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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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JDBC ArchitectureJDBC Architecture

Java Application JDBC

Oracle

DB2

Postgres

Oracle

Driver

DB2

Driver

Postgres

Driver

These are

Java classes

Network

We will

use this one…

JDBC Architecture (cont.)JDBC Architecture (cont.)

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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Seven StepsSeven Steps

• Load the driver

• Define the connection URL

• Establish the connection

• Create a Statement object stmt

• Execute a query using stmt

• Process the result

• Close the connection

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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 causes the Driver class

to be loaded

• When this class is loaded, it automatically

– creates an instance of itself

– registers this instance 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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An ExampleAn 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 oracle

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

imaginary1imaginary2

Registered Drivers

Oracle

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Connecting to the Connecting to the DatabaseDatabase

• Every database is identified by a URL

• Given a URL, DriverManager is asked

to find the driver that can talk to the

corresponding database

• DriverManager tries all registered

drivers, until a suitable one is found

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Connecting to the Connecting to the DatabaseDatabase

Connection con = DriverManager.

getConnection("jdbc.imaginaryDB1");

imaginary1imaginary2

Registered Drivers

Oracle

acceptsURL(“jdbc.imaginaryDB1”)?

Read more in DriverManager API

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The URLs in CSThe URLs in CSIn CS, a URL has the following structure:

jdbc:oracle:thin:name/password@sol4:1521:stud

For example:

jdbc:oracle:thin:snoopy/snoopy@sol4:1521:stud

A complete example

Your login Also, your login

The machine on which our DBMS runs

The standard port given to Oracle

on sol4

Interaction with the Interaction with the DatabaseDatabase

• We use Statement objects in order to

– Extract data from the database

– Update the database

• Three different interfaces are used:Statement, PreparedStatement, CallableStatement

• All are interfaces, thus cannot be instantiated

• They are created by the Connection

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Querying with StatementQuerying with Statement

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

•Will be discussed later…

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

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

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Changing DB with Changing DB with StatementStatement

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

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 (compiled) by the DBMS only once

• Column values can be set after compilation

• Instead of values, use ‘?’

• Hence, a Prepared Statement is statement that contains placeholders to be substituted later with actual values

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Querying with Querying with PreparedStatementPreparedStatement

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

PreparedStatement pstmt = con.prepareStatement(queryStr);

pstmt.setString(1, “t-shirt”);pstmt.setInt(2, 1000);

ResultSet rs = pstmt.executeQuery();

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Changing DB with Changing DB with PreparedStatementPreparedStatement

String deleteStr = “DELETE FROM Items " +"WHERE Name = ? and Cost > ?”;

PreparedStatement pstmt = con.prepareStatement(deleteStr);

pstmt.setString(1, “t-shirt”);pstmt.setInt(2, 1000);

int delnum = pstmt.executeUpdate();

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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 work?

• No!!! A ‘?’ can only be used to represent a column value

• WHY?

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

pstmt.setString(1, myFavoriteTableString);

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TimeoutTimeout

• 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?

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)

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ResultSet ExampleResultSet Example

Statement stmt = con.createStatement();

ResultSet rs = stmt.

executeQuery("select name,age from Employees");    

// Print the result

while(rs.next()) { System.out.print(rs.getString(1) + ”:“); System.out.println(rs.getShort(“age”)+”“);

}

Null ValuesNull Values

• 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)

• For example, getInt(column) will return

0 if the value is either 0 or null!!

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Null ValuesNull Values

• When inserting null values into

placeholders of Prepared Statements:

– Use the method setNull(index, sqlType) for

primitive types (e.g. INTEGER, REAL);

– You may also use the setXXX(index, null)

for object types (e.g. STRING, DATE).

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ResultSet Meta-Data 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

Many more methods in the ResultSetMetaData API

Mapping Java Types to SQL Mapping Java Types to SQL TypesTypes

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

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More InformationMore Information

A detailed overview of type mapping and type conversion can be found at

http://java.sun.com/j2se/1.3/docs/guide/jdbc/getstart/mapping.html

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

• An 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 BLOB; CLOBs are

similar

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Retrieving a BLOBRetrieving a BLOBcreate table userImages(

user varchar(50),image BLOB

);

ResultSet rs = stmt.executeQuery(“select image from

userImages”);while (rs.next) {

Blob b = rs.getBlob(“image”);InputStream stream =

b.getBinaryStream();doSomething(stream);

}

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Inserting a BLOBInserting a BLOB

PreparedStatement pstmt = con.prepareStatement(“insert into

userImages values(‘snoopy’, ?)”);

File file = new File(“snoopy.jpg”);InputStream fin = new FileInputStream(file);

pstmt.setBinaryStream (1, fin, file.length());pstmt.executeUpdate();

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Transactions and JDBCTransactions and JDBC

• 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

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?

• Possible problems:

– Dirty Reads: a transaction may read uncommitted

data

– Unrepeatable Reads: two different results are seen

when reading a tuple twice in the same transaction

– Phantom Reads: tuples are added to a table between

two readings of this table in a single transaction

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Isolation LevelsIsolation Levels

JDBC defines four isolation modes:

Level Dirty

Read

Unrepeatable

Read

Phantom

Read

Read Uncommited Yes Yes Yes

Read Commited No Yes Yes

Repeatable Read No No Yes

Serializable No No No

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Isolation LevelsIsolation Levels

• Set the transaction mode using

setTransactionIsolation() of class Connection

• Oracle only implements:

– TRANSACTION_SERIALIZABLE

• An exception may be thrown if serializability isn’t

possible

– TRANSACTION_READ_COMMITED

• This is the default

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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 A

select * from A

Table: A

1

2

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?