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Install the Java major version required by your game-server software, not an arbitrary version. On Ubuntu, update APT and install either the release-selected default-jdk or the exact versioned package (for example, openjdk-21-jdk). Use default-jre only when the server needs a runtime and no Java-based build or plugin tools. Verify the result with java -version, run the server as an unprivileged account, and allow only its documented listening port through Ubuntu’s firewall and any upstream cloud firewall.
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Decide which Java major version the server requires
Java packages are not interchangeable across all server builds. A server compiled for one Java major can refuse to start on an older runtime, while a newer runtime can expose compatibility problems in older libraries or plugins. Begin with the game-server project’s installation or compatibility page and identify the required Java major version (such as 17 or 21). That requirement takes precedence over Ubuntu’s package default.
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If the project does not specify a major version, start with Ubuntu’s archive default. The default-jdk package is a moving metapackage: its underlying OpenJDK major can change when you move to a different Ubuntu release. Consequently, record the Ubuntu release and the Java version you actually installed rather than assuming that “default” always means the same number.
Ubuntu 24.04 LTS provides TCK-certified OpenJDK 17 and 21 packages. Certification does not make either version universally correct for every game server; use the version listed by the server vendor or project.
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Default package versus a versioned package
| Choice | Install command | Compatibility behavior | Upgrade behavior |
|---|---|---|---|
| Ubuntu default JDK | sudo apt install default-jdk |
Follows the Java major selected for that Ubuntu release. Useful when the server has no pinned requirement. | Can move to a newer Java major when Ubuntu changes the metapackage or when you upgrade the operating system; test the server after such changes. |
| Versioned OpenJDK | sudo apt install openjdk-21-jdk (replace 21 with the required major) |
Pins the package family to the major your server documents. | Receives updates within that major while avoiding an automatic switch to a different major; you choose when to migrate. |
Package availability depends on the Ubuntu release and enabled repositories. If APT cannot find the exact version, confirm that the release is supported, the required repository components are enabled, and your package lists are current before changing installation sources.
JRE or JDK: what a game server actually needs
A Java Runtime Environment (JRE) contains the components needed to run Java programs. Ubuntu’s maintained runtime metapackage is default-jre. A Java Development Kit (JDK) includes that runtime plus development tools, including the Java compiler and diagnostic utilities.
| Need | Recommended package | Why |
|---|---|---|
| Run a prebuilt server only | default-jre or the matching versioned JRE package |
Smaller installation when no compiler or build tooling is required. |
| Compile plugins, mods or helper code; use build tooling; diagnose with JDK tools | default-jdk or the matching versioned JDK package |
Provides the runtime and javac plus the additional development tools. |
| Documentation says “Java X” but does not distinguish JRE and JDK | Matching JDK | It is the safer operational choice because it can run the server and supports tooling if a plugin or update process later needs it. |
Do not install an old Java 8 or 11 package merely because a tutorial is old. Install those majors only when the selected server explicitly requires them and the package is available for your Ubuntu release.
Check the Ubuntu host before installing
Use a supported 64-bit Ubuntu Server installation. Ubuntu documents amd64, arm64, ppc64el and s390x as supported Server architectures. The operating-system installation guidance lists at least 2 GB of RAM and 5 GB of disk space; those figures are installation minimums, not adequate targets for a populated game world.
Confirm the host architecture and release so that package and server binaries match:
uname -m
. /etc/os-release
printf '%s %sn' "$NAME" "$VERSION_ID"
Before installing, have the server’s Java major, world directory, documented launch procedure, listening port and protocol available. A port number cannot be chosen safely from Java alone; it belongs to the particular game-server software.
Install Java with APT
Option A: install Ubuntu’s default JDK
- Refresh package metadata:
sudo apt update - Install the JDK metapackage:
sudo apt install default-jdk - When APT shows the package list and disk usage, review it and confirm with
Y.
This route is appropriate when the server documentation accepts Ubuntu’s release default or does not pin a Java major.
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Option B: install a specific OpenJDK major
- Refresh package metadata:
sudo apt update - Substitute the documented major in the package name. For a server requiring Java 21, for example:
sudo apt install openjdk-21-jdk - Confirm the package transaction and wait for APT to finish.
Use the same pattern for another major only after checking that the package exists in your Ubuntu release. Installing a versioned JDK does not by itself guarantee that an already installed Java command will select it; verify the active alternative in the next section.
