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JDBCJDBC"Java Database Connectivity""Java Database Connectivity"

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• Getting Started Guide:

http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/

getstart/GettingStartedTOC.fm.html

• java.sql Package API:

http://java.sun.com/j2se/1.5.0/docs/api/java/sql/package-summary.html

Useful JDBC LinksUseful JDBC Links

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Introduction to JDBCIntroduction to JDBC

• JDBC allows for convenient database

access from Java applications

• Data is transferred from relations to

objects and vice-versa

– databases optimized for searching/indexing

– objects optimized for engineering/flexibility

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Why Access a Database from Why Access a Database from within a Program?within a Program?

• As we saw last week, some queries can’t be computed in SQL.

• PL/pgSQL includes more programming tools than SQL

• However, sometimes using PL/pgSQL will not be suitable:

– If we require object-oriented programming

– If accessing the database is a small part of a large Java application

– Etc.

• Why not keep all the data in Java objects?

• “Separation of concerns”: DBMSes concentrate on data storage and access; programs concentrate on algorithms, networking, etc.

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Packages to ImportPackages to Import

• In order to connect to a database from

java, import the following packages:

– java.sql.*; (usually enough)

– javax.sql.* (for advanced features, such

as scrollable result sets)

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accessaccess

• Add the following line to

your .classpath file (located in your

home directory):

setenv CLASSPATH ${CLASSPATH}:/usr/share/java/postgresql.jar

• And then open a new shell

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Six StepsSix Steps

• Load the driver

• Establish the Connection

• Create a Statement object

• Execute a query

• Process the result

• Close the connection

JDBC Architecture (1)JDBC Architecture (1)

Application

• DriverManager is provided by Java

Software as part of the Java 2

Platform.

• Drivers are provided by DBMS

vendors.

Driver

DriverManager

DBMS

JDBC Architecture (2)JDBC Architecture (2)

• The application creates a driver instance and registers it with the DriverManager.

• The DriverManager tells the driver to connect to the DB

• The DriverManager keeps track of registered driver instances and their connections to DB’s.

• The Driver “talks” to a particular DB through the connection

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ConnectingConnecting

1. Initializing a driver and registering it with the DriverManager: Class.forName(“org.postgresql.Driver");

2. Getting a connection:

DriverManager.getConnection(URL)

• The URL is:“jdbc:postgresql://dbserver/public”,”YOUR-

LOGIN”,null

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For exampleFor example

Class.forName(“org.postgresql.Driver"); DriverManager.getConnection

(“jdbc:postgresql://dbserver/public”,”gidi”,null);

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Interacting with the DBInteracting with the DB

• Once you have established a connection,

your would like to interact with the DB

• Interaction are done by sending Statements

and PreparedStatements to the DB

• These are used for 2 things (using different

methods):

1. Querying the DB (executeQuery)

2. Changing the DB (executeUpdate)

StatementStatement

Statement createStatement()

– returns a new Statement object

• Used to send SQL commands to the DB

• Created via the connection object

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Statement query methodsStatement query methods

1. stmt.executeQuery(String query): for

queries that return a single ResultSet

object (typically select)

2. stmt.executeUpdate(String query): for

INSERT, UPDATE, DELETE, and SQL DDL

directives

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CompilationCompilation

• When executing an SQL statement via

JDBC, it is not checked for errors until it

is run (Not checked during compilation)

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executeQueryexecuteQuery

String queryStr = "SELECT * FROM Sailors " +"WHERE Name = 'joe smith'";

Statement stmt = con.createStatement();ResultSet rs = stmt.executeQuery(queryStr);

• The executeQuery method returns a ResultSet object representing the query result.

No semi-colon(;)

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executeUpdateexecuteUpdate

String deleteStr = “DELETE FROM Sailors " +"WHERE sid = 15";

Statement stmt = con.createStatement();int delnum = stmt.executeUpdate(deleteStr);

•executeUpdate returns the number of rows modified

No semi-colon(;)

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PreparedStatement PreparedStatement motivationmotivation

• Suppose we would like to run the query

SELECT * FROM Emp

where name=‘moshe’;

• But we would like to run this for all employees (separately), not only ‘moshe’…

• Could we create a variable instead of ‘moshe’ which would get a different name every time??..

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PreparedStatementPreparedStatement

PreparedStatement

prepareStatement(String)

– returns a new PreparedStatement

object

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Prepared StatementsPrepared Statements

• Prepared Statements are used for queries that

are executed many times with possibly

different contents.

• A PreparedStatement object includes the query

and is prepared for execution (precompiled).

• Question marks can be inserted as variables.

-setString(i, value)

-setInt(i, value)The i-th question

mark is set to the

given value.

