RPM installing a Java "Hello Service" in systemd

🚨 This repository is archived and therefore no longer maintained.

This repo creates a RPM package explicitly depending on Java (pulling the installation of Java >= 1.8 as a dependency), and setting up a systemd service for Red Hat Linux family of systems (Fedora, RHEL,...).

It shows how to comply with Java exit values with systemd's expectations. This is done by coding Start and Stop commands that finish doing System.exit(status) calls with proper compliant statuses (0 for success, any value greater than that for error). These commands use JMX.

Contents

Java code

The module hello-service contains a Maven project of a plain Java application that executes a "Hello Task" every three seconds. This task (com.gvisoc.task.Hello) is a TimerTask and is wrapped in two operation commands that comply with the Exit codes expected by systemd (0 for OK, anything greater for other conditions).

The process wrappers are two classes, com.gvisoc.hello.Start and com.gvisoc.hello.Stop. The first one creates the task and controls the timer, exposing a Management Bean (see com.gvisoc.hello.StopMonitor) with a method stop. This method stop is called via JMX from com.gvisoc.hello.Stop.

The configuration of the process is dine via the Apache Commons library for configuration, in order to be able to refer environment variables written by an external file, set up in the Unit File (see sytemd unit file and Use). This file is touched by the RPM installation process (defined in the Maven POM file, see RPM configuration), so that is created if didn't exist. No environment variables are provided by default, that is something to be done with configuration managers like Ansible (to be explored).

Start and stop scripts

Two scripts are located under hello-service/src/main/resources: start.sh and stop.sh.

systemd unit file

The unit file is located under hello-service/src/systemd: hello.service.

RPM configuration

The file reponsible for the configuration is hello-service/pom.xml and uses MojoHaus' rpm-maven-plugin.

Use

1. Change the user in hello-service/pom.xml and in hello-service/src/systemd/hello.service from gabriel to a value that exists in your system.
2. Build the project and create the RPM package by executing the usual Maven tasks, mvn clen compile package
3. Install the package in your system (load it and run sudo yum install <rpm>). If there is no Java runtime installed, it should install one of version 11 or greater.
4. Execute the following command to refresh the systemd units without rebooting: sudo systemctl daemon-reload.
5. Start the service with sudo systemctl start hello
6. Check the output of the service with tail -f /tmp/hello.log:

10:26:07.603 [com.gvisoc.hello.Start.main()] DEBUG com.gvisoc.hello.Start - Scheduling greeting service with delay (ms): 3000
10:26:07.667 [com.gvisoc.hello.Start.main()] DEBUG com.gvisoc.hello.Start - Wait for exit command
10:26:10.612 [Timer-0] INFO  com.gvisoc.hello.task.HelloTask - Hello, gabriel!
10:26:13.612 [Timer-0] INFO  com.gvisoc.hello.task.HelloTask - Hello, gabriel!
10:26:16.611 [Timer-0] INFO  com.gvisoc.hello.task.HelloTask - Hello, gabriel!
10:26:19.613 [Timer-0] INFO  com.gvisoc.hello.task.HelloTask - Hello, gabriel!

7. Restart the service with sudo sytemctl restart hello. Stop the service with sudo systemctl stop hello. Check the result in the log (more logs from the Stop command may appear):

10:26:43.628 [Timer-0] INFO  com.gvisoc.hello.task.HelloTask - Hello, gabriel!
10:26:44.855 [com.gvisoc.hello.Start.main()] DEBUG com.gvisoc.hello.Start - Exiting with status: 0

8. Check the service status with sudo systemctl status hello.service -l.

References

Check this material:

1. MojoHaus' RPM Plugin
2. systemd unit file reference
3. For further steps and check / improve portability, check systemd unit packaging

Limitations

The last update fixes some dependency security issues and also reduces the complexity of this repo by avoiding issues with SeLinux and environment files' permissions on service start.

• No Java Tests (sorry). The focus of this repo is the RPM and systemd part.
• No SSL JMX communications. The stop() exposed operation has no parameters (hence no information had to be encrypted) and, regarding the authenticated execution through mTLS, it's out of the scope.
• The result was tested in Fedora 34.
• No further limitations other than those derived from the design itself, and Windows rpm building. Although not tested / explored, this last can be done with a Cygwin, a WSL, or a "dev-environment-in-a-container" environment or similar.

Feedback and pull requests are welcome.