JDBCJava and Databases

RHS – SOC 2

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

• Encapsulate what varies…

RHS – SOC 3

JDBC

• JDBC allows us to use data from a database with relative ease in a Java program

• Main steps– Connect to a data source– Execute queries on the database– Store result of query in a data structure– Process the data according to business logic

RHS – SOC 4

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

RHS – SOC 5

JDBC - connecting

• Making a connection to a data source (database) in done like this.

Connection conn = DriverManager.getConnection(url);

• So… what is the ”url”..?

RHS – SOC 6

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…?

RHS – SOC 7

JDBC - connecting

URL

RHS – SOC 8

JDBC - connecting

RHS – SOC 9

JDBC - connecting

• Simply copy the URL into the connection statement:

Connection conn = DriverManager.getConnection(

"jdbc:derby://localhost:1527/MovieInformation");

• In practice, URL can be read from a setup file, registry, etc..

RHS – SOC 10

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

RHS – SOC 11

JDBC - statements

• We can execute various types of statements on the connection object

• First, we must create a statement object

Statement stmt = conn.createStatement();

RHS – SOC 12

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

RHS – SOC 13

JDBC - queries

ResultSet srs = stmt.executeQuery("SELECT Title, prodYear FROM Movie");

RHS – SOC 14

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

RHS – SOC 15

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

RHS – SOC 16

JDBC - queries

• Typical processing of a result set:

while (srs.next())

{

// What should we do here…?

}

RHS – SOC 17

JDBC - queries

• The precise 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

RHS – SOC 18

JDBC - queries

while (srs.next())

{

String title = srs.getString("Title");

int year = srs.getInt("prodYear");

objectModel.addMovie(new Movie(title, year));

}

RHS – SOC 19

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 athttp://java.sun.com/docs/books/tutorial/jdbc/basics/retrieving.html

RHS – SOC 20

JDBC – exercise 1

In this exercise, you must use the methods available in the JDBC library

1. Make sure you have the MovieInformation database on your own computer. If not, create it and use the SQL code in the file Movie-InformationSQL.txt to create the tables and populate them with data

2. Obtain a connection to the MovieInformation database. You need the URL of the database to do this. You will also need to handle exceptions in the code

3. When you have successfully obtained a connection (no exceptions occurred), create a statement object. On the statement object, execute the query ’SELECT * FROM movie’

4. When you have successfully executed the query (no exceptions occurred), process the ResultSet object, by printing out some of the fields for each record in the set

RHS – SOC 21

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…?

RHS – SOC 22

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!

RHS – SOC 23

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

RHS – SOC 24

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);

RHS – SOC 25

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

RHS – SOC 26

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

RHS – SOC 27

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();

RHS – SOC 28

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

RHS – SOC 29

JDBC – exercise 2

In this exercise, you must (again) use the methods available in the JDBC library

1. First, you must have completed the steps in exercise 1

2. Change the statement object, such that it allows for result sets that can be scrolled and updated

3. In the processing of the records, now also update the movie records, by adding 2 to the numbers of Oscars won by each movie

4. After having successfully updated the records (no exceptions occurred), use Derby to check that the records were indeed updated

5. Finally, also try to insert a couple of new records into the movie and actor tables (follow the procedure described in the presentation)

6. After having successfully inserted the records (no exceptions occurred), use Derby to check that the records were indeed inserted

RHS – SOC 30

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…?

RHS – SOC 31

Keeping models in sync

Object model

Resultset

Appli-cation

DB

RHS – SOC 32

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

RHS – SOC 33

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

RHS – SOC 34

JDBC – exercise 3

1. Consider how a class model for movie information - containing information about movies, actors and casting – should be designed

2. How should the class model and the database interact? What happens, when the class model is updated (e.g by a user through a suitable GUI)?

3. Try to consider different alternatives with regards to when to update the database, and what to update

4. If you have time left, try to outline Java code for interaction between the class model and the database