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PreparedStatement.executeQueryPreparedStatement.executeQuery() ()

String queryStr = "SELECT * FROM Sailors " +"WHERE Name = ? and Rating < ?”;

PreparedStatement pstmt = con.prepareStatement(queryStr);

pstmt.setString(1, “Joe”);pstmt.setInt(2, 8);

ResultSet rs = pstmt.executeQuery();

1st question

mark

Value to insert

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PreparedStatement.executeUpdatePreparedStatement.executeUpdate()()

String deleteStr = “DELETE FROM Boats " +"WHERE Name = ? and Color = ?”;

PreparedStatement pstmt = con.prepareStatement(deleteStr);

pstmt.setString(1, “Fluffy”);pstmt.setString(2, "red");

int delnum = pstmt.executeUpdate();

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• Will this work?

PreparedStatement pstmt = con.prepareStatement(“select * from ?”);

pstmt.setString(1, "Sailors");

No! We may put ? only instead of values

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Why use Why use preparedStatement?preparedStatement?

• In most cases, you can use a regular statement and

just change the string you send to executeQuery

each time

• You have to be careful

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Why use Why use preparedStatement?preparedStatement?

• Suppose google worked without PreparedStatements,

they would implement search queries as something

like:

Statement s;

s.executeQuery(‘select URL,Title from internet where

content like ‘%”+searchString+”%’”);

• What would happen if a hacker searched for:

bla bla’ UNION select company as URL,

CreditCardNumber AS title from advertisingClients

where company like ‘Example taken from dbi course

ResultSet (1)ResultSet (1)• A ResultSet is an object which contains the

result of a query - a “table”.

• At most one ResultSet per Statement can be open at the same time(!!).

• A ResultSet maintains a cursor pointing to its current row of data.

• The 'next' method moves the cursor to the next row

• As of JDBC 2.0, scrollable ResultSets are available, which also include ‘previous’, ’first’, ‘last’, etc..

ResultSet (2)ResultSet (2)

• resultSet methods work on the

current row.

• The cursor is positioned before

the first row upon creation.

ResultSet (3)ResultSet (3)

Statement stmt=

con.createStatement();

ResultSet rs =

stmt.executeQuery (

"SELECT * FROM Table1");

while (rs.next()) {

//something…

}

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ResultSet methodsResultSet methods

• Getting the value in some column (for the current row):– getString(int columnNum);

– getString(String columnName);

– getInt(int columnNum);

– getInt(String columnName);

– Etc…

• To check if NULL was returned, you have to use wasNull() on the ResultSet after getting the value.

String s = rs.getString(“column1");

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Example revisitedExample revisited

Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1");

// retrieve and print the values for the current row

while (rs.next()) {

int i = rs.getInt("a");

String s = rs.getString("b");

float f = rs.getFloat("c");

System.out.println("ROW = " + i + " " + s + " " + f);

}

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ResultSetMetaData ResultSetMetaData

ResultSetMetaData rsmd = rs.getMetaData();int numcols = rsmd.getColumnCount();

for (int i = 1 ; i <= numcols; i++) {if (i > 1) System.out.print(",");System.out.print(rsmd.getColumnLabel(i));

}

An object created by the ResultSet which holds information about its columns

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Printing Query Output: Printing Query Output: Result Set (2) Result Set (2)

while (rs.next()) {for (int i = 1 ; i <= numcols; i++) {

if (i > 1) System.out.print(",");System.out.print(rs.getString(i));

}System.out.println("");

}

• getString() is allowed to access all simple JDBC types

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Cleaning Up After YourselfCleaning Up After Yourself

• Remember to close the Connections, Statements, PreparedStatements and ResultSets

con.close();stmt.close();pstmt.close();rs.close();

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Dealing With ExceptionsDealing With Exceptions

catch (SQLException e) {//human readable message about the

exceptionSystem.out.println(e.getMessage());

//String describing the reason of the exception

System.out.println(e.getSQLState());//driver-dependent code for the exceptionSystem.out.println(e.getErrorCode());

}

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Mapping SQL and Java Mapping SQL and Java Types Types

• SQL and Java data types are not identical

• There are significant variations between the SQL types supported by different database products

• JDBC defines a set of generic SQL type identifiers in the class java.sql.Types

• The driver is responsible for mapping between the DB SQL types and JDBC SQL types

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Transactions in JDBCTransactions in JDBC

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TransactionsTransactions

• Transaction = 2 or more statements which

must all succeed (or all fail) together

• If one fails, the system must reverse all

previous actions

• Aim: don’t leave DB in inconsistent state

halfway through a transaction

• COMMIT = complete transaction

• ROLLBACK = abort

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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

• 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()

con.setAutoCommit(boolean val)

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ExampleExamplecon.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();}