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JDBC – Java Database JDBC – Java Database ConnectivityConnectivity

Representation and Management of Data on the

Internet

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Introduction to JDBCIntroduction 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– objects optimized for engineering/flexibility

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Overview

TCP/IP

java.net

RMI JDBC CORBA

Network OS

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Working With OracleWorking With Oracle

Add to your .cshrc the following:if ($HOST == sol4) then

setenv ORACLE_HOME /opt/oracleelse

setenv ORACLE_HOME /usr/local/oracle8iendif

setenv PATH $ORACLE_HOME/bin:$PATHsetenv ORACLE_SID stud

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Using OracleUsing Oracle

• If a student whose login is Snoopy wants to work directly with Oracle:

sqlplus snoopy/snoopy@stud.cs

• Note: we use the login for a password! (Don’t change your password)

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Seven StepsSeven Steps

• Load the driver• Define the Connection URL• Establish the Connection• Create a Statement object• Execute a query• Process the result• Close the connection

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Packages to ImportPackages to Import

• 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)

JDBC ArchitectureJDBC Architecture

Application JDBC Driver

• Java code calls JDBC library• JDBC loads a driver • Driver talks to a particular database• Can have more than one driver -> more than one

database• Ideal: can change database engines without

changing any application code

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Loading the DriverLoading the Driver

• We can register the Driver indirectly using the Java statement:

Class.forName(“oracle.jdbc.driver.OracleDriver");

• Calling Class.forName, automatically – creates an instance of the driver– registers the driver with the DriverManager

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Another OptionAnother Option

• Another option is to create an instance of the driver and register it with the Driver Manager:

Driver driver = new oracle.jdbc.OracleDriver();

DriverManager.registerDriver(driver);

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The DriverManagerThe DriverManager

• The DriverManager tries all the drivers• Uses the first one that works• When a driver class is first loaded, it

registers itself with the DriverManager• Therefore, to load a driver, just register

it!

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Connecting to the Connecting to the DatabaseDatabase

String path = "jdbc:oracle:thin:";String host = "sol4";String port = "1521";String db = "stud";String login = "snoopy";String url = path + login + "/" + login +

"@" + host +":" + port + ":" + db;

Class.forName("oracle.jdbc.driver.OracleDriver");

Connection con = DriverManager.getConnection(conStr);

This is actually the password

Connection MethodsConnection Methods

Statement createStatement()– returns a new Statement object

PreparedStatement prepareStatement(String sql)– returns a new PreparedStatement object

CallableStatement prepareCall(String sql)– returns a new CallableStatement object

• Why all these different kinds of statements? Optimization.

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Querying with StatementQuerying with StatementString queryStr =

"SELECT * FROM Member " +"WHERE Lower(Name) = 'harry potter'";

Statement stmt = con.createStatement();ResultSet rs = stmt.executeQuery(queryStr);

• Statements are used for queries that are only issued once.

• The executeQuery method returns a ResultSet object representing the query result.

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Changing DB with Changing DB with StatementStatement

String deleteStr = “DELETE FROM Member " +"WHERE Lower(Name) = ‘lord voldemort’";

Statement stmt = con.createStatement();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.

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About Prepared About Prepared StatementsStatements

• Prepared Statements are used for queries that are executed many times.

• They are parsed only once. • Using setString(i, value) (setInt(i,

value), etc.) the i-th question mark is set to the given value.

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Querying with Querying with PreparedStatementPreparedStatement

String queryStr = "SELECT * FROM Program " +"WHERE Name = ? and Cost < ?”;

PreparedStatement pstmt = con.prepareStatement(queryStr);

pstmt.setString(1, “Unfogging the Future”);pstmt.setInt(2, 1000);

ResultSet rs = pstmt.executeQuery();

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Changing DB with Changing DB with PreparedStatementPreparedStatement

String deleteStr = “DELETE FROM Program " +"WHERE Name = ? and Cost < ?”;

PreparedStatement pstmt = con.prepareStatement(deleteStr);

pstmt.setString(1, “Unfogging the Future”);pstmt.setInt(2, 1000);

int delnum = pstmt.executeUpdate();

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TimeoutTimeout

• Use setQueryTimeOut(int seconds) 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?

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Statements vs. Statements vs. PreparedStatements: Be PreparedStatements: Be

Careful!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);

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Statements vs. Statements vs. PreparedStatements: Be PreparedStatements: Be

Careful!Careful!• 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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Statements vs. Statements vs. PreparedStatements: Be PreparedStatements: Be

Careful!Careful!• Will this work?

PreparedStatement pstmt = con.prepareStatement(“select * from ?”);

pstmt.setString(1, myFavoriteTableString);

ResultSetResultSet• A ResultSet provides access to a table of data

generated by executing a Statement. • Only one ResultSet per Statement can be open at

once.• The table rows are retrieved in sequence.• A ResultSet maintains a cursor pointing to its current

row of data. • The 'next' method moves the cursor to the next row.

