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• JDBC is used for accessing databases from Java applications
• Information is transferred from relations to objects and vice-versa
– databases optimized for searching/indexing
– objects optimized for engineering/flexibility
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• When executing an SQL statement via
JDBC, it is not checked for errors until it is run (Not checked during compilation)
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• There are queries that can not be computed in SQL:
– Given a table Bus(Source, Destination) find all pairs of places that it is possible to travel between (paths of any length)
• Java allows for a convenient user interface to the database
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• Load the driver
• Establish the Connection
• Create a Statement object
• Execute a query
• Process the result
• Close the connection
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• In order to connect to the Oracle database from java, import the following packages:
– java.sql.*; (usually enough)
– javax.sql.* (for advanced features, such as scrollable result sets)
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Driver
Manager
Application Driver
• The application creates a driver instance and registers it with the DriverManager.
• The DriverManager connects a driver to the DB
• The DriverManager keeps track of registered driver instances and their connections to DB’s.
• The Driver “talks” to a particular database through the connection

Driver
Manager
Application
Driver
1. Initializing a driver: new oracle.jdbc.driver.OracleDriver()
2. Registering it with the DriverManager:
DriverManager.registerDriver(…)
3. Getting a connection:
DriverManager.getConnection(URL)
Note:
• Stages 1+2 may be combined with
Class.forName(“oracle.jdbc.driver.OracleDriver");
• In Stage 3, the Manager tries all drivers and assigns a connection to the first driver that succeeds.
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String path = "jdbc:oracle:thin:";
String host = "sol4";
String port = "1521";
String db = "stud";
String login = “gidi";
String password = “password”;
String url = path + login + "/" + password +
"@" + host +":" + port + ":" + db;
Class.forName("oracle.jdbc.driver.OracleDriver");
Connection con = DriverManager.getConnection(url);
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1. Statement createStatement()
– returns a new Statement object
“prepared” because it already includes the query string
2. PreparedStatement prepareStatement(String sql)
– returns a new PreparedStatement object
• Both are used to send SQL commands to the DB
• Both are created by the connection

• Statements are used for queries that are only issued once.
• A statement can activate:
1.
executeQuery(String query) : for queries that don’t change the DB (select)
2.
executeUpdate(String query) : for queries which affect the DB (create, delete, drop, etc.)
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String queryStr =
"SELECT * FROM Sailors " +
"WHERE Name = 'joe smith'";
No semicolon(;) !!!
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(queryStr);
• The executeQuery method returns a ResultSet object representing the query result.
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String deleteStr =
“DELETE FROM Sailors " +
"WHERE sid = 15";
Statement stmt = con.createStatement(); int delnum = stmt.executeUpdate(deleteStr);
No semicolon(;) !!!
• executeUpdate returns the number of rows modified.
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• Suppose we would like to run the query
SELECT * FROM Emp where name=‘moshe’;
• But we would like to run this for all employees
(separately), not only moshe…
• Could we create a variable instead of ‘moshe’ which would get a different name every time??..
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• Prepared Statements are used for queries that are executed many times with possibly different contents.
• A PreparedStatement object includes the query and is prepared for execution.
• Question marks can be inserted as variables.
-setString(i, value)
-setInt(i, value)
The i-th question mark is set to the given value.
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String queryStr =
"SELECT * FROM Sailors " +
"WHERE Name = ? and Rating < ?”;
PreparedStatement pstmt = con.prepareStatement(queryStr); pstmt.setString(1, “Joe”); pstmt.setInt(2, 8);
Value to insert
1 st question mark
ResultSet rs = pstmt.
executeQuery ();
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String deleteStr =
“DELETE FROM Boats " +
"WHERE Name = ? and Color = ?”;
PreparedStatement pstmt = con.prepareStatement(deleteStr); pstmt.setString(1, “Fluffy”); pstmt.setString(2, "red"); int delnum = pstmt.
executeUpdate ();
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• Are these the same? What do they do?
String val = “Joe”;
PreparedStatement pstmt = con.prepareStatement(“select * from Sailors where sname=?”); pstmt.setString(1, val);
ResultSet rs = pstmt.executeQuery();
String val = “Joe”;
Statement stmt = con.createStatement( );
ResultSet rs = stmt.executeQuery(“select * from
Sailors where sname=” + val);
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• Will this always work?
Statement stmt = con.createStatement( );
ResultSet rs = stmt.executeQuery(“select * from R where A =‘ ” + val + “ ’ ”);
• The moral: When getting input from the user, always use a PreparedStatement
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• Will this work?
PreparedStatement pstmt = con.prepareStatement(“select * from ?”); pstmt.setString(1, "Sailors");
No! will attempt to insert ‘sailors’, and result in:
Select * from ‘sailors’
We can put ? only instead of values
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Statement
ExecuteQuery(String Q)
ResultSet createStatement()
Statement
Connection
ExecuteUpdate(String Q) int
Prepared
Statement setString(int num,String val)
ExecuteQuery() prepareStatement
(String Q)
Prepared
Statement
Connection
ExecuteUpdate()
ResultSet int
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• A ResultSet is an object which contains the result of a query- a “table”.
• Only one ResultSet per Statement can be open at once(!).
• A ResultSet maintains a cursor pointing to its current row of data.
• The ' next ' method moves the cursor to the next row
• As of JDBC 2.0, scrollable ResultSets are available, which also include ‘ previous ’, ’ first ’,
‘ last ’, etc..

