56.1 Unix/Linux services

Spring Boot application can be easily started as Unix/Linux services using either init.d or systemd.

56.1.1 Installation as an init.d service (System V)

If you’ve configured Spring Boot’s Maven or Gradle plugin to generate a fully executable jar, and you’re not using a custom embeddedLaunchScript, then your application can be used as an init.d service. Simply symlink the jar to init.d to support the standard start, stop, restart and status commands.

The script supports the following features:

• Starts the services as the user that owns the jar file
• Tracks application’s PID using /var/run/<appname>/<appname>.pid
• Writes console logs to /var/log/<appname>.log

Assuming that you have a Spring Boot application installed in /var/myapp, to install a Spring Boot application as an init.d service simply create a symlink:

$ sudo ln -s /var/myapp/myapp.jar /etc/init.d/myapp

Once installed, you can start and stop the service in the usual way. For example, on a Debian based system:

$ service myapp start

	Tip
If your application fails to start, check the log file written to /var/log/<appname>.log for errors.

You can also flag the application to start automatically using your standard operating system tools. For example, on Debian:

$ update-rc.d myapp defaults <priority>

Securing an init.d service

	Note
The following is a set of guidelines on how to secure a Spring Boot application that’s being run as an init.d service. It is not intended to be an exhaustive list of everything that should be done to harden an application and the environment in which it runs.

When executed as root, as is the case when root is being used to start an init.d service, the default executable script will run the application as the user which owns the jar file. You should never run a Spring Boot application as root so your application’s jar file should never be owned by root. Instead, create a specific user to run your application and use chown to make it the owner of the jar file. For example:

$ chown bootapp:bootapp your-app.jar

In this case, the default executable script will run the application as the bootapp user.

	Tip
To reduce the chances of the application’s user account being compromised, you should consider preventing it from using a login shell. Set the account’s shell to /usr/sbin/nologin, for example.

You should also take steps to prevent the modification of your application’s jar file. Firstly, configure its permissions so that it cannot be written and can only be read or executed by its owner:

$ chmod 500 your-app.jar

Secondly, you should also take steps to limit the damage if your application or the account that’s running it is compromised. If an attacker does gain access, they could make the jar file writable and change its contents. One way to protect against this is to make it immutable using chattr:

$ sudo chattr +i your-app.jar

This will prevent any user, including root, from modifying the jar.

If root is used to control the application’s service and you use a .conf file to customize its startup, the .conf file will be read and evaluated by the root user. It should be secured accordingly. Use chmod so that the file can only be read by the owner and use chown to make root the owner:

$ chmod 400 your-app.conf
$ sudo chown root:root your-app.conf

56.1.2 Installation as a systemd service

Systemd is the successor of the System V init system, and is now being used by many modern Linux distributions. Although you can continue to use init.d scripts with systemd, it is also possible to launch Spring Boot applications using systemd ‘service’ scripts.

Assuming that you have a Spring Boot application installed in /var/myapp, to install a Spring Boot application as a systemd service create a script named myapp.service using the following example and place it in /etc/systemd/system directory:

[Unit]
Description=myapp
After=syslog.target

[Service]
User=myapp
ExecStart=/var/myapp/myapp.jar
SuccessExitStatus=143

[Install]
WantedBy=multi-user.target

	Tip
Remember to change the Description, User and ExecStart fields for your application.

Note that unlike when running as an init.d service, user that runs the application, PID file and console log file behave differently under systemd and must be configured using appropriate fields in ‘service’ script. Consult the service unit configuration man page for more details.

To flag the application to start automatically on system boot use the following command:

$ systemctl enable myapp.service

Refer to man systemctl for more details.

56.1.3 Customizing the startup script

The default embedded startup script written by the Maven or Gradle plugin can be customized in a number of ways. For most people, using the default script along with a few customizations is usually enough. If you find you can’t customize something that you need to, you can always use the embeddedLaunchScript option to write your own file entirely.

Customizing script when it’s written

It often makes sense to customize elements of the start script as it’s written into the jar file. For example, init.d scripts can provide a “description” and, since you know this up front (and it won’t change), you may as well provide it when the jar is generated.

To customize written elements, use the embeddedLaunchScriptProperties option of the Spring Boot Maven or Gradle plugins.

The following property substitutions are supported with the default script:

Customizing script when it runs

For items of the script that need to be customized after the jar has been written you can use environment variables or a config file.

The following environment properties are supported with the default script:

	Note
The PID_FOLDER, LOG_FOLDER and LOG_FILENAME variables are only valid for an init.d service. With systemd the equivalent customizations are made using ‘service’ script. Check the service unit configuration man page for more details.

With the exception of JARFILE and APP_NAME, the above settings can be configured using a .conf file. The file is expected next to the jar file and have the same name but suffixed with .conf rather than .jar. For example, a jar named /var/myapp/myapp.jar will use the configuration file named /var/myapp/myapp.conf.

myapp.conf.

JAVA_OPTS=-Xmx1024M
LOG_FOLDER=/custom/log/folder

	Tip
You can use a CONF_FOLDER environment variable to customize the location of the config file if you don’t like it living next to the jar.

To learn about securing this file appropriately, please refer to the guidelines for securing an init.d service.