Java OOP Programming language (Part 8) - Java Database JDBC

Java Programming – JDBC
Oum Saokosal
Master’s Degree in information systems,Jeonju
University,South Korea
012 252 752 / 070 252 752
oumsaokosal@gmail.com

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3
Introduction to JDBC
• JDBC is used for accessing databases from Java
applications
• Information is transferred from relations to
objects and vice-versa
• databases optimized for searching/indexing
• objectsoptimized for engineering/flexibility

4
JDBC Architecture
Java
Application JDBC
Oracle
DB2
MySQL
Oracle
Driver
DB2
Driver
MySQL
Driver
Network

JDBC Architecture (cont.)
Application JDBC Driver
• Java code calls JDBC library
• JDBC loads a driver
• Driver talks to a particular database
• An application can work with severaldatabases by
using all correspondingdrivers
• Ideal: can change databaseengines without
changing any application code (not always in
practice)

6
JDBC Driver for MySQL (Connector/J)
• DownloadConnector/J using binary
distribution from :
http://dev.mysql.com/downloads/connector/j/
• To install simply unzip (or untar) and put
mysql-connector-java-[version]-bin.jar

7
Seven Steps
• Load the driver
• Define the connection URL
• Establish the connection
• Create a Statement object
• Execute a query using the Statement
• Process the result
• Close the connection

8
Loading the Driver
• We can register the driver indirectly using the
statement
Class.forName("com.mysql.jdbc.Driver");
• Class.forName loads the specified class
• When mysqlDriver is loaded, it automatically
• creates an instanceof itself
• registers this instancewith the DriverManager
• Hence, the driver class can be given as an
argument of the application

9
An Example
// A driver for imaginary1
Class.forName("ORG.img.imgSQL1.imaginary1Driver");
// A driver for imaginary2
Driver driver = new ORG.img.imgSQL2.imaginary2Driver();
DriverManager.registerDriver(driver);
//A driver for MySQL
Class.forName("com.mysql.jdbc.Driver");
imaginary1 imaginary2
Registered Drivers
MySQL

10
Connecting to the Database
•Every database is identified by a URL
•Given a URL, DriverManager looks for the
driver that can talk to the corresponding
database
• DriverManager tries all registered drivers,
until a suitable one is found

11
Connecting to the Database
Connection con = DriverManager.
getConnection("jdbc:imaginaryDB1");
imaginary1 imaginary2
Registered Drivers
Oracle
a r r
acceptsURL("jdbc:imaginaryDB1")?

Interaction with the Database
•We use Statementobjects in order to
• Query the database
• Update the database
•Three different interfaces are used:
Statement, PreparedStatement, CallableStatement
• All are interfaces, hence cannot be instantiated
• They are created by the Connection

13
Querying with Statement
• The executeQuery method returns a ResultSet object
representing the query result.
•Will be discussed later…
String queryStr =
"SELECT * FROM employee " +
"WHERE lname = ‘Wong'";
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(queryStr);

14
Changing DB with Statement
String deleteStr =
"DELETE FROM employee " +
"WHERE lname = ‘Wong'";
int delnum = stmt.executeUpdate(deleteStr);
• executeUpdate is used for data manipulation: insert, delete,
update, create table, etc. (anything other than querying!)
• executeUpdate returns the number of rows modified

15
About Prepared Statements
• PreparedStatements are used for queries that are
executed manytimes
• They are parsed (compiled) by the DBMS only
once
• Column values can be set after compilation
• Instead of values, use ‘?’
• Hence, Prepared Statementscan be though of as
statementsthat contain placeholdersto be
substituted later with actual values

16
Querying with PreparedStatement
String queryStr =
"SELECT * FROM employee " +
"WHERE superssn= ? and salary > ?";
PreparedStatement pstmt =
con.prepareStatement(queryStr);
pstmt.setString(1, "333445555");
pstmt.setInt(2, 26000);
ResultSet rs = pstmt.executeQuery();

