CSCI 6962: Server-side Design and Programming JDBC Database Programming

Outline Introduction to JDBC Connecting to a database server Executing queries and reading result sets Prepared statements Executing update statements Synchronized database access

JDBC Definition Java Database Connectivity (JDBC): set of classes that provide methods to –Connect to a database through a database server (using a driver) –Query database using SQL syntax, getting “list” of records that match query –Manipulate database by executing SQL commands to modify, insert, and delete records web container JSF page Managed bean database database driver DBMS database server JDBC

JDBC Components Major objects involved: – Connection : represents connection to a database through a server – Statement : represents SQL statement executed on database via that connection –ResultSet : represents “list” of records matching a query Database server database Statement object select * from widgets ResultSet object ID name price ID name price ID name price

Connecting to the Database Server Load the database driver –Not necessary in most recent version, but safe thing to do Syntax: Class.forName("driver class").newInstance(); Name of driver class based on provider (see their documentation) –Derby: org.apache.derby.jdbc.ClientDriver –MySQL: com.mysql.jdbc.Driver

Connecting to the Database Server Need to provide url of database Form: jdbc:servertype:serverURL:port/databasename –Derby: jdbc:derby://localhost:1527/DBname –MySQL: jdbc:mysql://localhost:3306/Dbname

Connecting to the Database Server Syntax: connectionobject = DriverManager.getConnection("databaseURL", "username", "password"); Derby example: Should close connection when done connectionobject.close();

Exception Handling in JDBC Any database-related statement may throw SQLException –Your code must put in try/catch block –May also need to catch other exceptions ClassNotFoundException for missing database driver Diagnostic message displayed Better idea: Redirect to an error page

Connecting to the Database Server

Executing Queries Create new statement object using the connection Execute an SQL query using that statement Store results in a ResultSet object Syntax: statement = connection.createStatement(); statement.executeQuery(“SQL query”);

Reading ResultSets Can only do simple access: –Read in field values from current record –Move to next record Syntax to move to next record: ResultSetObject.next(); –Returns false if no next record, true otherwise –Must execute once before reading first record –Usually while loop to read until no more records while(ResultSetObject.next()) { code to read in current record }

Reading ResultSets Syntax to read field from current record: value = ResultSetObject.getType(fieldname); Specify field name used in database Specify type data is to be read in as varChar  getString int  getInt double  getDouble

Widget Example Goal: Display all Widgets in datatable –Same as before, but no longer hardwired Database access in Widget class getAllWidgets: –Queries for all widgets, extracts ID of each widget –Constructs widget with that ID, adds to list Constructor: –Queries for widget with given ID –Extracts name, price to set its properties

getAllWidgets Code Execute SQL query for all widgets Loop through all results Get the value of the ID field Close connection and return list Use it to construct a widget and add to the list

Inserting Parameter Values Queries often based on variables –Example: finding widget with given ID Must insert values into query –If value is string, must make sure quote marks ‘ ‘ surround the value! Insert given ID into the query

Constructor Code Execute SQL query for widgets with given ID Advance to first (and only) result Extract name as string and price as double

Prepared Statements Tell database server basic form of statements in advance –Database server can do all work for that type of statement once “Fill in blanks” for actual values when actually execute statement –Easier syntax than inserting manually –More secure against SQL injection attacks! Example: Extracting widget with given ID –All statements of form: "SELECT * FROM widgets WHERE ID = ____“

Prepared Statements Declare as PreparedStatement PreparedStatement lookup = null; Define prepared statement using connection.prepareStatement(template); Place ‘?’ where actual values will be inserted lookup = connection.prepareStatement("SELECT * FROM widgets WHERE ID = ?");

Prepared Statements Use setType (index, value) to insert value into statement lookup.setString(1, ID); Execute query on the prepared statement resultsConstructor = lookup.executeQuery(); Type of field (like get method in ResultSet) Which ‘?’ to insert the value into Insert ISBN into first (and only) ‘?’ in lookup

Executing Update Statements Syntax: int chng = statement.executeUpdate(SQL) or int chng = preparedstatement.executeUpdate() Returns number of records changed by statement –Often used for validation

Updating Price in Widget Class

Performing Update from Bean Call static Widget method with ID, price If price changed, display new inventory If no change, display error message in JSF page

Synchronized Database Access Database updates occur “simultaneously” on busy sites Can interfere with one another Example: Quantity update after purchase –Query for previous quantity –Subtract 1 –Update database with new quantity

Synchronized Database Access Java runs separate clients as “parallel” threads which execute “simultaneously” –Processor swaps back and forth between threads Problem if following sequence occurs: –Current quantity = 100 –Client 1 code to get current quantity executes (value = 100) –Processor swaps to client 2 thread –Client 2 code to get current quantity (value still = 100) –Client 2 code sets new quantity to 99 and stores in database –Processor swaps back to client 1 thread –Client 1 code also sets new quantity to 99 and stores in database!

Synchronized Database Access Get quantity Quantity = 100 Client 1 thread Get quantity Quantity = 100 Client 2 thread Set quantity = 99 Store 99 in database Set quantity = 99 Store 99 in database Problem: this code should not be interrupted!

Synchronized Database Access Can declare sections of code to be synchronized –Only one thread may execute it at a time –Another thread cannot start the code until the first has finished it Syntax: synchronized(object) { code } Only one thread at a time should be able to execute this code on this object

Synchronized Database Access