Download - ppt
1
JDBC – Java Database JDBC – Java Database ConnectivityConnectivity
Representation and
Management of Data on the
Internet
2
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
3
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
5
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
6
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
7
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);
8
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
9
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
10
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
11
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
13
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);
14
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
15
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
16
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();
17
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();
18
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);
19
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);
20
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)
26
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!!
28
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).
29
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
31
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
33
Cleaning Up After YourselfCleaning Up After Yourself
• Remember to close the Connections, Statements, PreparedStatements and ResultSets
con.close();stmt.close();pstmt.close();rs.close()
34
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();
}}
35
Advanced TopicsAdvanced Topics
36
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
37
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);
}
38
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();
39
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
40
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?
41
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
42
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)
43
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();}
44
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
45
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
46
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
47
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?