Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsInstall the Java major version required by your game-server project, then verify it before you copy in server files. If the project does not specify a version, Ubuntu’s default-jdk is the safest starting point: run sudo apt update followed by sudo apt install default-jdk. Use default-jre only when the server needs a runtime and no Java-based build, plugin or diagnostic tools. Finally, permit only the documented game port in Ubuntu’s firewall and in any cloud-provider firewall.
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1. Check the server’s Java requirement first
Java compatibility is determined by the server software, not by Ubuntu alone. Read the project’s installation page, release notes or compatibility matrix and identify the required major version, such as 17 or 21. A server compiled for one major release can refuse to start on another, producing an “unsupported class version” or similar error.
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If the project explicitly requires a major version, install that versioned OpenJDK package. For example:
sudo apt update
sudo apt install openjdk-21-jdkThe package name must match the Java release available for your Ubuntu version. Do not copy an old tutorial that installs Java 8 or Java 11 unless the server documentation specifically requires it. Canonical states that OpenJDK 17 and OpenJDK 21 are TCK-certified on Ubuntu 24.04 LTS; another Ubuntu release can have a different default Java major.
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When no major version is pinned, install Ubuntu’s moving default:
sudo apt update
sudo apt install default-jdkdefault-jdk follows the Java version selected as the default for that Ubuntu release. That makes it convenient for a new installation, but it also means a later Ubuntu package update can move the default to a newer major. A server with a strict compatibility range should use a versioned package instead.
2. JRE or JDK: which package belongs on a game server?
A Java Runtime Environment (JRE) supplies what is needed to run Java applications. Ubuntu’s maintained runtime package is default-jre:
sudo apt update
sudo apt install default-jreA Java Development Kit (JDK) contains the runtime plus development tools, including the javac compiler. Most dedicated game servers can run with a JRE, but a JDK is often the more practical administration choice when you install plugins, compile extensions, run build scripts or need Java diagnostic tools.
| Choice | Install command | Use it when |
|---|---|---|
| Ubuntu default JRE | sudo apt install default-jre |
The server is runtime-only and does not require Java tooling. |
| Ubuntu default JDK | sudo apt install default-jdk |
The server needs a runtime and you may need compilers or administration tools; no fixed major is required. |
| Versioned JDK | sudo apt install openjdk-21-jdk (example) |
The server documentation pins a Java major and that package is available for your Ubuntu release. |
The JRE-versus-JDK choice does not override the required major version. If a project requires Java 17, install the Java 17 runtime or JDK rather than whichever major happens to be Ubuntu’s current default.
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3. Prepare Ubuntu and install Java with APT
Update package metadata
Refresh APT’s package index before every installation attempt:
sudo apt updateThis downloads the current package lists from the repositories configured for your Ubuntu release. It does not upgrade the operating system or restart the game server.
Install the selected package
Choose exactly one package path based on the compatibility decision above. APT will show the packages and disk space it plans to use; confirm only after checking that the package name and release are correct.
Confirm the installed Java
Run:
java -versionRead the major version in the output and compare it with the server project’s requirement. The exact wording and patch number are release-dependent, so use the major version for compatibility decisions. If you installed a JDK or your tooling needs a compiler, also run:
javac -versionjavac should report the compiler supplied by the installed JDK. A successful java -version check proves that Java is available to your shell; it does not prove that a particular server build, mod set or plugin is compatible.
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4. When the default package is not the right package
Use a versioned package for a pinned server
If the project’s documentation names Java 21, install openjdk-21-jdk (or the corresponding runtime package when the project explicitly supports runtime-only deployment). Replace 21 with the required major and use the package naming available in your Ubuntu archive.
Understand default-package upgrades
default-jdk and default-jre are metapackages that track Ubuntu’s chosen default. They are useful when the project follows the Ubuntu-supported default, but they are not a promise that every Ubuntu release uses the same Java major. Record the major shown by java -version after installation and after planned operating-system upgrades.
Resolve an unexpected major version
If java -version reports a different major than the server requires, inspect the installed alternatives and packages before launching the server. Install the required versioned package, then select the intended Java executable through Ubuntu’s alternatives mechanism if more than one version is installed. Re-run both version commands and test the server against its documented compatibility matrix.
5. Create a safer account and directory for the server
Do not run a network-facing game process as root. Use a dedicated, unprivileged account and keep the server files in a directory owned by that account. This limits the damage if the server, a plugin or an uploaded world file is compromised.
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- The game process has no administrative privileges.
- Its worlds, configuration, logs and downloaded components are outside unrelated users’ home directories.
- Backups and updates can be performed without changing ownership to
root. - Only the files and directories required by the server are writable by its service account.
After Java is verified, follow the game project’s own instructions to upload or download its server files, accept its EULA where applicable, configure the world and launch it. Because no particular game is named here, there is no universal JAR filename, launch command or Java flag that can be stated safely.
