Ok. Back to basics. It’s rare these days that any developer working in the real world has to write a basic HelloWorld.java, compile the file with the core Java Compiler (javac, the Java Programming Language Compiler) and run the file using the command line ‘java’ command. But hey, sometimes things are just fiddly to debug and you need to get back to the real basics and say HelloWorld.
Honestly, I’m mainly writing this blog post for myself as I can never remember how to do this as it is something that I haven’t had to do in many years, because you know, we have things called IDEs, Web Applications, Automated Deployments and just modern technology in general which is the space I tend to work in.
Just a general note, the common reason why you’ll need to do things like this is because your Local à Dev à Test à Live Environments are slightly different. Different enough to cause issues that you’ll only notice while you migrate your applications through the development lifecycle.
How to Create a HelloWorld.java File with a Main Method
For the purpose of this blog post, let’s forget we’re in the 20th Century using a modern IDE, let’s get Notepad open.
For the purpose of this blog post, I’m going to use a real world example that I was debugging, specifically when trying to figure out why the ‘reasonably recent’ Java 8 functionality on one application wasn’t working on one application server, which had recently had Java 8 installed with Tomcat 7 which was installed via WHM > cPanel’s Easy Apache 3. The LocalDate.now() function is only available from Java 8, and a specific release of Java 8, just to add to the confusion. None of which is really relevant for this blog post, but it puts into context why you sometimes need to go back to real basics and to try and figure out where the problem ultimately lies.
Sometimes you just need to strip back the many layers of applications, get back to the core and build up from there to identify where the problem is. Anyhow, back to where we were.
Basically create this file and save the filename as HelloWorldMain.java;
package PlayGround;
import java.time.LocalDate;
 
public class HelloWorldMain {
public static void main(String[] args) {
System.out.println("LocalDate.now(): " + LocalDate.now());
}
}
Yes that’s right, pure, core, Java. No frameworks. No libraries. No helpers. Nothing. Welcome to the 80s.
The reason why we’re creating a Main method is to ensure that when we run the compiled code, as detailed later on, it will automatically run the Main method and print the information you require to the command line console.
How to Compile Your HelloWorld.java File to Create a HelloWorld.class File
Ok, now you’ve created your HelloWorldMain.java file as outlined in the previous step, it’s time to compile this into executable code. As you’ll know, you can’t run .java files as they are (unlikely say .html files), instead you need to compile your .java files into .class files using the Java Development Kit (aka. JDK) that is installed on your machine (local machine or server).
To compile your .java file, firstly you need to open the Command Prompt (on Windows) or SSH into your Linux server. Using your command line skills (simple right…?), navigate to the location of your file and compile your .java file by running the following command;
javac HelloWorldMain.java
Awesome, you should now have a HelloWorldMain.class file created in the same directory you are currently in.
But… things aren’t always quite as they seem. We’ve not gone into any of the details yet about where you’ve just run the Java compiler command. Windows? Linux? Now this is where things get interesting. Let’s say you’re developing on a Windows machine, and you’re deploying on a Linux server. Immediately you’ve got a discrepancy.
What version of Java are you running on your Windows machine?
What version of Java are you running on your Linux machine?
What other crazy-ass things have you got configured that may be getting in the way in either environment? Hopefully nothing, but keep that in mind. Often things are configured in the background that you may not be aware of, particularly whenever you’ve used handy installers and other software to manage the installation of your environments.
So, let’s get back to the basics and find out the environments we’re working in.
Check the Java Version
On the machines you’re working on, check what version of Java is currently the priority by running the command;
java -version
You can also view on Linux the Environment Variables which can help to identify what is setup by running the command;
printenv
(aka. the Print Environment Variables command)
And on Windows you can view your Environment Variables by opening the Command Prompt ( aka. Windows Start Menu > Run > Then type ‘cmd’ without the quotes and press go ) or ( Just press Windows Key + R ), then type;
set
Which will output your Environment Variables.
Check the Java Alternatives
On Linux machines you can configure multiple versions of Java to run via the ‘alternatives’ feature. This essentially allows you to install multiple Java versions alongside each other without having to worry about the specifics. This allows you to have multiple versions of Java installed, and still configure a default.
Which basically means that when you run any default java command that it will run against whatever alternative has been configured to point at the different versions of Java installed on your Linux box.
To check this, simply run the command;
sudo alternatives --config java
Which will allow you to configure which is the default Java version – according to the Linux environment.
This is an important point to note, since an application (for example Tomcat…) can actually still override this locally within the application configuration files and hence completely ignore what you set at the base server level. The joys and flexibility (aka. frustrations…) of dealing with Linux.
How to Run Your HelloWorld.class File From the Command Line
Ok, so now we’ve covered a fair amount of the ‘basics’, let’s take a look at actually running the file we’ve just created.
Basically just run this command (with tweaks) against the file that is hosted against your own Linux server;
java -cp /home/{Root Directory for User }/public_html/WEB-INF/classes/ PlayGround.HelloWorldMain
Michael Cropper