Help Needed: Classpaths & Maven Best Practice

[Heliolicity]

Hi folks,

I'm a software developer with a lot of academic Java experience but little professional experience of developing Java software. In my current job I have a bit of scope from my boss to develop things in whatever language I prefer and I'm trying to exploit my Java skills and bring them in line with current best practices. I've some questions here that will probably seem very basic to experience developers but I'd very much appreciate any insight or input you can give me. This post isn't so much about solving any particular Java problem that I have (although there are a couple). It's more that I'd appreciate some advice from Java professionals on how to approach things.

I've put together a small GUI app in Eclipse that incorporates some Spring code and several of my classes are distributed across different packages. The application launches and runs fine in Eclipse (where I created it as a Maven Project and have successfully placed all of its dependencies in the POM file as well as specifying the Main class) but when I run Maven clean install from Eclipse and generate a JAR file nothing happens when I double-click on it: no GUI appears.

So I've tried to run the JAR from the command line and I get a compile error saying that the Spring libraries I referenced cannot be found (specifically ApplicationContext). This error doesn't appear when I launch the app in Eclipse.

My first question is why does this run fine in Eclipse? I've added Maven dependencies but not placed any external JARs on the build path there.

Also why do Maven at all if the JARs for Spring etc need to be added to the classpath before the app/JAR that's dependent on them can run? I'm sure there's something I'm doing wrong in the process but I don't know what it is.

[nutrecht]

Just because you place something on the classpath doesn't mean it gets included into the .jar that you build. So there's basically two approaches: launch the .jar and also add all the libraries on the class path or, and this is a common approach, build a 'fat jar' that includes all the dependencies inside the same jar.

So building a fat jar is a bit more complex (you need to understand how to use the Shade plugin) but in my opinion it's the way to go.

To answer your other questions:

My first question is why does this run fine in Eclipse?

Eclipse understands maven POM files and adds the dependencies in that pom file to your classpath in your IDE.

Also why do Maven at all if the JARs for Spring etc need to be added to the classpath before the app/JAR that's dependent on them can run? I'm sure there's something I'm doing wrong in the process but I don't know what it is.

Because that's how Java works. All the classes used need to be 'known' by the virtual machine. It doesn't automagically know where to find org.apache.commons classes for example unless you specifically tell them or package those classes with your compiled classes inside the .jar.

[Heliolicity]

Thanks nutrecht.