Download driver

Java Database Connectivity: JDBC
Representation and Management of Data
on the Internet
Database Connectivity

Relational databases
◦ Manage all information generated by businesses
◦ Centric to business applications
◦ Established and Mature

Application developers require standard
interface to relational databases
◦ Change in database vendor should not require
change in application programs
Relational Database Properties

Fundamental Properties
◦ Tables, Indexes, Triggers, Stored Procedures, Transactions
◦ ACID Properties – Atomicity, Consistency, Isolation, Durability

Data Definition (DDL)
◦ Manage Database Objects
◦ For example, Create and delete tables, indexes, stored procs

Queries (DML)
◦ Read, write, or delete information between one or more related
tables
◦ SQL
ACID Properties

Atomic
◦ Updates in a transaction either occur or don’t occur

Consistent
◦ Transaction always preserves the system state

Isolated
◦ Concurrent transactions occur as if occurring in isolation
(there is no partial view of another transaction)

Durable
◦ Committed transactions survive failures of any type (e.g.,
communication, server). Transaction logging allows
recovery.
SQL Statements (DML)

Insert
INSERT into Course(CourseID, Name, ProfessorID)
values (162, ‘Java101’, 12);

Update
 UPDATE Course set NumStudents = 0;

Delete
 DELETE from Course where (ProfessorID = 100 );

Select
 SELECT Name from Course where CourseID = 162;
Relational DBs and Objects

Persistent objects must be mapped to relation
tables
Relation Tables
Class Diagram
Professor
ProfID
Name
Student
0..*
0..*
teaches
Professor
1
takes
Course
1..*
0..*
prereq
Course
CourseID
ProfID
Name
Location
Time
Student
StudentID
Name
BillingInfo
0..*
Prereq
CourseID
PrereqCourseID
Takes
StudentID
CourseID
What is JDBC?

A pure Java API for database communication similar to
ODBC (JDBC created in 1995)
◦ JDBC and ODBC are based on the same standard: X/Open's SQL
Call-Level Interface

A set of classes that perform database transactions
◦ Connect to relational databases
◦ Send SQL commands
◦ Process results

JDBC2 has been defined (part of Java 2 in 1998)
◦ NOT universally supported
◦ Additional features such as scrollable cursors, batch updates
Benefits of JDBC
No proprietary DB code (although passthrough is supported)
 Don’t have to rely on single vendor...ever
 Don’t need to make DB vendor decision
early
 Easier for DB vendors

◦ Don’t need to provide a query language, only
implement API
◦ Only low-level support
JDBC Architecture
Java Application
JDBC API
JDBC Driver Manager
JDBC Driver API
(T1) JDBCODBC Bridge
ODBC Driver
(T2) Java
Portion
Native
Portion
(T3) Java
Client
Server
Component
Proprietary, vendor-specific
database access protocol
(T4) Java
Driver to
vendor's
protocol
JDBC Drivers
Provided with
Sun's JDK
JDBC Classes

java.sql.* provides classes used by an
application
<<Interface>>
Statement
<<Singleton>>
DriverManager
<<Interface>>
Connection
<<Interface>>
PreparedStatement
<<Interface>>
ResultSet
<<Interface>>
DatabaseMetaData
<<Interface>>
CallableStatement
<<Interface>>
ResultSetMetaData
JDBC Statement Types (2)

Callable Statements
◦ Vendor specific invocation of stored procedures
String sql = “begin ? := Class.registerStudent(?, ?, ?, ?);
CallableStatement stmt = connection.prepareCall(sql);
stmt.registerOutParameter(1,Types.Integer);
stmt.setInt(2, studentID);
stmt.setString(3, classList);
stmt.registerOutParameter(3, OracleTypes.CURSOR);
stmt.registerOutParameter(4, OracleTypes.CURSOR);
stmt.execute();
int numClasses = stmt.getInt(1);
ResultSet classList = stmt.getObject(3);
ResultSet unavailableClassList = stmt.getObject(4);
…
classList.close();
unavailableClassList .close();
stmt.close();
- Close ALL result sets !!!
Result Sets

Contain rows from query results

Greatly enhanced in JDBC 2.0 (Java 2)
◦ ResultSet maintains the current row
◦ Can obtain an item from any column in the current row
◦ Can move forward, not backward
◦ Scrollable
◦ Updatable
the database
◦ Batch Updates
- move forward and backward
- write through the ResultSet to
- submit multiple update statements for
performance
improvement
◦ RowSets
- disconnect then reconnect to
Database
using
optimistic concurrency control
◦ Others Features
- Connection Pooling, JNDI Naming
Statement SQL

