Convert a java application into a Linux service

First of all, a Linux Service is an application that runs in the background and performs an essential task. One of the advantages to use a service is that you do not need to start it manually, you can configure it to start with the system using this command:

sudo systemctl enable <service_name>

Some of the most commonly used services are:

• httpd
• mysql
• chronyd
• sshd

You can list all the services inside your Server using

systemctl list-units --type=service

Also you can see the status of an specific service using:

systemctl status <service_name>

Let see an example:

[ernesto.lopez@test-vm ~]$ systemctl status chronyd.service
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: active (running) since Tue 2021-11-02 12:31:04 MST; 3 weeks 3 days ago
     Docs: man:chronyd(8)
           man:chrony.conf(5)
 Main PID: 1212 (chronyd)
    Tasks: 1 (limit: 98556)
   Memory: 1.1M
   CGroup: /system.slice/chronyd.service
           └─1256 /usr/sbin/chronyd

To explain this output:

• chronyd.service - NTP client/server: is the name of the service and a small description of it.
• Loaded: loaded (/usr/lib/systemd/system/chronyd.service; : means that the services located in the path that appears was loaded in memory
• enabled; vendor preset: enabled) : this part indicate that the service is enable, so it will start at boot, this is achieved using the command sudo systemctl enable
• Active: active (running) : this means that the service is up and running in the system, this line usually appears on green if you are using a normal user. This line also notifies the user when this service was started.
• Main PID: 1212 (chronyd) : this line shows the process ID for this service. you can then use ps 1212 to get the command associated with the service.

These lines represent an important aspect to evaluate a service, cGroup deserve a single post entry.

NOTE Before converting anything to a service, make sure you have installed java on your server. A quick using dnf and java 8 example:

sudo dnf -y update
sudo dnf -y install java-1.8.0-openjdk-devel
java –version

 #Find your java home path
sudo alternatives --config java 

Add this line into your profile file (/etc/profile) to easier the following steps:

export JAVA_HOME=$(dirname $(dirname $(readlink $(readlink >$(which javac)))))
export PATH=$PATH:$JAVA_HOME/bin
export >CLASSPATH=.:$JAVE_HOME/jre/lib:$JAVA_HOME/lib:$JAVA_HOME/lib/tools.jar

Save the file and set the system to use this profile

sudo source /etc/profile

You can verify that the configurations were correct, by running:

echo $JAVA_HOME
echo $PATH
echo $CLASSPATH

Back to the starting our java app, suppose our app is in a .jar file called my-own-app.jar

JAR stands for Java ARCHIVE, it is a file format used to aggregate many java files into one.

More on the official documentation from Oracle

And for example we have our application stored inside /opt/my-own-app/ directory

First thing that we are going to do is to create a script that start the app, imagine we are going to convert our java app in a bash script (Just an imagination exercise!)

cd /opt/my-own-app
touch my-own-app.sh
vi !$

Then, we need to tell the script to start the app in the .jar file

#!/bin/bash 
# Script to start MyOwnApp
echo "--------------------WAIT-------------------" 
# Start the app 
# minimum heap 512MB, maximum heap 1GB 
cd /opt/my-own-app 
java -Xms512M -Xmx1G -jar my-own-app.jar &

Here we are creating a script that move tho the directory where the .jar file is located and then execute the java command which is used to launch a java application. The elements to consider:

• -Xms512M specifies the initial sizeo of the memory allocation pool, in this case we append M to indicates it is megabyte. This value must be determined based on app consumption.
• -Xmx1G is the maximum size of the memory allocation pool, this case indicates 1G but can vary depending on your app.
• -jar define the location of the jar file, because in the previous step in the script we move to the /opt/my-own-app dir, we use just the name of the file in this case.

Save the file and change the permissions to make it an executable:

sudo chmod u+rwx my-own-app.sh
ls -l | grep my-own-app.sh 

Next step is to create a service unit:

vi /etc/systemd/system/my-own-app.service 

Note A service unit describes how to manage a service or application on the server.

Insert the following information on the file:

[Unit]
Description=my own app starting
After=network.target

[Service] 
Type=forking
ExecStart=/opt/my-own-app/my-own-app.sh
TimeoutStartSec=0

[Install] 
WantedBy=default.target

The important aspects of this:

• WantedBy=default.target defines how a unit should be enabled, this represents the default and will create a file .want append inside /etc/systemd/directory
• Type=forking tells the systemd that the process is still running even though the parent process exited.
• ExecStart=/opt/my-own-app/my-own-app.sh is the file to be executed.
• After=network.target the service must be started after network services.

The following step is to reload the systemd daemon

sudo systemctl daemon-reload

And finally enable and start the service

sudo systemctl start my-own-app.service
sudo systemctl enable my-own-app.service

You can verify if the service started ok or if there was any trouble using

sudo systemctl status my-own-app.service