Runtime-only installation
For a prebuilt server that explicitly needs no compiler or JDK tooling, Ubuntu documents:
sudo apt update
sudo apt install default-jre
If the project requires a pinned major, select its matching versioned runtime package instead of the default metapackage. A JRE is sufficient only when every server, plugin and maintenance task you will run is compatible with a runtime-only installation.
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Verify the active Java installation
Run the executable that the server will use:
java -version
The output format and patch number vary by Ubuntu release and package updates. Check the major number in the output against the server’s requirement; do not rely on the package name alone. If you installed a JDK and need build or plugin tooling, also run:
javac -version
javac should report the expected major. If it is missing after a JDK installation, inspect the package transaction and your command path rather than replacing files manually.
When more than one Java major is installed
Multiple JDKs can coexist. Ubuntu uses alternatives to select the command reached through the normal path. Inspect and, if necessary, select the required executable:
update-alternatives --list java
sudo update-alternatives --config java
Choose the path whose major matches the server. Repeat the check with update-alternatives --config javac when compilation tools must use the same major. Run java -version again after changing the selection. A service started by a supervisor can have a different environment from your interactive shell, so test the command in the service’s actual launch context.
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Create a safer account and directory for the server
Do not run a network-facing game process as root. Create a dedicated unprivileged account and keep the server files separate from system directories and from unrelated applications:
sudo adduser --system --home /srv/game-server --group game-server
sudo install -d -o game-server -g game-server /srv/game-server
The account name and path are examples; use a name that fits your host. Copy or download the server files according to that project’s documentation, then assign ownership only to the dedicated account. Avoid granting it sudo access. Keep backups, world data and logs in known locations, and ensure the account can write the directories the server needs without making the whole filesystem writable.
Accept a server EULA only when that project requires it and only after reading its terms. The EULA file name, acceptance setting and launch command differ by game, so follow the selected project’s instructions rather than copying a command from another server.
Start the game server with the project’s documented command
Change to the server directory and run the exact command supplied by the server project. Since no particular game is specified here, there is no universal filename, memory flag or startup command to substitute:
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sudo -u game-server -H sh -c 'cd /srv/game-server && <documented-server-command>'
Replace the text in angle brackets with the project’s real command; do not type the brackets. If the documentation specifies JVM options, place them exactly where it requires. Memory settings, garbage-collection options, mod loaders and Java module flags are server-specific and should not be invented from a generic tutorial.
For an unattended host, run the process under a service manager or a terminal multiplexer as documented for your server. Whichever method you choose, configure log retention, restart behavior and a clean shutdown so that world data is not damaged during maintenance.
Open only the required network port
A server that works locally but cannot be reached remotely usually has a binding or firewall problem, not a Java installation problem. First identify the port and protocol in the game-server documentation and confirm that the process is listening on the intended address.
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sudo ss -ltnup
Look for the server’s process and documented port. Binding to a loopback address such as 127.0.0.1 makes it reachable only from the host; a remote-access server generally needs to bind to the host’s appropriate interface, as the game documentation explains.
Configure Ubuntu’s UFW firewall
Ubuntu identifies ufw as its default firewall configuration tool. Allow only the documented port and protocol, then inspect the active rules:
sudo ufw allow <port>/tcp
sudo ufw status
Use /udp instead when the server documentation specifies UDP, or create separate rules when it genuinely uses both protocols. Replace <port> with the real number; never open a broad range “just in case.” If UFW is inactive, enable it only after allowing your administration method (for example, SSH) so you do not lock yourself out:
sudo ufw allow OpenSSH
sudo ufw enable
sudo ufw status verbose
The SSH profile name can vary if you use a nonstandard configuration. Verify your existing remote-access rule before enabling the firewall.
Check the provider or upstream firewall
Cloud VPS and hosted-network platforms often place a security group, network ACL or external firewall in front of the Ubuntu host. The port must be allowed there as well as in UFW, with the correct protocol and source range. For a private server behind a home router, configure port forwarding to the host’s private address and account for the ISP’s inbound restrictions. Limit source addresses when the game supports a private player group; otherwise the port is exposed to the public internet and should be protected with the game’s own authentication and update practices.
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The documented Ubuntu minimum of 2 GB RAM and 5 GB disk describes installing Ubuntu Server. A game world can need considerably more, depending on concurrent players, world size, mods, simulation distance, logging and backups. Treat those variables as capacity inputs rather than promising a universal Java heap size.