Insert
 String insertStmt = "INSERT into Course(CourseID, Name,
ProfessorID) " +"values (162, ‘Java101’, 12)";
 stmt.executeUpdate(insertStmt);

Update
 String depositStmt = "UPDATE Course set NumStudents = 0";
 stmt.executeUpdate(depositStmt);

Delete
 String deleteStmt = "DELETE from Course where (ProfessorID = "
+ terminatedProfessor + ”)";
 stmt.executeUpdate(deleteStmt);
Example - Schema
Assume the tables shown below. Write a program
that displays all the students and the courses they
take.
JDBC Summary

Three Statement Types
◦ Statement – simplest, slowest (for repeated calls), Difficult handling of
quotes
◦ PreparedStatement – Allows DB to cache parsed SQL
◦ CallableStatement – Executes DB stored Procedure

Basic Steps
◦ Load Driver(s) (typically with Class.forName)
◦ Obtain a connection (DriverManager.getConnection())
◦ Get Statement from connection
 connection.createStatement - Statement
 connection.prepareStatement - PreparedStatement
 connection.prepareCall - CallableStatement
◦ stmt.executeQuery , stmt.executeUpdate, or stmt.execute
What is ODBC?
ODBC is (Open Database Connectivity):
 A standard or open application programming interface
(API) for accessing a database.
 SQL Access Group, chiefly Microsoft, in 1992
 By using ODBC statements in a program, you can access
files in a number of different databases, including Access,
dBase, DB2, Excel, and Text.
 It allows programs to use SQL requests that will access
databases without having to know the proprietary
interfaces to the databases.
 ODBC handles the SQL request and converts it into a
request the individual database system understands.

More on ODBC
You need:
◦ the ODBC software, and
◦ a separate module or driver for each database
to be accessed. Library that is dynamically
connected to the application.
 Driver masks the heterogeneity of DBMS
operating system and network protocol.
 E.g. (Sybase, Windows/NT, Novell driver)

ODBC Architecture
Application
ODBC driver
manager
Driver
(DBMS/OS/network)
Data Source
JDBC Architecture
Oracle
Driver
Oracle
Java
Application
JDBC
DB2
Driver
DB2
Network
Postgres
Driver
Postgres
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
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
◦ objects optimized for engineering/flexibility
What is JDBC?

JDBC is: Java Database Connectivity
◦ is a Java API for connecting programs written in Java to the data in
relational databases.
◦ consists of a set of classes and interfaces written in the Java
programming language.
◦ provides a standard API for tool/database developers and makes it
possible to write database applications using a pure Java API.
◦ The standard defined by Sun Microsystems, allowing individual
providers to implement and extend the standard with their own
JDBC drivers.

JDBC:
◦ establishes a connection with a database
◦ sends SQL statements
◦ processes the results.
JDBC vs ODBC
ODBC is used between applications
 JDBC is used by Java programmers to
connect to databases
 With a small "bridge" program, you can
use the JDBC interface to access ODBCaccessible databases.
 JDBC allows SQL-based database access
for EJB persistence and for direct
manipulation from CORBA, DJB or other
server objects

JDBC API
The JDBC API supports both two-tier and threetier models for database access.
 Two-tier model -- a Java applet or application
interacts directly with the database.
 Three-tier model -- introduces a middle-level
server for execution of business logic:

◦ the middle tier to maintain control over data access.
◦ the user can employ an easy-to-use higher-level API
which is translated by the middle tier into the
appropriate low-level calls.
JDBC Architectures
Java Application
JDBC driver manager
JDBC/native
bridge
JDBC/ODBC
bridge
Native driver
(DBMS specific)
ODBC Driver
JDBC Driver
(DBMS Specific)
DBMS
JDBC middleware
(various DBMS)
The JDBC Steps
1. Importing Packages
2. Registering the JDBC Drivers
3. Opening a Connection to a Database
4. Creating a Statement Object
5. Executing a Query and Returning a Result Set
Object
6. Processing the Result Set
7. Closing the Result Set and Statement Objects
8. Closing the Connection
Loading 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
Another 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);
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 oracle
Class.forName("oracle.jdbc.driver.OracleDriver");
imaginary1 imaginary2
Oracle
Registered Drivers
1: Importing Packages
//
// Program name: LecExample_1a.java
// Purpose: Basic selection using
prepared statement
//
//Import packages
import java.sql.*; //JDBC packages
import java.math.*;
import java.io.*;
import oracle.jdbc.driver.*;
2: Registering JDBC Drivers
class LecExample_1a {
public static void main (String args []) throws
SQLException {
// Load Oracle driver
DriverManager.registerDriver (new
oracle.jdbc.driver.OracleDriver());
Connecting to the Database



