The Java Persistence API

Edel Sherratt

Contents

• Revisit applications programming• Using Java Persistence API

Applications Programming Alternatives

• Extend a high level language by embedding SQL statements in it

• Extend SQL with programming language constructs

• Provide a call level interface (CLI) from a programming language

Applications Programming with a Call Level Interface

• Obtain a handle on the database• Send SQL queries to the database

management system using query functions• Process the results of those queries– Results are tables and must be transformed into

types that the application program can use– Use a cursor to access rows of the result set– Fetch each row in turn; represent as an array or

associative array or other suitable structure

PHP connection to a database

• Obtain a database handle: e.g., pg_connect, mysql_connect, sqlite_open

• Execute queries: e.g. pg_query, sqlite_query, mysql_query

• Process results of the query: e.g. mysql_fetch_array, pg_fetch_array, sqlite_fetch_array, and many others

Java JDBC

• Driver manager provides implementations of Connection, Statement and ResultSet

• Connection acts as database handle• Statement enables creation and execution of

SQL queries• ResultSet maintains a cursor, enabling access

to current row of data returned by query

Example from the Java Dungeon

• this.connection = DriverManager.getConnection("jdbc:sqlite:"+dbname);

• statement.executeUpdate("create table character (" +

"name varchar(20) primary key," + "description text," +"kind varchar(20)," +"location varchar(20) references

location(name));");

Processing the result set

• ResultSet things = whats_at(my_location); if (things.next()) {

System.out.println("\nYou consider taking:");

do {System.out.println(

things.getString("name") );} while (things.next());

}

Java Persistence

• With a CLI, there is a clear and explicit translation between database tables and programming language constructs

• The database and the application program are designed separately and made to work together.

• The Java Persistence API allows us to design with objects, and have the library functions deal with the translation to and from tables.

From CS12220 – Contacts.java

• http://www.aber.ac.uk/~dcswww/Dept/Teaching/CourseNotes/current/CS12230/code/1-week1-in-lab/0-simple-examples/

• A simple example, but wouldn’t it be good to have contacts persist in a database

Java Persistence API – main elements

• Entity – like Contact; instances are represented as rows in a table

• And an EntityManager – to interact with the database

• And a Persistence Unit – to group related entities together

Files, Directories and Jars

• Contact.java – a class definition with some extra annotations

• AddContacts.java – includes an EntityManager that allows me to place contacts in the database

• META-INF – a directory containing MANIFEST.MF and persistence.xml

• lib – a directory containing necessary jar files

Contact.java• Original example by Lynda, with annotations: @blah• @Entity(name= "Contact")

public class Contact { @Id // the primary key @Column (name = "name", nullable = false) private String name; //name of contact

public String getName() { return this.name; }

@Column (name="phone") private String phone; //phone of contact… etc.

• The annotations are defined in javax.persistence

AddContact.java

• EntityManagerFactory – creates an entity manager factory for the persistence unit – must match the persistence unit named in META-INF/persistence.xml

• EntityManager – interacts with the database• A loop that reads in names and numbers and

stores them in the database• Notice how transactions are defined and used

META-INF/persistence.xml

• Names the persistence unit• And the persistence provider • And the class to be persisted• And various properties like those we saw in

connection strings previously

META-INF/MANIFEST.MF

• The class path• The main class

Compiling, Packaging and Running the Application

• java <source files> -cp <classpath to javax.persistence> <destination for classes>

• jar cvmf META-INF/MANIFEST.MF <jar to be created> <classes to be packaged> META-INF

• java –jar <the jar that was created>• NB: you can run the jar anywhere, but do

make sure the library jars are where MANIFEST/META-INF says they will be!

Changing to another database

• The java sources stay the same• The Class Path entry in

META-INF/MANIFEST.MF changes to reflect the new database connection jar

• Some properties in META-INF/persistence.xml are changed to reflect the new database

Using an IDE

• Normally, you would use an IDE like Netbeans or Eclipse to build your JPA application

• Netbeans will create a directory called dist containing your executable jar and a lib directory

• You can zip the jar and the lib into a single file that can be run anywhere

Annotating relationships

• @OneToOne• @OneToMany• @ManyToOne• @ManyToMany• None of these is bidirectional

Bidirectional Relationships

• Every relationship has an owning side • and an inverse side that maps to the owning

side• Annotate both sides to form a bidirectional

relationship• Design options like those for object-oriented

database systems

A more complete example with relationships

• Neil Taylor• Department-Employee• Run as a Netbeans project • Or zip the distribution to run standalone

Online tutorials

• http://www.javaworld.com/javaworld/jw-01-2008/jw-01-jpa1.html

• http://www.javaworld.com/javaworld/jw-01-2008/jw-01-jpa2.html

• http://www.roseindia.net/jpa/

In Summary

• There are many ways to construct database applications

• The Java Persistence API allows us to focus on object oriented design

• Modern IDE’s automate much of the process of creating database applications