17
Updating with PreparedStatement
String deleteStr =
“DELETE FROM employee " +
"WHERE superssn = ? and salary > ?";
PreparedStatement pstmt = con.prepareStatement(deleteStr);
pstmt.setString(1, "333445555");
pstmt.setDouble(2, 26000);
int delnum = pstmt.executeUpdate();

18
Statements vs. PreparedStatements:
Be Careful!
•Are these the same?What do they do?
String val = "abc";
PreparedStatement pstmt = con.prepareStatement("select * from R where A=?");
pstmt.setString(1, val);
ResultSet rs = pstmt.executeQuery();
String val = "abc";
Statement stmt = con.createStatement( );
ResultSet rs =
stmt.executeQuery("select * from R where A=" + val);

19
Statements vs. PreparedStatements:
Be Careful!
• Will this work?
• No!!! A ‘?’ can only be used to represent a
column value
PreparedStatement pstmt = con.prepareStatement("select * from ?");
pstmt.setString(1, myFavoriteTableString);

20
Timeout
•Use setQueryTimeOut(int seconds) of
Statement to set a timeout for the driver
to wait for a statement to be completed
•If the operation is not completed in the
given time, an SQLException is thrown
•What is it good for?

ResultSet
• ResultSet objects provide access to the tables
generated as results of executing a Statement queries
• Only one ResultSet per Statement can be open at the
same time!
• The table rows are retrieved in sequence
• A ResultSet maintainsa cursor pointingto its current row
• The next() method moves the cursor to the next row

ResultSet Methods
• boolean next()
• activatesthe next row
• the first call to next() activatesthefirst row
• returns false if there are no more rows
• void close()
• disposesof the ResultSet
• allows you tore-use the Statementthat createdit
• automatically called by most Statement methods

ResultSet Methods
• Type getType(int columnIndex)
• returns the given field asthe given type
• indicesstart at 1 and not 0!
• Type getType(String columnName)
• same, but uses name offield
• less efficient
• For example: getString(columnIndex), getInt(columnName),
getTime, getBoolean, getType,...
• int findColumn(String columnName)
• looksup column indexgiven column name

24
ResultSet Methods
•JDBC 2.0 includes scrollable result sets.
Additional methods included are : ‘first’,
‘last’, ‘previous’, and other methods.

25
ResultSet Example
ResultSet rs = stmt.
executeQuery("select lname,salary from Employees");
// Print the result
while(rs.next()) {
System.out.print(rs.getString(1) + ":");
System.out.println(rs.getDouble(“salary"));
}

Mapping JavaTypes to SQLTypes
SQL type Java Type
CHAR, VARCHAR, LONGVARCHAR String
NUMERIC, DECIMAL java.math.BigDecimal
BIT boolean
TINYINT byte
SMALLINT short
INTEGER int
BIGINT long
REAL float
FLOAT, DOUBLE double
BINARY, VARBINARY, LONGVARBINARY byte[]
DATE java.sql.Date
TIME java.sql.Time
TIMESTAMP java.sql.Timestamp

NullValues
•In SQL, NULL means the field is empty
•Not the same as 0 or ""
•In JDBC, you must explicitly ask if the
last-read field was null
• ResultSet.wasNull(column)
•For example, getInt(column) will return 0
if the value is either 0 or NULL!

28
NullValues
•When inserting null values into
placeholders of Prepared Statements:
• Use the method setNull(index, Types.sqlType)
for primitive types (e.g. INTEGER, REAL);
• You may also use the setType(index, null) for
object types (e.g. STRING, DATE).

29
ResultSet Meta-Data
ResultSetMetaData rsmd = rs.getMetaData();
int numcols = rsmd.getColumnCount();
for (int i = 1 ; i <= numcols; i++) {
System.out.print(rsmd.getColumnLabel(i)+" ");
}
A ResultSetMetaData is an object that can be used to get
information about the properties of the columns in a
ResultSet object
An example: write the columns of the result set

DatabaseTime
• Times in SQL are notoriously non-standard
• Java defines three classesto help
• java.sql.Date
• year, month,day
• java.sql.Time
• hours, minutes, seconds
• java.sql.Timestamp
• year, month,day,hours, minutes, seconds,nanoseconds
• usually use this one

