Java Database Connectivity JDBC

ICW Lecture 12

Errol Thompson

Last Time

• URLs

• A reminder of Sockets.

• Threads: running processes at the same time on the same computer.

• They can be tricky to use.

This lecture

• Relational databases

• Accessing relational databases from Java using JDBC and SQL

• Manipulating database structure from Java

References

• Java JDBC Documentation http://java.sun.com/javase/6/docs/technotes/guides/jdbc/

• JDBC Basics Tutorial http://java.sun.com/docs/books/tutorial/jdbc/basics/index.html

• JDBC Advanced Tutorial http://java.sun.com/developer/Books/JDBCTutorial/index.html

• mySQL Connector http://dev.mysql.com/downloads/connector/j/3.1.html

• Cay Horstmann (2008) Big Java, 3rd Edition. Wiley.

What is a Relational Database?

Tables and Relationships

Riders

Teams

Events

A rider is a member of a team. A team comprises many riders.

A rider competes in an event. A event has many riders competing.

A table contains many records. Each record has a unique primary key. Tables are linked using the keys.

Accessing a book

• If I want to read a book, what do I need to do?

• What do you need to do to access a file?

• Do these same ideas apply to a database?

To access a relational database

• Need to know where the database is• Need to know the structure of the

database– What tables– What columns and keys

• Use SQL to form queries and to manipulate data

How do you open (connect) to a database?

JDBC Database Access

Accessing a database

• What do we need to be able to do to access a database from our program code?• Obtain access (i.e. Open the database)

• Read data from the database

• Write data to the database

• Relinquish access (i.e. Close the database)

• Have transaction control• Group a set of updates

• Rollback changes if errors occur

Linking to database driver

Set the class path to find the driver

Database DriverWhere do you find driver names?

Should be in database vendor documentation

For driver in assignment: “com.mysql.jdbc.Driver” or"org.gjt.mm.mysql.Driver"

Database URLHow is a database URL constructed?

“jdbc:<vendor sub protocol name>:<vendor specific options or commands>

For mySQL: “jdbc:mysql://<Server URL>/<database name>”

There are variations

Java code

Loading the driver classClass.forName(" com.mysql.jdbc.Driver ");

Get a connection object (open the database connectionConnection con = DriverManager.getConnection(url,

username, password);

Use try {…} catch {…} around these

Methods available on connection objectcon.isClosed() returns true if connection is closed

con.close() closes the connection

How do you obtain data from a database?

JDBC Database Access

Building and executing a query

SQL Select statement – Simple formSelect <fields> from <table> where <condition>

Create a statementStatement <variable> = <connectionObject>.createStatement();PreparedStatement <variable> =

<connectionObject>.prepareStatement(<parameterised query>);<preparedStatementObject>.set<dataType>(<parameter number>,

<value>);

Execute queryResultSet <variable> = <statementObject>.executeQuery(<SQL

Select Statement>);

Remember to use try {…} catch {…}

Retrieving data from result set

Stepping through data records<resutSetObject>.next();

Retrieving individual fields<varaible> = <resutSetObject>.get<DataType>(<column

number>);

<varaible> = <resutSetObject>.get<DataType>(<column name>);

dataType can be int, double, String, Date, etc.

Close the result set<resutSetObject>.close();

How can you write data to a database?

JDBC Database Access

SQL commands

To add dataInsert Into <Table> (<field names>) Values

(<values>)

To modify dataUpdate <Table> Set <Assignments> Where

<condition>

JDBC statement execution

Prepare as for SQL SelectExecute

int <var> = <statementObject>.executeUpdate(<SQL statement>);

Returns number of records changed

<statementObject>.execute(<SQL statement>)Returns true if a result set has been created by SQL

statement

<statementObject>.getResultSet()Returns result set

<statementObject>.getUpdateCount()Returns the number of records updated

How do I handle sets of data from a database in my

program?Scrollable and updatable result sets

Changes to the createStatement

Additional options on the createStatement and prepareStatementType

TYPE_FORWARD_ONLY

TYPE_SCROLL_INSENSITIVE

TYPE_SCROLL_SENSITIVE

ConcurrencyCONCUR_READ_ONLY

CONCUR_UPDATEABLE

Scrollable result set

Step forward and backwardsrs.next()

rs.previous()

rs.relative(n)

Skip to a specific rowrs.absolute()

rs.getRow()

Other methodsfirst, last, beforeFirst, afterLast

isFirst, isLast, isBeforeFirst, isBeforeLast

Updateable result set

Must create or prepare statement with CONCUR_UPDATABLEAble to update field values

updateXxx(column, value)updateRow()

Able to add rowsmoveToInsertRow()updateXxx(column, value)insertRow()moveToCurrentRow()

Able to delete rowsdeleteRow()

How do I ensure that no other program is updating the same

data as my program?Transaction control

What is the issue?

Should we treat a group of database updates as though they were one update?Purchase transaction updates

Product inventory

Sales history

Customer transaction history

What constitutes a complete transaction?

Implementation of transaction control

Switch off automatic commitconn.setAutoCommit(false);

Update the result set

Commit the changesconn.commit();

Oops! Something has gone wrong.conn.rollback();

Why is transaction control associated with the

connection and not the statement?

Batch updates

Improves the performance of update operationsstat.addBatch(command);

stat.executeBatch();

Might also treat the batch as a single transaction

Can I access information about the structure of the

database?Metadata

What is Metadata

Data about the structure of the databaseWhat tables are in the database?

How many columns are in a table?

What columns are in the tables?

What are the data types of the columns?

Metadata is held in a result set like conventional data

SQL data management statements

All that are supported by the database software in use.

IncludesCreate Database

Use <database>

Create Table

Create Relationships

Modify Table

Drop table

Drop Database

Conclusion

• Reviewed the nature of a relational database• Connection to a database using JDBC• Retrieving data using SQL and JDBC• Using SQL to update the database• Using a result set to update the database• Database metadata using SQL• Using SQL commands to modify the

database structure

Next Time

• Servlets