The . syntax can only be used to run (by "sourcing") shell scripts.

You'll need to use the java command to run a .jar file:

java -jar Minecraft.jar

If you don't have java installed, you can fix that by installing the default-jre¹ package. You can see if you already have java installed by running in a terminal:

java -version 

[1]: This will install the default openjdk Java runtime. You can use openjdk-8-jre, or openjdk-7-jre, or openjdk-6-jre instead, if you prefer - whichever is available on your version of Ubuntu.

Running a from class inside your JAR file load.jar is possible via

java -jar load.jar

When doing so, you have to define the application entry point. Usually this is done by providing a manifest file that contains the Main-Class tag. For documentation and examples have a look at this page. The argument load=2 can be supplied like in a normal Java applications:

java -jar load.jar load=2

Having also the current directory contained in the classpath, required to also make use of the Class-Path tag. See here for more information.

First, try running it on the command-line, with

java -jar <file.jar>

The user.dir property is cross-platform (see here) so it should not be the problem. However, are you using correct file separators? Remember it's '/' on UNIX and '\' on Windows.

several ways:

1. appending & at the back. However, using this, the program will still be terminated if you closed the terminal that started the program.

2. Start a screen session, and start the program inside it; you can detach the screen session and close the terminal. Later on, you can attach to the session again and found yourself back on the console as if you've been there all along. However, you will need to start a screen session before running the program, and if you forgot to do that, you can't do anything about it.

3. Use disown job control from your shell. This will detach the task from your tty and your program won't be terminated when the tty is closed. However, I don't think there's any way to reattach a disowned job.

Using systemd directives in place of shell results in a more consistent environment. This includes not relying on $PATH or the working directory by providing full paths where they are known.

Neither & for job control, nor managing SIGHUP is needed. systemd service units are already backgrounded.

Assuming the program doesn't fork, in other words the java exec keeps running, that's Type=simple

[Unit]
Description=something

[Service]
ExecStart=/usr/bin/java -jar /var/www/test.com/something.jar
StandardOutput=file:/var/log/something.out.txt
StandardError=file:/var/log/something.err.txt
Type=simple
WorkingDirectory=/var/www/test.com

[Install]
WantedBy=multi-user.target