- RAM: leave room for Ubuntu, the Java process, native memory, file cache and monitoring. Increase allocation when player count or mod complexity grows, while avoiding a heap so large that the host swaps.
- Storage: budget for the world, generated chunks, logs, downloads, snapshots and at least one off-host backup. Monitor free space because a full filesystem can stop both the server and its logs.
- CPU: check the server’s tick or simulation guidance. Some workloads depend more on single-thread performance than on total core count; mods and plugins can change that balance.
- Architecture: use a server build and native dependencies that support the host architecture. amd64 is common, while arm64, ppc64el and s390x are also listed for Ubuntu Server; compatibility still belongs to the game project.
Measure during realistic play sessions. Record memory pressure, CPU saturation, tick-time warnings and disk growth before changing Java flags or moving to a larger host.
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Self-hosting or a VPS: operational trade-offs
| Factor | Self-hosted Ubuntu machine | VPS |
|---|---|---|
| Administration | You maintain hardware, power, operating-system updates and local networking. | The provider supplies the virtual hardware; you still maintain Ubuntu, Java, the game server and backups. |
| Resources | Capacity depends on the machine and what else runs on it; noisy neighboring applications are under your control. | Plan size gives more predictable allocated resources, subject to the provider’s performance policy. |
| Connectivity | You may need router forwarding, a stable address and ISP support for inbound traffic. | Usually includes a public network interface and provider-level security groups, but those rules must be configured. |
| Recovery | Hardware failure can require local replacement unless you maintain off-site backups. | Snapshots and replacement instances may simplify recovery, but they are not a substitute for tested game-world backups. |
Both choices use the same APT installation and Java verification process. Select based on the player community, uptime expectations, network control and your willingness to operate hardware.
Keep Java and Ubuntu maintained without surprising the server
APT security updates are important, but a Java major change can affect an older server or plugin set. Apply routine package updates during a maintenance window:
sudo apt update
sudo apt upgrade
Before upgrading Ubuntu or intentionally changing Java majors, make a restorable backup of the world and configuration, record the current java -version, and test the server build and plugins on a staging copy. Schedule a controlled restart rather than terminating a live process. Keep the server’s compatibility matrix with your maintenance notes so you can roll back to the previously supported major if testing fails.
Troubleshooting APT, Java and connections
APT says it cannot locate the package
- Check the spelling and package name for your Ubuntu release.
- Run
sudo apt updateand look for repository or signature errors. - Confirm that the release is supported and the required archive components are enabled.
- Use
apt policy default-jdkorapt policy openjdk-21-jdkto see whether APT has a candidate version.
Do not download an unrelated binary installer simply to bypass an APT error; resolve the release or repository issue first, or choose a package supported by that release.
java -version reports an unexpected major
Another JDK may already be installed, or the alternatives selection may still point to it. Inspect update-alternatives --list java, select the required path with sudo update-alternatives --config java, and verify again. If a service manager launches a different Java binary, inspect its environment and use an explicit path when the server documentation permits it.
The server reports an unsupported class-file version
This error means the server or one of its components was compiled for a Java major that does not match the active runtime. Read the server project’s compatibility matrix, install or select the required major, and repeat java -version. Do not “fix” the message by randomly trying newer versions; the project may require an older supported runtime.
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- Confirm that the process is running and listening with
sudo ss -ltnup. - Check the configured bind address and documented port and protocol.
- Confirm the matching UFW rule with
sudo ufw status verbose. - Check the cloud security group, provider firewall or router forwarding rule.
- Test from a network outside the server’s LAN; a local test can succeed even when inbound traffic is blocked upstream.
Change one layer at a time and record the result. Opening every port or disabling the firewall hides the fault and creates unnecessary exposure.
Quick Recap
Installation checklist
- Identify the server’s documented Java major; if none is specified, use Ubuntu’s
default-jdk. - Install with
sudo apt updatefollowed by the appropriate JDK, versioned JDK, or runtime package. - Verify both
java -versionand, when tooling is needed,javac -version. - Run the process as a dedicated unprivileged account in its own directory.
- Follow the selected server’s EULA and launch instructions instead of using a generic command.
- Match UFW and upstream firewall rules to the documented port and protocol only.
- Provide resources based on players, world data and mods; Ubuntu’s 2 GB RAM and 5 GB disk figures are not game-performance targets.
- Back up the world, test Java or Ubuntu upgrades, and schedule controlled restarts.
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