6. Size Ubuntu for the workload, not just the installation
Ubuntu Server documentation lists 64-bit amd64, arm64, ppc64el and s390x architectures. It recommends at least 2 GB of RAM and 5 GB of disk for the Ubuntu Server installation itself. Those figures are operating-system installation minimums, not performance targets for a game world.
A game server’s actual needs vary with concurrent players, world or map size, mods, plugins, simulation distance and view distance. Leave room for the operating system, Java process, world files, logs, backups and temporary update files. Monitor memory and disk usage under a realistic player load before deciding that the machine is adequately sized.
Ubuntu Server is intended as a platform for server applications, and Ubuntu’s documentation targets the latest LTS. Keep the operating system and Java packages updated, but schedule restarts around the game server. Test a Java or Ubuntu upgrade against the server build before applying it during a live event.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. Open only the game port
Find the port and protocol in the server documentation
There is no universal game-server port. Use the port and protocol documented by your specific project and the value configured in its server settings. If the server uses a non-default port, the firewall rule must use that exact number. Permit TCP, UDP or both only when the server documentation requires them.
Allow the port with UFW
Ubuntu identifies ufw as its default firewall configuration tool. After confirming the port, allow it with:
sudo ufw allow <port>Replace <port> with the documented numeric port. Do not open a broad range “just in case.” If you need a protocol-specific rule, use the UFW syntax documented for your Ubuntu release and server protocol.
Check the active rules
sudo ufw statusConfirm that the intended rule appears and that unrelated management services remain restricted. Keep your remote administration method available before enabling or tightening firewall rules, or you can lock yourself out.
Check upstream firewalls too
A local UFW rule is only one layer. A VPS or hosted instance may also have a cloud security group, provider firewall or network access-control list. Permit the same documented port and protocol there, and remove temporary broad rules after testing. If the machine is behind a home router, configure port forwarding to the server’s internal address and consider whether exposing the service to the public internet is appropriate.
8. Test the complete path before inviting players
- Start the server using the vendor’s documented command under the dedicated account.
- Read the startup log for a successful bind and for Java compatibility errors.
- On the Ubuntu host, verify that the process is listening on the configured address and port using your normal Linux socket-inspection tool.
- From the same host, test a local client connection if the game supports one.
- From a separate network, test the public address and documented port. This distinguishes a working process from a blocked provider firewall, router or NAT rule.
- Stop the server cleanly and confirm that the world or map saves before making configuration changes.
Keep the listening address deliberate. Binding only to a private interface can be correct for a local network; binding to all interfaces may be required for public access but increases exposure. Use the setting recommended by the game project and your network design.
9. Troubleshoot the failures that occur most often
APT cannot find the package
- Check the Ubuntu release and architecture.
- Confirm that the package name is spelled correctly and is available in repositories enabled for that release.
- Run
sudo apt updateagain and read any repository errors instead of ignoring them. - Use the versioned package only when that major is published for your Ubuntu release; otherwise choose a supported package documented by the project.
The Java major is wrong
Inspect installed Java alternatives and package versions. A default package can map to a different major on another Ubuntu release. Install the required versioned OpenJDK package, select the intended executable when multiple versions coexist, and verify with java -version and, for a JDK, javac -version.
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This error indicates a mismatch between the server build and the installed Java major. Do not fix it by randomly trying older Java packages. Consult the server project’s compatibility matrix, install the stated major, select it as the active executable and repeat the startup test.
The server starts locally but nobody can connect
- Confirm that the server is listening on the expected address and port.
- Check the UFW rule with
sudo ufw status. - Verify the cloud provider’s security group or external firewall.
- For a home connection, verify router port forwarding and the server’s internal address.
- Confirm that players are using the same address, port and protocol configured by the server.
Java works in your shell but not for the service
A service can run with a different environment or executable path than an interactive shell. Check the service definition or launch script used by the game project, make its Java path explicit when supported, and test again as the dedicated account. Avoid granting the service administrator privileges merely to make an environment problem disappear.
Quick Recap
10. A repeatable installation checklist
- Read the game server’s required Java major and supported operating systems.
- Choose
default-jdkwhen no major is pinned and development tools are useful. - Choose
default-jrefor a runtime-only installation with no tooling requirement. - Choose a versioned
openjdk-XX-jdkwhen the project pins majorXX. - Run
sudo apt updatebefore installation. - Verify with
java -version; runjavac -versionwhen a compiler is needed. - Run the server as a dedicated unprivileged account in its own directory.
- Accept the EULA and use the project’s documented launch procedure where applicable.
- Size RAM and storage for players, worlds, mods, logs and backups rather than Ubuntu’s 2 GB RAM and 5 GB disk installation minimums.
- Allow only the documented listening port and protocol in UFW and any upstream firewall.
- Test locally and from an external network before sharing the address.
- Schedule Ubuntu and Java updates around the server, then retest compatibility after upgrades.
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