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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