Chapter 4 Database Connectivity

Instead of accessing the database directly, we can access it via a Data Source Name DSN... Click Add and select the Microsoft Access Driver *.mdb Click Add and select the Microsoft Acce

CHAPTER 4

DATABASE

CONNECTIVITY

What Is JDBC? (cont.)

Portability is one of the key aspects of JDBC

Can you imagine using Java for writing an application once and executing it on a large number of platforms—and then having to

5

number of platforms—and then having to

change the code when your organization

switches from Microsoft SQL Server to

What Is JDBC? (cont.)

7

Application programmers use the JDBC API

Database vendors and tool providers use the JDBC Driver API

This architecture follows the successful model

This architecture follows the successful model

of Microsoft’s ODBC, which provided a C

programming interface for database access

JDBC vs ODBC

9

Why not just use ODBC from Java?

Answer: You can use ODBC from Java, but

with the help of JDBC in the form of the ODBC Bridge

JDBC-ODBC Bridge

The question now becomes, "Why do you

need JDBC?"

JDBC vs ODBC (cont.)

10

There are several reasons to this question:

ODBC is hard to learn It mixes simple and advanced features together, and it has complex options even for simple queries.

ODBC is not appropriate for direct use from Java

because it uses a C interface Calls from Java to native C code have a number of drawbacks in the security, implementation, robustness, and

automatic portability of applications.

JDBC vs ODBC (cont.)

11

There are several reasons to this question:

ODBC relies on the use of void* pointers and other C features that are not natural in Java.

An ODBC-based solution is inherently less safe and harder to deploy than a pure Java solution.

Typical Uses of JDBC

12

The JDBC API supports both two-tier and

three-tier models for database access

In the two-tier model, a Java applet or

application talks directly to the database This

application talks directly to the database This requires a JDBC driver deployed on the client

Figure 1: Two-tier Architecture for Data Access.

Typical Uses of JDBC (cont.)

13

A user's commands are delivered to the

database, and the results of those statements are sent back to the user

The data source may be located on another

The data source may be located on another machine to which the user is connected via a network

This is client/server model, with the user's

machine as the client, and the machine

housing the data source as the server

Typical Uses of JDBC (cont.)

14

Java applet or HTML browser Client machine (GUI)

HTTP, RMI, CORBA, or other calls

Figure Three-tier Architecture for Data Access.

Application Server

(Java)

JDBC

Server machine (business logic) HTTP, RMI, CORBA, or other calls

DBMS-proprietary protocol

The Structured Query Language

16

A database: a bunch of named tables with

rows and columns Each column has a column name The rows contain the actual data or

records

The Authors Table

Author_ID Name Fname

driver to access data.

Instead of accessing the database directly, we can access it via a Data Source Name (DSN)

JDBC Installation (cont.)

19

Set up a DSN and get a connection through that:

Choose Control Panel > Administrative Tools > Data Sources.

In the ODBC Data Source Administrator dialog box, click the System DSN tab.

Click Add and select the Microsoft Access Driver (*.mdb)

Click Add and select the Microsoft Access Driver (*.mdb)

Type in the name mdbTEST for the Data Source Name

Click Create and select a file to save the database to (I chose "d:\java\mdbTEST.mdb") - this creates a new blank

MS Access database!

Click “OK" all the way out

Database URLs

21

All classes used for JDBC programming are

contained in the java.sql and javax.sql

packages.

Database URLs:

jdbc:subprotocol : other stuff

jdbc:subprotocol : other stuff

subprotocol selects the specific driver for connecting

Making the Connection

22

To set up our DriverManager and let it know that we want to communicate with ODBC data sources via the JDBC:ODBC bridge, we call the forName() method of the Class class

Connect to MS SQL Server 2000 & MySQL

23

MS SQL Server 2000

Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver" );

String dburl = "jdbc:sqlserver://localhost:1433;" +

Executing SQL Commands

24

Firstly, create a Statement object To do that, use the Connection object that you obtained from the call to

String command = “UPDATE Books”

+ “ SET Price = Price – 5.00”

+ “ WHERE Title NOT LIKE

‘%Introduction%’”;

Executing SQL Commands (cont.)

25

Then call the executeUpdate method:

stat.executeUpdate(command);

This method returns a count of the rows

affected by the SQL command

The executeUpdate method can execute actions such as:

INSERT , UPDATE , and DELETE

Data definition commands: CREATE TABLE ,

Executing SQL Commands (cont.)

* FROM Books”);

while (rs.next())

look at a row of the result set

}

There is also a catch-all execute statement to

execute arbitrary SQL statements.

Executing SQL Commands (cont.)

