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
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