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JDBC – JJDBC – Java ava DDataatabbase ase CConnectivityonnectivity

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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

• An application can work with several databases by using all

corresponding drivers

• Ideal: can change database engines without changing any

application code (not always in practice)

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Seven StepsSeven 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

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Registering the DriverRegistering the Driver

• To use a specific driver, we need to instantiate it

and register it within the driver manager:

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

DriverManager.registerDriver(driver);

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A Modular AlternativeA Modular Alternative

• We can register the driver indirectly using the statement

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

• Class.forName loads the specified class

• When OracleDriver is loaded, it automatically

- creates an instance of itself

- registers this instance with the DriverManager

• Hence, the driver class can be given as an argument of

the application

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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 DatabaseConnecting 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

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Connecting to the DatabaseConnecting to the Database

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 CS

In 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

Your login Also, your login

The machine on which our

Oracle runs

The standard port of Oracle

Interaction with the DatabaseInteraction with the Database

• We use Statement objects 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

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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 StatementChanging DB with Statement

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 StatementsAbout Prepared Statements

• 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, Prepared Statements can be though of as statements that contain 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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Updating with Updating 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. PreparedStatements: Be Statements vs. 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. PreparedStatements: Be Statements vs. PreparedStatements: Be Careful!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);

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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

• 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 maintains a cursor pointing to its current row

- 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

- indices start at 1 and not 0!

• Type getType(String columnName)

- same, but uses name of field

- less efficient

• For example: getString(columnIndex), getInt(columnName), getTime,

getBoolean, getType,...

• int findColumn(String columnName)

- looks up column index given column name

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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"));

}

Mapping Java Types to SQL TypesMapping Java Types to SQL Types

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

Null ValuesNull Values

• 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!

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Null ValuesNull Values

• 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).

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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

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,

Prepared Statements and Result Sets

con.close();

stmt.close();

pstmt.close();

rs.close()

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Dealing With ExceptionsDealing With 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();

}

}

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Transaction ManagementTransaction Management

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Transactions and JDBCTransactions and JDBC

• Transaction: more than one statement that must all

succeed (or all fail) together

- e.g., updating several tables due to customer 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

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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()

• Note: DDL statements (e.g., creating/deleting tables) 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 Example

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 (SQLException 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: one transaction reads data written by another

uncommitted transaction

- Unrepeatable Reads: two different results are seen when reading

the same row twice in the same transaction

- Phantom Reads: rows are added to (or deleted from) 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?

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Large ObjectsLarge Objects

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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

• Oracle does not support these objects as in the specification,

so a special treatment is required

• We will see how BLOBs are managed

- Handling CLOBs is similar

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Storing BLOBsStoring BLOBs

• Suppose that we have a binary source (e.g., a

file, a socket, etc.) that is readable through a Java

InputStream object istream

• Suppose that we want to store the source content

in a table MyBlobs(name varchar, content BLOB)

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Storing BLOBs (cont)Storing BLOBs (cont)

• First, we set AutoCommit to false:con.setAutoCommit(false);

• Next, we insert a row with an empty BLOB:Statement stmt = con.createStatement();

stmt.executeUpdate("insert into myblobs values('b1',empty_blob()")

• Now, retrieve the BLOB: ResultSet rs =

stmt.executeQuery("select content from myblobs where name = 'b1'");

rs.next(); BLOB bl = (BLOB) (rs.getBlob(1));

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Storing BLOBs (cont)Storing BLOBs (cont)

• We can now get the BLOB's output streamOutputStream blStream = bl.getBinaryOutputStream();

• Next, we write the content into the stream: int bytesRead = 0; byte[] data = new byte[4096];

while ((bytesRead = fileStream.read(data)) >= 0)

blStream.write(data,0,bytesRead);

• Finally, we close the resources and commitrs.close(); stmt.close(); blStream.close();

con.commit();

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Retrieving BLOBsRetrieving BLOBs

• BLOB retrieval is simpler that storage

• Suppose that we want to write our BLOB to ostream

• First, we get the BLOB:Statement stmt = con.createStatement();

ResultSet rs =

stmt.executeQuery("select content from binaryFiles where name ='b1'");

rs.next(); BLOB bl = (BLOB) (rs.getBlob(1));

• Next, get the input stream of the BLOB:InputStream blStream = bl.getBinaryStream();

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Retrieving BLOBs (cont)Retrieving BLOBs (cont)

• Now, we read the BLOB content through the stream:int bytesRead = 0; byte[] data = new byte[4096];

while ((bytesRead = blStream.read(data)) >= 0)

ostream.write(data, 0, bytesRead);

• Finally, we close the resources and commitrs.close(); stmt.close(); blStream.close();

con.commit();