27

To know the contents of the fields:

String isbn = rs.getString(1);

double price = rs.getDouble(“Price”);

There are accessors for various types:

getString, getDouble,…

getString, getDouble,…

Each accessor has two form: one with a

numeric argument (column number), one with

a string argument (column name).

Unlike array indexes, database column

numbers start at 1

Executing SQL Commands (cont.)

28

SQL data types and Java data types are not exactly the same See the below table

SQL Data Type Java Data Type

Executing SQL Commands (cont.)

29

SQL Data Type Java Data Type

CHARACTER(n) or CHAR(n) String

Data Types in MS Access

30

Advanced SQL Types

31

Many databases can store large objects such as

images or other data.

Binary large objects: BLOBs.

Character large objects: CLOBs.

The getBlob and getClob methods return

The getBlob and getClob methods return

objects of type java.sql.Blob and

java.sql.Clob

A SQL ARRAY is a sequence of values.

Ex: In a Student table, you can have a Scores

column that is an ARRAY OF INTEGER

Managing Connections, Statements, and Result Sets

32

Every Connection object can create one or

more Statement objects

You can use the same Statement object for

Managing Connections, Statements, and Result Sets (cont.)

Call the close method on Statement object:

Call the close method on Statement object:

closes this statement object and its associated result set.

These objects use large data structures, and

you don’t want to wait for the garbage collector

to deal with them

Managing Connections, Statements, and Result Sets (cont.)

to deal with exceptions

Example 1 (cont.)

38

“Any”.

Click the Query

button; all books

matching your

selection will be

displayed in the text area.

Click the button,

Click the button,

all prices of that

publisher are

adjusted by the

amount you

entered

publisher, independent of the author:

SELECT Books.Price, Books.Title

FROM Books, Publishers

WHERE Books.Publisher_Id = Publishers.Publisher_Id AND Publishers.Name = the name from the list box

Prepared Statements (cont.)

42

Instead of using a separate query statement every time the user launches such a query, we

can prepare a query with a host variable.

Each host variable is indicated with a ?

Each host variable is indicated with a ?

String publisherQuery =

“SELECT Books.Price, Books.Title” +

“ FROM Books, Publishers” +

“ WHERE Books.Publisher_Id = Publishers.Publisher_Id AND Publishers.Name = ?”;

Prepared Statements (cont.)

Once all variables have been set to values,

you can execute the query:

publisherQueryStat. setString(1, publisher);

// The position 1 denotes the first ?

// The second argument is the value that we want

to assign to the host variable

ResultSet rs = publisherQueryStat executeQuery() ;

Prepared Statements (cont.)

44

The price update feature is implemented as an

UPDATE statement

UPDATE Books

SET Price = Price + ?

WHERE Books.Publisher_Id = (SELECT Publisher_Id

We call executeUpdate, not executeQuery The return value of executeUpdate is the

count of changed rows

WHERE Books.Publisher_Id = (SELECT Publisher_Id

FROM Publishers WHERE Name = ?)

Summary of Necessary APIs

To execute a query void setXxx(int n ,

Xxx x ) Xxx x ) ResultSet

executeQuery(String

sql )

ResultSet executeQuery()

int executeUpdate(String

sql )

int executeUpdate()

boolean execute(String sql )

boolean execute()

Summary of Necessary APIs

one Returns false after the last row.

•Xxx getXxx( int columnNumber)

•Xxx getXxx (String columnName)

(Xxx is a type such as int, double, String, Date, etc.) return the value of the column with the given column number or name, converted to the specified type Not all type conversions are legal.

Note: The column number belongs to the column in the result set, not the one in the database.

Scrollable Result Sets

48

Suppose that you want to move backward in the result set But JDBC 1 had no previous

method

Since JDBC 2.0 API (JavaTM 2 SDK, Standard

Since JDBC 2.0 API (Java 2 SDK, Standard Edition, version 1.2), you can move forward and backward and jump to any position in the result set

Scrollable Result Sets (cont.)

49

To obtain scrollable result sets from your

queries, you must use:

Statement stat = conn createStatement (type,

Scrollable Result Sets (cont.)

50

You have the following choices:

Do you want the result set to be scrollable or not?

If not, use ResultSet.TYPE_FORWARD_ONLY

If the result set is scrollable, do you want it to be able to reflect changes in the database occurred after the query that yielded it?

Do you want to be able to update the database by editing the result set?

Scrollable Result Sets (cont.)

51

ResultSet Type Values

TYPE_FORWARD_ONLY The result set is not scrollable.

TYPE_SCROLL_INSENSITIVE The result set is scrollable but not

sensitive to database changes.

TYPE_SCROLL_SENSITIVE The result set is scrollable and

ResultSet Concurrency Values

TYPE_SCROLL_SENSITIVE The result set is scrollable and

sensitive to database changes.

