JDBC Java and Databases

SWC – JDBC JDBC – Java DataBase Connectivity An API (i.e. a set of classes and methods), for working with databases in Java code Primary goal: to make the Java code (almost) independent of the actual data- base being used

SWC – JDBC JDBC allows us to use data from a database with relative ease in a Java program Main steps 1.Connect to a data source 2.Execute queries on the database 3.Store result of query in a data structure 4.Process the data according to business logic

SWC – JDBC There is nothing ”magical” about the JDBC classes and methods – it is just yet another part of the library Is found in java.sql.* packages

SWC – JDBC - setup Before starting, we need to install a so- called ”driver” for MySQL The driver program enables NetBeans to communicate with MySQL databases Comes as a.jar file (Java archive)

SWC – JDBC - setup The MySQL driver is called a ”MySQL connector” 1.Go to 2.Download the ”JDBC Driver for MySQL” 3.Dowload the.ZIP file 4.Unzip somewhere proper, e.g. in the ”MySQL” folder under ”Program Files”

SWC – JDBC - setup

SWC – JDBC - setup After having downloaded the driver, we must make a reference to it from the project that will use the driver We use the NetBeans Library Manager for this 1.Add MySQL driver to a library 2.Refer to the library from the project

SWC – JDBC - setup

SWC – JDBC - setup

SWC – JDBC - setup MySQL driver

SWC – JDBC - setup

SWC – JDBC - setup

SWC – JDBC - setup

SWC – JDBC - setup

SWC – JDBC - setup You’re done!

SWC – JDBC - connecting Making a connection to a data source (database) in done like this. Connection conn = DriverManager.getConnection( url, user, password); What are proper values for these three parameters?

SWC – JDBC - connecting In general, we can connect to a data source ”anywhere on the web” Therefore, the data source is specified as a URL (uniform resource locator) What is the URL for a database on our own computer…?

SWC – JDBC - connecting URL

SWC – JDBC - connecting

SWC – JDBC - connecting Simply copy the URL into the connection statement: Connection conn = DriverManager.getConnection( "jdbc:mysql://localhost:3306/MovieInformation”,…,…); In practice, URL can be read from a setup file, registry, etc..

SWC – JDBC - connecting The ”user” and ”password” are those values you also use when connecting ”directly” to the database In practice, they should not be hard-coded into the Java code…

SWC – JDBC - connecting If connection succeeds, we have a connection object available Actual data source is now abstracted away… We can now execute statements on the connection object

SWC – JDBC - statements We can execute various types of statements on the connection object First, we must create a statement object Statement stmt = conn.createStatement();

SWC – JDBC - statements On the statement object, we can call various methods, that perform an operation on the database In particular, we can execute an SQL query on the statement object

SWC – JDBC - queries ResultSet srs = stmt.executeQuery("SELECT Title, prodYear FROM Movie");

SWC – JDBC - queries The return value of executeQuery is a result set (type ResultSet ) The result set object contains the result of the query, i.e the set of rows returned by the query A result set is somewhat similar to an ArrayList, but with some important differences

SWC – JDBC - queries A result set object has a cursor, which points to a specific row in the set The cursor can be moved in the result set, by calling certain methods For instance the next() method –Returns a boolean value (any more rows?) –Advances the cursor one row

SWC – JDBC - queries Typical processing of a result set: while (srs.next()) { // Process the record... }

SWC – JDBC - queries The actual processing of the result set is – of course – application dependent Typically, we will here map the result to the object model in the application This is a topic in itself… Typically, some application-specific class acts as a ”bridge” between relational model and object model

SWC – JDBC - queries while (srs.next()) { String title = srs.getString("Title"); int year = srs.getInt("prodYear"); objectModel.addMovie(new Movie(title, year)); }

SWC – JDBC - queries The ResultSet methods enable us to retrieve the actual field values from each row in the result set Many more details about result set methods are found at

SWC – JDBC - updates It is fairly straightforward to retrieve data from a database using JDBC –Create a connection object, using a URL –Create a statement object –Execute a query on the statement object –Process the result set returned from the query What if we wish to update some data…?

SWC – JDBC - updates It it very important to realise, that the result set object and the underlying database are still ”connected” to each other If we update a value in the result set, the corresponding field in the database is also updated! This is the beauty of JDBC!

SWC – JDBC - updates A few technicalities… A result set has some properties with regards to –Freedom of cursor movement –Ability to update database These properties are specified when we create the statement object

SWC – JDBC - updates In order to let us update the data, we must create the statement object like: Statement stmt = conn.createStatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);

SWC – JDBC - updates while (srs.next()) { String title = srs.getString("Title"); int year = srs.getInt("prodYear");... srs.updateInt("prodYear", year + 30); srs.updateRow(); } This also updates the DB

SWC – JDBC - insertion A result set also allows us to insert a new record into the result set – and thereby into the database The procedure requires a few steps: –Move the cursor to a special row called the ”insert row” –Set field values by using the update… methods –Finally, the row is actually inserted

SWC – JDBC - insertion srs.moveToInsertRow(); srs.updateString("TITLE", "District 9"); srs.updateString("COUNTRY", "UK"); srs.updateString("GENRE", "Sci-Fi"); srs.updateInt("MOVIEID", 11); srs.updateInt("PRODYEAR", 2009); srs.updateInt("OSCARS", 0); srs.insertRow();

SWC – JDBC - deletion Finally, it is also possible to delete a row from the result set – and thereby also from the database srs.deleteRow(); Deletes the row to which the result set cursor currently points

SWC – Keeping models in sync We could – in principle – just execute queries, updates, etc. directly on the database, without using result sets It is much more efficient to use result sets But…when and how do we update…?

SWC – Keeping models in sync Object model Result set Appli- cation DB

SWC – Keeping models in sync Questions to ponder –When do we load the data initially? Application start-up When object model is accessed User initiative –When (how often) is data saved? Application shutdown Upon every change to object model User initiative

SWC – Keeping models in sync Questions to ponder (continued) –How much data do we save, when we save Everything (brute force). Inefficient, but easy to implement in code Difference between current object model, and object model at last save. Efficient, but requires more book-keeping