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
Connecting to the Database
Connection con = DriverManager.
getConnection("jdbc.imaginaryDB1");
acceptsURL(“jdbc.imaginaryDB1”)?
a
r
imaginary1 imaginary2
r
Oracle
Registered Drivers
Read more in DriverManager API
3: Opening connection to a Database
//Prompt user for username and password
String user;
String password;
user
= readEntry("username: ");
password = readEntry("password: ");
// Connect to the local database
Connection conn =
DriverManager.getConnection
("jdbc:oracle:thin:@aardvark:1526:teach
", user, password);
Interaction with the Database

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

Querying with Statement
String queryStr =
"SELECT * FROM Member " +
"WHERE Lower(Name) = 'harry potter'";
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery(queryStr);
• The executeQuery method returns a ResultSet
object representing the query result.
•Will be discussed later…
Changing 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
4. Creating a Statement Object
// Query the hotels table for resort =
'palma nova’
// Please notice the essential trim
PreparedStatement pstmt =
conn.prepareStatement ("SELECT
hotelname, rating FROM hotels WHERE
trim(resort) = ?");
pstmt.setString(1, "palma nova");
About 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, a Prepared Statement is statement that
contains placeholders to be substituted later
with actual values
Querying with PreparedStatement
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();
Changing DB with PreparedStatement
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();
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);
Statements vs. PreparedStatements:
Be Careful!

Will this work?
PreparedStatement pstmt =
con.prepareStatement(“select * from ?”);
pstmt.setString(1, myFavoriteTableString);
 No!!! A ‘?’ can only be used to represent
column value

WHY?
a
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?

5. Executing a Query,
Returning a Result Set Object &
6. Processing the Result Set
ResultSet rset = pstmt.executeQuery();
// Print query results
while (rset.next())
System.out.println (rset.getString (1)+"
"+ rset.getString(2));
7. Closing the Result Set and Statement Objects
8. Closing the Connection
// close the result set, statement,
and the connection
rset.close();
pstmt.close();
conn.close();
}
Mapping Data Types
There are data types specified to SQL that need
to be mapped to Java data types if the user
expects Java to be able to handle them.
 Conversion falls into three categories:

◦ SQL type to Java direct equivalents
SQL INTEGER direct equivalent of Java int data type.
◦ SQL type can be converted to a Java equivalent.
SQL CHAR,VARCHAR, and LONGVARCHAR can all
be converted to the Java String data type.
◦ SQL data type is unique and requires a special Java data
class object to be created specifically for their SQL
equivalent.
SQL DATE converted to the Java Date object that is
defined in java.Date especially for this purpose.
ResultSet

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 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 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 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 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)

ResultSet 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 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!!

Null 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).
ResultSet Meta-Data
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
ResultSetMetaData rsmd = rs.getMetaData();
int numcols = rsmd.getColumnCount();
for (int i = 1 ; i <= numcols; i++) {
System.out.print(rsmd.getColumnLabel(i)+” “);
}
Many more methods in the ResultSetMetaData API
Database 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
Cleaning Up After Yourself

Remember to close the Connections,
Statements, PreparedStatements and
ResultSets
con.close();
stmt.close();
pstmt.close();
rs.close()
Dealing 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();
}
}
LOBs: 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

Retrieving a BLOB
create 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);
}
Inserting 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();
Transactions 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
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.setInt(2, 13);
pstmt.executeUpdate();
What happens if this
pstmt.setInt(1, 100);
update fails?
pstmt.setInt(2, 72);
pstmt.executeUpdate();
Transaction 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
AutoCommit
setAutoCommit(boolean val)



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
Fixed 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 (Exception e) {
con.rollback();
}
Isolation 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
Isolation Levels
JDBC defines four isolation modes:
Level
Read Uncommited
Dirty
Read
Yes
Unrepeatable
Read
Yes
Phantom
Read
Yes
Read Commited
No
Yes
Yes
Repeatable Read
No
No
Yes
Serializable
No
No
No
Isolation 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
Level: READ_COMMITED
Transaction 1:
insert into A values(1)
insert into A values(2)
commit
Transaction 2:
select * from A
select * from A

Question: Is it
possible for a
transaction to see 1
in A, but not 2?

1
2
Table: A
Question: Is it
possible for the 2
queries to give
different answers
for level
SERIALIZABLE?
Mapping 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