While persistent data can be handled in any number of ways, using a
relational database is common.
Java provides JDBC for low-level access to a DBMS,
allowing the use of SQL.
This works (if you are good at SQL and configuring
various DBMSes), but has a drawback with Java:
the results of queries is a table (actually, a
raw SQL data).
To process effectively in Java, this data must be read one field at a time,
for each row, and objects constructed and added to a collection.
This is because Java works best with collections of objects.
More recent versions of Java include the Java Persistence API, or “JPA”. This includes a much simpler (and more Java-ish) way to access databases, and includes its own query language. Long available in Java EE, JPA is now available in Java SE. However, while Java servers generally include an implementation of JPA, neither Java SE JRE or the Oracle JDK implement the classes required. (The JDK does include the Apache Derby RDBMS.)
To use JPA with Java SE, you must configure an
implementation of JPA.
Initially known as “TopLink”, the reference implementation
is the open source “EclipseLink”.
To use JPA, download the EclipseLink zip archive from
www.eclipse.org/eclipselink/ and unpack it
Next, create an environment variable named
with the location of the
Finally, you must make sure
eclipselink.jar is found in the
EclipseLink internally uses
log4j for logging (even though
the documentation says it doesn't).
persistence.xml JPA configuation file,
logging is disabled for this demo.
log4j still launches, finds it isn't configured
correctly, and displays annoying error messages.
To prevent this, the file
log4j.properties is used with
the simple directive to disable logging.
(This may not be a problem in a future version.)
This demo uses the JavaDB (Apache Derby) that is bundled with the JDK. For simplicity, the demo uses an embedded database, so you don't have to set up any server. The downside of using embedded databases is that it becomes difficult to bundle your application in a Jar file. This is because any read-write database must be outside of that jar file, which requires extra careful configuration, and permission to create files and folders on the user's hard disk.
To run this demo, save the files shown below (except the PNG graphics), preserving the folder structure as shown. Then you can compile and run it, like so:
C:\Temp\JPADemo>javac -Xlint:all com\wpollock\jpademo\Main.java C:\Temp\JPADemo>java com.wpollock.jpademo.Main Titles of books in our collection: ID: 1, Title: Underwater Basket-Weaving for Dummies ID: 2, Title: Brain Surgery: Self-Taught Number of books: 2 C:\Temp\JPADemo>
(This is easier using some IDE or a build tool such as Maven. But all the demos I can find online do that, so I wanted to show the basic way using nothing but the JDK and EclipseLink.)
The two graphics show the resulting database table.