (0.05 sec)

mysql> grant all on *.* to eran@localhost identified by '1234';

Query OK, 0 rows affected (0.14 sec)

mysql>create table phones (name varchar(255) not null unique key, phone varchar(25) not null);

mysql>describe phones;+-------+--------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+--------------+------+-----+---------+-------+| name | varchar(255) | | PRI | | || phone | varchar(25) | | | | |+-------+--------------+------+-----+---------+-------+mysql> insert into phones values ('Eran Toch', '+972-4-9831894');Query OK, 1 row affected (0.11 sec)

Creating the DB

Creating user account

Creating the ‘phones’ table

Is everything alright? Let’s see…

Inserting some data


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Example – ConnectionExample – Connectionimport java.sql.*;

public class SQLConnect { Connection conn = null; Statement stmt = null; ResultSet rs = null;

public SQLConnect(){} public void createConnection(){ try{ Class.forName("com.mysql.jdbc.Driver").newInstance(); } catch (Exception E){ System.out.println(E); } try{ conn = DriverManager.getConnection("jdbc:mysql://localhost/mytest?user=test master&password=1234"); } catch (SQLException E){ System.out.println(E); }}

Importing java.sql.* that contains all the classes we need

Connection, Statement and ResultSet are defined as class variables

Dynamically loading the specific JDBC driver. The runtime environment must know where the library is located!

Connecting to the database using the url


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Example – Locating LibrariesExample – Locating Libraries

• If the following error message occurs:java.lang.ClassNotFoundException: com.mysql.jdbc.Driverjava.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


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Example – Access and QueryExample – Access and Query

public String getPhones(){ String output = ""; try { stmt = conn.createStatement(); rs = stmt.executeQuery("SELECT * FROM phones"); if (rs != null){ while (rs.next()){ output += rs.getString("phone") + "\n"; } } } catch (Exception E){ System.out.println(E.getMessage()); }

Creating a statement

Creating a ResultSet, based on a SQL statement

Going through the ResultSet by using rs.next(). Remember – you need to call the next method before you start reading from the ResultSet

Reading a field from the ResultSet


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Example – Cleaning offExample – Cleaning offfinally { if (rs != null) { try { rs.close(); } catch (SQLException sqlEx) {} rs = null; } if (stmt != null) { try { stmt.close(); } catch (SQLException sqlEx) {} stmt = null; } } return output;}

public void closeConnection(){ if (conn != null){ try { conn.close(); } catch (SQLException sqlEx){} conn = null; }}

Cleaning off is best done in the “finally” clause

Cleaning off ResultSet

Cleaning off Statement, after the ResultSet

Cleaning off the connection, in a different method (why?)


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Example – Test ClientExample – 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); }}

Outputphones:+972-4-9831894


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AgendaAgenda



• References


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ReferencesReferences

• 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-0922-access.html

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