CONCUR_READ_ONLY The result set cannot be used to

update the database.

CONCUR_UPDATABLE The result set can be used to

update the database.

Scrollable Result Sets (cont.)

ResultSet rs = stat.executeQuery( query );

Scrollable Result Sets (cont.)

Scrollable Result Sets (cont.)

the 1 row.

To move backward or forward by a number of rows:

rs.relative(n);

If n > 0, the cursor moves forward.

If n < 0, the cursor moves backward.

Scrollable Result Sets (cont.)

55

To set the cursor to a particular row number:

rs.absolute(n);

To get the current row number:

int currentRow = rs.getRow();

int currentRow = rs.getRow();

1 st row in the result set has number 1

If the return value is 0, the cursor is not on a row

It is either before the first row or after the last row.

Scrollable Result Sets (cont.)

Using a scrollable result set is very simple

The hard work of caching the query data is

carried out by the database driver

Updatable Result Sets

57

Using this mechanism, you can edit result set

and the changes are automatically reflected in the database

To obtain updatable result sets:

The result sets returned by a call to

executeQuery are then updatable

Statement stat =

conn.createStatement(ResultSet.TYPE_SCROLL_INSENSI

TIVE, ResultSet.CONCUR_UPDATABLE );

Updatable Result Sets (cont.)

double price = rs.getDouble("Price");

rs.updateDouble ("Price", price + increase);

rs.updateRow ();

} }

Updatable Result Sets (cont.)

59

java.sql.ResultSet:

Xxx data )

(Xxx is a type such as int , double , String ,

Date , etc.) update a field in the current row of the

Updatable Result Sets (cont.)

60

To make the updates effective:

void updateRow() Sends all updates in the current row to the database.

To cancel the updates to the current row:

void cancelRowUpdates() Call this method after calling an updater method(s) and before calling the

method updateRow to roll back the updates

Updatable Result Sets (cont.)

Build up a new row

Move the cursor to a special position,

called the insert row.

Deliver the new row to the database

Updatable Result Sets (cont.)

62

To delete the row:

void deleteRow() Removes the row from both the result set and the database.

give you the same power as executing

commands

64

In SQL, data that describe the database or one

of its parts are called metadata

You can get three kinds of metadata: about a database, about a result set, and about

parameters of prepared statements

parameters of prepared statements

To get metadata of a database:

DatabaseMetaData meta = conn.getMetaData();

ResultSet mrs = meta.getTables(null, null, null, new String[] { “TABLE” });

return a result set that contains information about

column’s name, type, and field width.

String query = "SELECT * FROM Books";

ResultSet rs = stat.executeQuery(query);

ResultSetMetaData meta = rs.getMetaData ();

for (int i = 1; i <= meta.getColumnCount (); i++) {

String columnName = meta.getColumnLabel (i);

int columnWidth = meta.getColumnDisplaySize (i);

68

A set of statements can be grouped to form a

transaction

The transaction can be committed when all

has gone well Or, if an error has occurred in has gone well Or, if an error has occurred in

one of them, it can be rolled back.

Transactions (cont.)

69

Main reason for grouping commands into

transactions is database integrity

Example: Suppose we want to transfer money

from one bank account to another

from one bank account to another

Simultaneously debit one account and credit another.

If the system fails before crediting the other

account, the debit needs to be undone.

Transactions (cont.)

70

By default, a database connection is in

autocommit mode

This means that each SQL command is

committed to the database as soon as it is

committed to the database as soon as it is

executed and you cannot roll it back.

To check the current autocommit mode setting:

To turn off autocommit mode:

Transactions (cont.)

71

Now you create a statement object:

Call executeUpdate any number of times:

Save Points

72

You can gain more control over the rollback

process by using save points

Creating a save point marks the place where you can later return without having to return to you can later return without having to return to the start of the transaction

Save Points (cont.)

73

Example:

Statement stat = conn.createStatement(); // start

transaction; rollback() goes here

stat.executeUpdate(command1);

stat.executeUpdate(command1);

Savepoint svpt = conn.setSavepoint(); // set

savepoint; rollback(svpt) goes here

The commands in a batch can be:

INSERT , UPDATE , and DELETE

CREATE TABLE and DROP TABLE

Batch Updates (cont.)

75

To execute a batch:

Statement stat = conn.createStatement();

// Instead of calling executeUpdate, call addBatch

String command = “CREATE TABLE ”;

stat.addBatch(command);

Finally, you submit the entire batch:

int[] counts = stat.executeBatch();

while ( ) {

command = “INSERT INTO VALUES (” + … + “)”; stat.addBatch(command);

}