JDBC – Java Database Concentricity

Representation and Management of Data on the Internet

Relational Databases

• Data stored in records (tuples) which live in tables (relations)

• Maps row (record) to column (field) in a single table

A B C D

321 4

4 3 2 1

A (very short) Reminder of SQL

• Creating tables:

CREATE TABLE books (isbn INTEGER primary key,title VARCHAR2(40) not null,abstract TEXT);

A (very short) Reminder of SQL

• Inserting tuples into the tables

INSERT INTO books(isbn, title)VALUES (12345, “Foundations of Database”)

A (very short) Reminder of SQL

• Updating tuples in the tables

UPDATE books SETtitle=“Foundation of Databases”WHERE isbn=12345;

A (very short) Reminder of SQL

• Deleting tuples from the tables

DELETE FROM booksWHERE title like ‘%goblin%;

A (very short) Reminder of SQL

• Queries

SELECT titleFROM booksWHERE abstract like ‘%hobbit%;

Transactions

• 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

Working With Oracle

Add to your .cshrc the following:if ($HOST == sol4) then

setenv ORACLE_HOME /opt/oracle

else

setenv ORACLE_HOME /usr/local/oracle8i

endif

setenv PATH $ORACLE_HOME/bin:$PATH

setenv ORACLE_SID stud

Calling Oracle

• If a student whose login is Snoopy wants to work directly with Oracle:

sqlplus snoopy/snoopy@stud.cs

• Note: we use the login for a password!

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

Overview

TCP/IP

java.net

RMI JDBC CORBA

Network OS

JDBC Drivers (Fig.)

JDBC

Type I“Bridge”

Type II“Native”

Type III“Middleware”

Type IV“Pure”

ODBCODBCDriver

CLI (.lib)

MiddlewareServer

Java Package

JDBC is implemented in Java in the packagejava.sql

Seven Steps

• Load the driver

• Define the Connection URL

• Establish the Connection

• Create a Statement object

• Execute a query

• Process the result

• Close the connection

Important Classes

• DriverManager– loads, chooses drivers

• Driver– connects to actual database

• Connection– a series of SQL statements to and from the DB

• Statement– a single SQL statement

• ResultSet– the records returned from a Statement

DriverManager

Driver

Connection

Statement

ResultSet

Loading the Driver

• Registering the Driver directly automatically:

Class.forName(

“oracle.jdbc.driver.OracleDriver");

• Calling Class.forName, automatically – creates an instance of the driver– registers the driver 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);

The DriverManager

• The DriverManager tries all the drivers

• Uses the first one that works

• When a driver class is first loaded, it registers itself with the DriverManager

• Therefore, to load a driver, just register it!

Creating a Connection

• Use getConnection on the DriverConnection getConnection

(String url, String user, String password)

• Connects to given JDBC URL with given user name and password

• Throws java.sql.SQLException• returns a Connection object

URLS

• Gives the required information for making the connection to the database

• For example, Snoopy will use the URLjdbc:oracle:thin:snoopy/snoopy@sol4:1521:stud

Path to driver User name

Password

Host

PortDatabase name

Connection

• A Connection represents a session with a specific database

• Within the context of a Connection, SQL statements are executed and results are returned

Connections

• There can be multiple connections to a database

• A connection provides “metadata”, i.e., information about the database, tables, and fields

• Connection object has methods to deal with transactions

Creating a Connection

String url = "jdbc:oracle:thin:snoopyo/snoopy@sol4:1521:stud";

try {

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

Connection con = DriverManager.getConnection(url);

}

catch (ClassNotFoundException e)

{ e.printStackTrace(); }

catch (SQLException e)

{ e.printStackTrace(); }

Statements

Statement createStatement()– returns a new Statement object

PreparedStatement prepareStatement(String sql)– returns a new PreparedStatement object

CallableStatement prepareCall(String sql)– returns a new CallableStatement object

• Why all these different kinds of statements?

Statements

• A Statement object is used for executing a static SQL statement and obtaining the results produced by it

• executeQuery is used for statements that return an output result

• executeUpdate is used for statements that need not return an output

Executing Queries and Updates

ResultSet executeQuery(String) – Execute a SQL statement that returns a single

ResultSet

int executeUpdate(String) – Execute a SQL INSERT, UPDATE or

DELETE statement – Used for CREATE TABLE, DROP TABLE

and ALTER TABLE– Returns the number of rows changed

Timeout

• Use setQueryTimeOut 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?

Cursor

• What is the result of a query?

• Is a remote connection different from a direct access to the database?

• How can a database send the result of a query through communication lines?

• The answer: using a cursor

Result Set

• 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

– you can’t rewind

Working with ResultSet

• 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

Getting Values from Rows

• Type getString(int columnIndex)– returns the given field as the given type– fields indexed starting at 1 (not 0)

• Type getString(String columnName)– same, but uses name of field– less efficient

• int findColumn(String columnName)– looks up column index given column name

getType

getType

Example

Code Example

Optimized Statements

• Prepared Statements– SQL calls that you make again and again– allows driver to optimize (compile) queries– created with Connection.prepareStatement()

• Stored Procedures– written in DB-specific language– stored inside database– accesed with Connection.prepareCall()

Metadata

• Connection:– DatabaseMetaData getMetaData()

• ResultSet:– ResultSetMetaData getMetaData()

Useful Methods of Metadata

• getColumnCount

• getColumnDisplaySize

• getColumnName

• getColumnType

• isNullabale

Imagine the case where you want to print the result

Transaction Management

• Transactions are not explicitly opened and closed

• Instead, the connection has a state called AutoCommit mode

• if AutoCommit is true, then every statement is automatically committed

• default case: true

AutoCommit

Connection.setAutoCommit(boolean)

• if AutoCommit is false, then every statement is added to an ongoing transaction

• you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback()

Connection Manager• For a large threaded database server, create a

Connection Manager object• It is responsible for maintaining a certain number of

open connections to the database• When your applications need a connection, they ask

for one from the CM’s pool• Why? Because opening and closing connections

takes a long time• Warning: the CM should always

setAutoCommit(false) when a connection is returned