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JDBC – Java Database
Connectivity
Representation and Management
of Data on the Internet
1
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
2
JDBC Architecture
We
willare
These
use
thisclasses
one…
Java
Java
Application
JDBC
Oracle
Driver
Oracle
DB2
Driver
DB2
Network
Postgres
Driver
Postgres
3
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
Seven 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
5
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
6
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);
7
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
8
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
9
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
10
The URLs in CS
In CS, a URL has the following structure:
jdbc:oracle:thin:name/password@sol4:1521:stud
Also, your login
Your login
The
machine
on port
The
standard
which
ourOracle
given to
DBMSonruns
sol4
For example:
jdbc:oracle:thin:snoopy/snoopy@sol4:1521:stud
A complete example
11
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…
13
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
14
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
15
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();
16
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();
17
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);
18
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 a
column value
• WHY?
19
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?
20
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”)+”“);
}
26
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).
28
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
29
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
More 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
31
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()
33
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();
}
}
34
Advanced Topics
35
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
36
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);
}
37
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();
38
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
39
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();
40
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
41
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
42
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();
}
43
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
44
Isolation Levels
JDBC defines four isolation modes:
Level
Dirty
Unrepeatable
Phantom
Read
Read
Read
Read Uncommited
Yes
Yes
Yes
Read Commited
No
Yes
Yes
Repeatable Read
No
No
Yes
Serializable
No
No
No
45
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
46
Level: READ_COMMITED
• Transaction 1:
• Transaction 2:
insert into A values(1)
select * from A
insert into A values(2)
select * from A
commit
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?
47