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Install the Java major version required by your game server, not simply the newest one. If the server documentation does not specify a version, update APT and install Ubuntu’s default JDK with sudo apt update followed by sudo apt install default-jdk. Verify the result with java -version. Use default-jre only when the server needs a runtime and no compiler or Java development tools. Run the server as an unprivileged account, keep its files in a dedicated directory, and allow only its documented network port through Ubuntu’s firewall and any upstream cloud firewall.
Confirm the Java version before changing Ubuntu
Use the server’s compatibility requirement as the authority
Java compatibility is determined by the server build, mod loader, plugins and sometimes the world-management tools around it. Find the project’s current installation or compatibility documentation and identify the required major version, such as 17 or 21. A message about an unsupported class-file version almost always means that the server was built for a different Java major than the one being used.
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When no major version is specified
Start with Ubuntu’s default-jdk package. The package follows the default Java development kit selected for your Ubuntu release, so the major version can change when you move to a different release. This is convenient for a new installation, but it is not a promise that every Ubuntu version supplies the same Java major.
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Canonical states that the Ubuntu archive provides OpenJDK packages for supported long-term-support releases. On Ubuntu 24.04 LTS, OpenJDK 17 and OpenJDK 21 are TCK-certified. That certification does not make either version universally correct for every server; the server’s compatibility statement still wins.
Check the Ubuntu release and architecture
Before installing, establish which Ubuntu release you are administering. The release determines what default-jdk points to and which versioned packages are available. You can inspect the release information with:
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Choose between a JRE and a JDK
| Package choice | What it provides | Use it when | Trade-off |
|---|---|---|---|
default-jre |
The Java runtime maintained in Ubuntu’s archive | The server only executes Java and its documented tooling does not compile code | Smaller installation, but no javac compiler or full development tool set |
default-jdk |
The runtime plus the Java compiler and development tools | You need to build plugins, compile code, use build tools, or want the broadest compatibility with server administration utilities | Larger installation than a runtime-only package |
openjdk-XX-jdk |
A specific OpenJDK major version, such as openjdk-21-jdk |
The server documentation pins a major version | More predictable compatibility, but you must manage that version deliberately when upgrading Ubuntu |
Ubuntu’s Java runtime guidance documents default-jre for runtime-only use. A JDK also contains the runtime, so installing a JDK is the safer choice when the requirement is unclear and the modest additional disk usage is acceptable.
Install Java with APT
1. Refresh package metadata
- Log in with an account that can use
sudo. - Refresh the local package index:
sudo apt update - Read any repository errors before continuing. A failed update can leave APT unaware of packages that are available in the enabled Ubuntu repositories.
2. Install the default JDK when the server has no pinned major
sudo apt install default-jdkAPT resolves this metapackage to the default JDK for the Ubuntu release. The exact Java major shown after installation is release-dependent, so always verify it rather than inferring it from an old guide.
3. Install a pinned major when the server requires one
Use the versioned OpenJDK package pattern documented by Ubuntu and the server project. For example, a server that requires Java 21 can use:
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sudo apt install openjdk-21-jdkReplace 21 only with a major that the server explicitly supports and that your Ubuntu release provides. If APT reports that the package cannot be found, do not substitute a different major at random; troubleshoot the release and repositories first.
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4. Install only a runtime when that is sufficient
sudo apt install default-jreThis is appropriate when the server’s documented launcher needs only the Java runtime. If you later need to compile a plugin or a build tool reports that javac is missing, install the matching JDK rather than mixing an unrelated Java major.
Verify the installation and selected major
Check the runtime
java -versionRead the major version in the output and compare it with the game server’s compatibility documentation. The complete output varies by Ubuntu release and package revision; record it when asking for support because the vendor may need the precise build string.
Check development tools when needed
javac -versionA successful result confirms that the compiler from a JDK is available. A runtime-only installation normally will not provide this command.