ResultSet MethodsResultSet Methods• boolean next()

– activates the next row– the first call to next() activates the first row– returns false if there are no more rows

• void close() – disposes of the ResultSet– allows you to re-use the Statement that created it– automatically called by most Statement methods

ResultSet MethodsResultSet Methods

• Type getType(int columnIndex)– returns the given field as the given type– fields indexed starting at 1 (not 0)

• Type getType(String columnName)– same, but uses name of field– less efficient

• int findColumn(String columnName)– looks up column index given column name

ResultSet MethodsResultSet Methods• String getString(int columnIndex) • boolean getBoolean(int columnIndex) • byte getByte(int columnIndex) • short getShort(int columnIndex) • int getInt(int columnIndex) • long getLong(int columnIndex) • float getFloat(int columnIndex) • double getDouble(int columnIndex) • Date getDate(int columnIndex) • Time getTime(int columnIndex) • Timestamp getTimestamp(int columnIndex)

ResultSet MethodsResultSet Methods• String getString(String columnName) • boolean getBoolean(String columnName) • byte getByte(String columnName) • short getShort(String columnName) • int getInt(String columnName) • long getLong(String columnName) • float getFloat(String columnName) • double getDouble(String columnName)• Date getDate(String columnName) • Time getTime(String columnName) • Timestamp getTimestamp(String columnName)

isNullisNull

• In SQL, NULL means the field is empty• Not the same as 0 or “”• In JDBC, you must explicitly ask if a

field is null by calling ResultSet.isNull(column)

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Printing Query Output: Printing Query Output: Result Set (1) Result Set (1)

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));

}

Print Column Headers:

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Printing Query Output: Printing Query Output: Result Set (2) Result Set (2)

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("");

}

• To get the data in the i-th column: rs.getString(i)• To get the data in column Abc: rs.getString(“Abc”)

Mapping Java Types to SQL Mapping Java Types to SQL TypesTypes

SQL type Java Type CHAR, VARCHAR, LONGVARCHAR StringNUMERIC, DECIMAL java.math.BigDecimalBIT booleanTINYINT byteSMALLINT shortINTEGER intBIGINT longREAL floatFLOAT, DOUBLE doubleBINARY, VARBINARY, LONGVARBINARY byte[]DATE java.sql.DateTIME java.sql.TimeTIMESTAMP java.sql.Timestamp

Database TimeDatabase Time• Times in SQL are notoriously non-standard• Java defines three classes to 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

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Cleaning Up After YourselfCleaning Up After Yourself

• Remember to close the Connections, Statements, PreparedStatements and ResultSets

con.close();stmt.close();pstmt.close();rs.close()

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Dealing With ExceptionsDealing With Exceptions

• A exception can have more exceptions in it.catch (SQLException e) { while (e != null) {

System.out.println(e.getSQLState());System.out.println(e.getMessage());System.out.println(e.getErrorCode());e = e.getNextException();

}}

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Advanced TopicsAdvanced Topics

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LOBs: Large OBjectsLOBs: Large OBjects

• Two types:– CLOB: Character large object (a lot of characters)– BLOB: Binary large object (a lot of bytes)

• Actual data is not stored in the table with the CLOB/BLOB column. Only a pointer to the data is stored there

• I will show how to use a CLOB; BLOBs are similar

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Retrieving a CLOBRetrieving a CLOBcreate table userComments(

user varchar(50),comment CLOB)

);ResultSet rs =

stmt.executeQuery(“select comment from userComments”);while (rs.next) {

Clob c = rs.getClob(“comment”);Reader reader =

c.getCharacterStream();doSomething(reader)

}

We can also use getAsciiStream()which returns an

InputStream

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Inserting a CLOBInserting a CLOB

PreparedStatement pstmt = con.prepareStatement(“insert into

userComments values(‘sara’, ?)”);

Reader reader = new FileReader(fileName);pstmt.setCharacterStream(1, reader,

Integer.MAX_VALUE);pstmt.executeUpdate(); We can also

use setAsciiStream

()which gets an InputStream

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TransactionsTransactions

• Transaction = more than one statement 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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ExampleExample• 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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Transaction ManagementTransaction Management

• Transactions are not explicitly opened and closed

• 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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AutoCommitAutoCommit

• If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback()

• In order to work with LOBs, you usually have to set AutoCommit to false, while retrieving the data

• Note: DDL statements in a transaction may be ignored or may cause a commit to occur. The behavior is DBMS dependent

Connection.setAutoCommit(boolean val)

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Fixed ExampleFixed Examplecon.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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Isolation LevelsIsolation Levels

• How do different transactions interact? Do they see what another has written?

• JDBC supports 4 “isolation modes” • Set using:

Connection.setTransactionIsolation• Oracle only implements:

– TRANSACTION_SERIALIZABLE– TRANSACTION_READ_COMMITED

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Isolation Levels (cont.)Isolation Levels (cont.)

• TRANSACTION_SERIALIZABLE: transactions are equivalent to serial transactions

• TRANSACTION_READ_COMMITED: A transaction can only read values that have been committed

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Level: READ_COMMITEDLevel: READ_COMMITED

• Transaction 1:insert into A values(1)insert into A values(2)commit

• Transaction 2:select * from Aselect * from A

Table: A

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Question: Is it possible for a transaction to see 1 in A, but not 2?

Question: Is it possible for the 2 queries to give different answers for level SERIALIZABLE?