Advanced Java Programming – Eran Toch Methodologies in Information System Development Tutorial: Advanced Java Programming and Database connection Eran Toch Methodologies in the Development of Information Systems November 2003

2 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

3 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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

4 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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 ExceptionWith Exception

5 Advanced Java Programming – Eran Toch Methodologies in Information System Development The try 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));

6 Advanced Java Programming – Eran Toch Methodologies in Information System Development The catch 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()); }

7 Advanced Java Programming – Eran Toch Methodologies in Information System Development Catching 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

8 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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()); }

9 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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: Used frequently for cleanup processes. finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } else { System.out.println("PrintWriter not open"); }

10 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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"); }

11 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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; }

12 Advanced Java Programming – Eran Toch Methodologies in Information System Development Exceptions and JavaDoc Exception can be documented by Javadoc using statement /** * regular javadoc text… the Driver was not found. the the DriverManager.getConnection * method returned an error. */ public void createConnection()throws SQLException, Exception{

13 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

14 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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.reference

15 Advanced Java Programming – Eran Toch Methodologies in Information System Development Database Install Download the MySQL database from: Install it Create a specific database: create database mytest; Create a user account: grant all on mytest.* to eran identified by ‘1234’

16 Advanced Java Programming – Eran Toch Methodologies in Information System Development JDBC Install Download Connector/J from: 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%

17 Advanced Java Programming – Eran Toch Methodologies in Information System Development Accessing Database 1.Load the driver 2.Creating a connection object 3.Create a statement object 4.Execute an SQL query and get results using the ResultSet object

18 Advanced Java Programming – Eran Toch Methodologies in Information System Development Example – Database Management mysql> create database mytest; Query OK, 1 row affected (0.05 sec) mysql> grant all on *.* to 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', ' '); 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

19 Advanced Java Programming – Eran Toch Methodologies in Information System Development Example – Connection import 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

20 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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

21 Advanced Java Programming – Eran Toch Methodologies in Information System Development Example – 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

22 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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; } 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?)

23 Advanced Java Programming – Eran Toch Methodologies in Information System Development 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:

24 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

25 Advanced Java Programming – Eran Toch Methodologies in Information System Development References Exception handling in the Java tutorial: JDBC Tutorial: MySQL Tutorial: MySQL JDBC Connector/J Tutorial: Using Microsoft Access with JDBC: access.html access.html