Inspect installed packages and alternatives
If more than one Java major is installed, inspect the packages and the command alternatives:
apt list --installed 2>/dev/null | grep -E 'openjdk|default-jre|default-jdk'
sudo update-alternatives --config java
sudo update-alternatives --config javacThe alternatives menu lets you select an installed executable. Select the same major for java and javac when you use build tooling. Changing the alternative does not install a missing major; install the required version first.
Prepare an isolated account and directory
Use an unprivileged service account
Do not run a network-facing game server as root. Create a dedicated system account and group, then give it ownership of only the server directory. One possible layout is:
sudo adduser --system --home /opt/game-server --group game
sudo install -d -o game -g game /opt/game-serverThe account name and path are choices, not requirements imposed by Java. Use a different path if your operating procedures require it, but keep server data separate from operating-system files and from unrelated applications.
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Place files and protect permissions
Copy the server files into the dedicated directory and set ownership so the service account can read and write its world, logs and configuration:
sudo chown -R game:game /opt/game-server
sudo -u game find /opt/game-server -maxdepth 1 -type f -printf '%fn'Keep backups outside the live directory or in storage that the game process cannot overwrite. Never make the entire server tree world-writable merely to work around a permission error.
Accept the EULA where the server requires it
Many game servers will not start until their publisher’s EULA is accepted in a configuration file. Read that server’s terms and follow its documented method. The exact filename, setting and startup command differ by project, so do not copy a command intended for another server.
Launch and test the server without inventing flags
Follow the vendor’s launch command
Use the server project’s documented command from its own directory and run it as the dedicated account. Java flags, the JAR filename, memory settings and first-run options vary by server; there is no universal command or safe universal heap size.
cd /opt/game-server
sudo -u game -- java -versionThe second command confirms which Java executable the service account sees. Start the actual server with the project’s documented launcher after completing its EULA and configuration steps.
Confirm local listening before debugging the internet path
After startup, check the process and listening sockets from the Ubuntu host:
ps -u game -f
sudo ss -lntupMatch the listening address, protocol and port to the server documentation. A process bound only to a loopback address can be reachable from the host but invisible to remote players; a process bound to all interfaces still requires firewall and provider rules.
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Open only the documented game port
Identify protocol and port from the server documentation
Do not guess a port or assume that every server uses TCP. Some servers require UDP, some require both, and some use additional query or voice ports. Permit only the ports the selected server documents, and list each protocol explicitly.
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Ubuntu identifies ufw as its default firewall configuration tool. Check its current state first:
sudo ufw status verboseAdd a rule for the documented port and protocol, replacing the example values with the server’s actual settings:
sudo ufw allow <port>/tcp
sudo ufw allow <port>/udp
sudo ufw statusRun only the rule that the server needs. If UFW is inactive, review the existing SSH rule and your recovery access before enabling it; otherwise you can lock yourself out of a remote machine. The exact command to enable UFW is sudo ufw enable, but apply it only after confirming that your administration path is allowed.
Configure the upstream firewall too
A cloud security group, hosting-panel firewall or separate edge device can block traffic before it reaches Ubuntu. Mirror the same narrow port-and-protocol policy there. Opening a port in UFW cannot override a provider rule that denies it, and allowing it at the provider does not bypass a local UFW denial.
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Test from the correct network
Test locally first, then test from a different network rather than relying on a client inside the same LAN. If local connections work but remote players cannot connect, inspect the listening address, UFW status, the provider security group and any router or NAT forwarding. Do not expose administrative ports just to make the game connection work.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Size Ubuntu and Java for the workload
| Figure | What it means | What it does not mean |
|---|---|---|
| 2 GB RAM | Ubuntu Server’s documented installation recommendation | Not a guaranteed amount for a game world, player population, mods or view distance |
| 5 GB disk | Ubuntu Server’s documented installation recommendation | Not enough to estimate world growth, logs, backups, snapshots or mod files |
| amd64, arm64, ppc64el, s390x | Architectures listed in Ubuntu Server documentation | Not proof that a particular game server or native dependency supports every architecture |
Allocate resources according to concurrent players, world size, simulation distance, mods, generated terrain, backups and log retention. Monitor memory pressure, disk consumption and tick or latency metrics supplied by the server rather than treating Ubuntu’s installation figures as performance targets.