31
Cleaning UpAfterYourself
• Remember to close the Connections,
Statements, PreparedStatements and Result
Sets
con.close();
stmt.close();
pstmt.close();
rs.close()

32
DealingWith Exceptions
• An SQLException is actually a list of exceptions
catch (SQLException e) {
while (e != null) {
System.out.println(e.getSQLState());
System.out.println(e.getMessage());
System.out.println(e.getErrorCode());
e = e.getNextException();
}
}

33
Transactions and JDBC
• Transaction: more than one statementthat must
all succeed (or all fail) together
• e.g.,updating several tablesdue tocustomer purchase
• If one fails, the system must reverse all previous
actions
• Also can’t leave DB in inconsistent state halfway
through a transaction
• COMMIT = complete transaction
• ROLLBACK = cancel all actions

34
Example
• Suppose we want to transfer money from bank
account 13 to account 72:
PreparedStatement pstmt = con.prepareStatement("update BankAccount
set amount = amount + ?
where accountId = ?");
pstmt.setInt(1,-100);
pstmt.executeUpdate();
pstmt.executeUpdate(); What happens if this
update fails?

35
Transaction Management
• Transactions are not explicitly opened and closed
• The connection has a state called AutoCommit
mode
• if AutoCommit is true, then every statementis
automatically committed
• if AutoCommit is false, then every statement is
added to an ongoing transaction
• Default: true

36
AutoCommit
• If you setAutoCommit to false, you must explicitlycommitor
rollbackthe transactionusing Connection.commit() and
Connection.rollback()
• Note: DDL statements (e.g., creating/deletingtables)in a
transactionmay be ignored or may cause a commit to occur
• The behavior is DBMS dependent
setAutoCommit(boolean val)

37
Scrollable ResultSet
• Statement createStatement(
int resultSetType,
int resultSetConcurrency)
•resultSetType:
• ResultSet.TYPE_FORWARD_ONLY
• -default; same as in JDBC 1.0
• -allows only forward movement of the cursor
• -when rset.next() returnsfalse, the data is no
longer available and the result set is closed.

• ResultSet.TYPE_SCROLL_INSENSITIVE
• -backwards, forwards, random cursormovement.
• -changes made in the database are not seen in the
result set object in Java memory.
• ResultSetTYPE_SCROLL_SENSITIVE
• -backwards, forwards, random cursormovement.
• -changes made in the database are seen in the
• result set object in Java memory.
Scrollable ResultSet (cont’d)

39
• resultSetConcurrency:
• ResultSet.CONCUR_READ_ONLY
• This isthe default (and same as in JDBC 1.0) and allowsonly data to be
read from the database.
• ResultSet.CONCUR_UPDATABLE
• This option allowsfor the Java program to make changesto the database
based on newmethodsand positioning ability ofthe cursor.
• Example:
• Statement stmt = conn.createStatement(
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
• ResultSetrset= stmt.executeQuery( “SHOW TABLES”);

40
public boolean absolute(int row) throws SQLException
• -If the given row number is positive, this method moves the
cursor to the given row number (with the first row numbered 1).
• -If the row number is negative, the cursor moves to a relative
positionfrom the last row.
• -If the row number is 0, anSQLException will be raised.
public boolean relative(int row) throws SQLException
• This method callmoves the cursor a relative number of
rows, either positive or negative.

• An attempt to move beyond the last row (or before
the first row) in the result set positions the cursor
after the last row (or before the first row).
public boolean first() throws SQLException
public boolean last() throws SQLException
public boolean previous() throws SQLException
public boolean next() throws SQLException

42
public void beforeFirst() throws SQLException
public void afterLast() throws SQLException
public boolean isFirst() throws SQLException
public boolean isLast() throws SQLException
public boolean isAfterLast() throws SQLException
public boolean isBeforeFirst() throws SQLException
public int getRow() throws SQLException
• getRow() methodretrieves the current row number:
The first row is number 1, the secondnumber 2, andso on.

Java OOP Programming language (Part 8) - Java Database JDBC

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