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public class DemoApplication {
 
    public static void main(String[] args) throws Exception {
        DemoApplication calc = new DemoApplication();
        System.out.println(calc.divide(calc.add(17, 19), 2));
    }
 
    public int add(int i, int j) {
        return i + j;
    }
 
    public int divide(int i, int j) {
        return i / j;
    }
}

In earlier times, I stepped one step further to get into the method divide. Then I got the result of add(17,19) in my parameter.

But Eclipse is offering a better solution. In the Debugging Perspective, there is a view called Expressions. Just add a new expression add(17,19) and the Debugger is printing out the result.

The result is directly printed out:

A simple, but helpful feature in Eclipse!

I started about two months with working a little bit with OSGi. I want to write here these parts, which were unclear for me at the beginning. It should be an introduction for everyone, who has never heard about OSGi.

Okay, let’s start.

The setup of OSGi is something like EJB.

1. EJB has a specification from Sun.
2. OSGi has specification from the OSGi Alliance.
3. EJB need’s an application server as a platform (Weblogic, Glassfish, Websphere, JBoss, and so on) to run an “EJB” application.
4. OSGi need’s also a platform to run “OSGi” application. Like an application server, which is implementing the EJB contract, an OSGi platform implements the OSGi framework specification. Currently, there some OSGi implementation out there. The most known are Equinox, Apache Felix or Knopflerfish.

Okay, now, we know, how OSGi is built. But what does it? In one sentence, it is something like a JVM with some extra features. OSGi extends the JVM and a Java program is running on that OSGi Platform instead of running directly on the JVM. OSGi offers some advantages, which I will mention shortly.

 
(Source: http://de.wikipedia.org/wiki/OSGi)

 
The difference between a usual Java program (a JAR file) and an OSGi program (let us call that a bundle) are some information in the META-INF\MANIFEST.MF file. These information looks like:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Example OSGi bundle
Bundle-SymbolicName: ch.stefanjaeger.osgi.example
Bundle-Version: 1.0.0

That’s it. An OSGi program (called a bundle) is nothing more than a JAR file with the MANIFEST.MF. Now, you can run this bundle in an OSGi environment like Equinox or Apache Felix.

Okay, know, we come to the advantages of OSGi. The biggest advantage is the ability to run a bundle in different versions at the same time. For example, bundle A has a dependency to bundle B in version 1.0.0 and bundle C has a dependency to bundle B in version 1.1.0.

With the usual JVM, you get a problem when running A and C in the same VM. The JVM will take the first “B” in the classpath it found. This means, it is possible, that for C gets the files from B in version 1.0.0 (instead of 1.1.0), which could result in “MethodNotFoundException” and such stuff.

With OSGi, this problem doesn’t exists anymore. In the MANIFST.MF you can define all dependencies explicitly with the version. Following example means, that our bundle has a dependency to the Apache Commons Lang Package in version between 2.0.0 and 2.1.0.

Require-Bundle: org.apache.commons.lang;bundle-version="[2.0.0,2.1.0]"

The OSGi platform checks at the beginning, that all required bundles are available and solves the classpath problem for us.

Okay, we’re almost finished. There is one important point I want to mention. Packages are not “open” as usual in JAR files. If we want to make some classes accessible for other bundles, we do have to export these packages:

Export-Package: ch.stefanjaeger.communiction.interfaces

Of course, OSGi can do more than that. But for this introduction, I think, it’s enough

• Study Guide
  http://java.boot.by/scbcd5-guide/
• Free Training Test:
  http://www.sun.com/training/certification/assessment/

I also wrote some “Factsheets”, which I used to sum up all topics. Besides Factsheet 0, they can also be used as an assistance on the daily EJB 3.0 work.

• Factsheet 0 – some stuff, which is only for the SCBCD relevant
• Factsheet 1 – Stateful and Stateless Session Beans
• Factsheet 2 – MDBs and JMS
• Factsheet 3 – Transactions and Security
• Factsheet 4 – JPA, part 1
• Factsheet 5 – JPA, part 2

If you find any mistakes in the Factsheet, just let me know.

I am currently preparing for the SCBCD 5.0 and in my opinion, it’s better than the book “Enterprise JavaBans 3.0” from O’Reilly. “Mastering EJB 3.0” covers many topics about Java EE much deeper. For example, it mentions poison messages or clustering as  has a very useful overview of all possible annotations in the appendix.

Only the part JPA is not covered as deep as in the O’Reilly book. EmbededId is only mentioned, but not described in detail.

There are interesting clean ups like “Remove unnecessary casts” or to add missing Annotations like “@Override”.

After creating your profile, simply right click to the project and choose Source – Clean up.

Nice feature. If you want to know more about this feature, you will find here more information’s.

This is not really a helpful information. Today, I found a small, simple but incredible program (open source), which is the best battery program I’ve seen so far. It’s called Batti and it’s hosted on Google (http://code.google.com/p/batti/).

This program adds a 2-3 pixel deep line at the top. The line is coloured and has a gradient. With this line, I see know always, how many battery I’ve left. This is just nice.

Unfortunately, the built in UML templates are not quite good. But fortunately, you can download UML 2.0 stencils for Visio from the internet. These stencil are perfectly UML 2.0 conform and helps to draw simple UML diagrams.

Try it out!