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Self-hosting versus a VPS
| Consideration | Self-hosted machine | VPS |
|---|---|---|
| Administration | You maintain hardware, power, operating-system updates and router configuration | The provider supplies the virtual machine platform; you still maintain Ubuntu, Java and the game server |
| Resources | Capacity depends on the hardware you own and what else runs on it | Plan size and CPU sharing depend on the selected provider offering; verify the advertised limits |
| Network controls | Router NAT, ISP policy and local firewall may all matter | UFW and a provider security group or external firewall usually both need configuration |
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Troubleshoot common installation failures
APT cannot find the package
- Confirm the Ubuntu release with
cat /etc/os-release. - Run
sudo apt updateand read repository errors. - Check that the required Ubuntu repositories are enabled for that release.
- Verify the package spelling, for example
openjdk-21-jdk. - Search available packages with
apt-cache search openjdkand choose only a major supported by the server.
Do not download an unrelated installer or replace the required major simply because an old tutorial used a different package name.
java -version shows an unexpected major
default-jdk follows the Ubuntu release default, which can differ between releases. Inspect installed packages and run sudo update-alternatives --config java. If the needed major is not listed, install its matching versioned JDK, then select it. Recheck the result as the same service account that will launch the server.
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The server reports an unsupported class-file version
The server build and selected Java major are incompatible. Compare the error, java -version output and the project’s compatibility matrix. Install or select the required major; changing memory flags or firewall rules will not resolve a class-file mismatch.
The server starts locally but remote players cannot connect
- Use
sudo ss -lntupto verify that the process is listening on the documented port and protocol. - Confirm it is not bound only to loopback.
- Check
sudo ufw status verboseand remove accidental rules that do not match the documented service. - Check the cloud security group, hosting firewall, router forwarding and any ISP restrictions.
- Test from an external network and distinguish a failed connection from an authentication or server-list problem.
The service account cannot read or write files
Check ownership and permissions on the server directory, its parent directories and the specific world or configuration files. Correct ownership with a targeted chown; do not run the entire process as root or make files globally writable.
Keep Java, Ubuntu and the server maintainable
Patch deliberately
Keep Ubuntu packages current, but schedule upgrades around the game server. A Java or operating-system update can change the selected default major or expose an incompatibility with a particular server build. Before a maintenance window, back up worlds and configuration, record java -version, and test the server build with the proposed Java package.
Pin the major when compatibility matters
If the project documents a specific LTS major, use the corresponding versioned package instead of relying on default-jdk. Test upgrades in a copy of the world first. You can retain more than one installed major and select the one required by the service with the alternatives system, but document that choice so a later administrator does not silently switch it.
Back up before upgrades and world changes
Stop or quiesce the server using its documented procedure before copying world data. Keep at least one backup outside the live server directory and verify that a restore is possible. Include configuration, plugins or mods, the Java major, and any service or firewall rules needed to recreate the installation.
Quick Recap
Installation checklist
- The server’s documented Java major is recorded; if none is specified, Ubuntu’s release default is acceptable.
sudo apt updatecompleted without repository errors.- The chosen package is installed:
default-jdk,default-jreor the requiredopenjdk-XX-jdk. java -versionmatches the server requirement, andjavac -versionworks when build tooling is needed.- The server runs under a dedicated unprivileged account with a separate data directory.
- Any required EULA acceptance and vendor configuration steps are complete.
- The process listens on the documented address, port and protocol.
- UFW and any provider firewall allow only those required ports.
- RAM, disk, architecture support and backup procedures are appropriate for the expected workload.
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