• Any methods on the resultSet will occur on the current row.
• The cursor is positioned before the first row upon creation.
Statement stmt=con.createStatement();
ResultSet rs = stmt.executeQuery
("SELECT * FROM Table1"); while (rs.next()) {
//something…
}

• Getting the value in some column (for the current row):
– getString(int columnNum);
String s = rs.getString(“column1");
– getString(String columnName);
– getInt(int columnNum);
– getInt(String columnName);
– Etc…
• To check if null was returned, you have to use wasNull() on the ResultSet after getting the value.
• The driver maps SQL types (varchar, number,…) to the appropriate Java method (getString, getInt…)
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SQL type
Java method
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Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM
Table1");
// retrieve and print the values for the current row while (rs.next()) { int i = rs.getInt("a");
String s = rs.getString("b"); float f = rs.getFloat("c");
System.out.println("ROW = " + i + " " + s + " " + f);
}
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1. TYPE_FORWARD_ONLY: The default, simple RS.
2. TYPE_SCROLL_INSENSITIVE : Scrollable RS, does not modify when the DB is modified
3. TYPE_SCROLL_SENSITIVE : Scrollable RS, if its values are changed in the DB, they will also change in the RS.
In addition, a ResultSet may be either:
1. CONCUR_READ_ONLY: the programmer cannot change the ResultSet (default)
2. CONCUR_UPDATABLE: the programmer can change the ResultSet
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For Statement:
Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE);
ResultSet rs = stmt.executeQuery( "SELECT * from emp");
For PreparedStatement:
PreparedStatement pstmt= con.prepareStatement("SELECT * FROM Emp where Eid=?” ,
ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE); pstmt.setInt(1, "1000010");
ResultSet rs = stmt.executeQuery();
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An object created by the ResultSet which holds information about its columns
ResultSetMetaData rsmd = rs.getMetaData(); int numcols = rsmd.getColumnCount();
} for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(",");
System.out.print(rsmd.getColumnLabel(i));
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} while (rs.next()) { for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(",");
System.out.print(rs.getString(i));
}
System.out.println("");
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• Remember to close the Connections,
Statements, PreparedStatements and
ResultSets
Highly recommended con.close(); stmt.close(); pstmt.close(); rs.close();
Recommended
Optional (Will otherwise be closed by its calling statement)
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• An exception may be chained: catch (SQLException e) { while (e != null) {
//human readable message about the exception
System.out.println(e.getMessage());
//String describing the reason of the exception
System.out.println(e.getSQLState());
} }
//driver-dependant code for the exception
System.out.println(e.getErrorCode()); e = e.getNextException();
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String table1=“create table table1(col1 integer, col2 integer)”;
Statement st=con.createStatement(); int resCreate=st.executeUpdate(table1);
PreparedStatement ps1 = con.prepareStatement(“insert into Table1 values(?,?)”); for(int i=0;i<10;i++){ ps1.setInt(1,i); ps1.setInt(2,i*i); ps1.executeUpdate();
}
Statement st2=con.createStatement();
ResultSet rs=executeQuery(“select col2 from Table1”); while(rs.next()){
System.out.println(rs.getInt(1)); }
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• You can use Stmnt.setQueryTimeOut(int secs) 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
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• Transaction = 2 or more statements which must all succeed (or all fail) together
• 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 = abort
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• 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.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate();
What happens if this update fails?
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• The connection has a state called AutoCommit mode
• if AutoCommit is true, then every statement is automatically committed
• if AutoCommit is false, then every statement is added to an ongoing transaction
• Default: true
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con.setAutoCommit(boolean val)
• If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and
Connection.rollback()
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con.setAutoCommit(false); try {
PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ?
where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate();
} pstmt.setInt(1, 100); pstmt.setInt(2, 72); pstmt.executeUpdate(); con.commit(); catch (Exception e) { con.rollback();
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• Useful link regarding JDBC: http://java.sun.com/j2se/1.3/docs/guide/jdbc/ getstart/GettingStartedTOC.fm.html
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