Aikar's Flags: Minecraft Startup Flags and Java Version

Aikar’s flags are a set of Java startup options, recommended by PaperMC, that tune garbage collection (G1GC) so Minecraft servers have shorter lag spikes. On a FREAKHOSTING Minecraft server they are already in the Startup Command, together with your plan’s memory (-Xmx), so you don’t need to add them. What you can change is the Java version, under Docker Image on the Startup page.

Difficulty	Time	Works on	Panel page
Beginner	5 minutes	Every Minecraft server type	Startup

What does the Startup page show?

Startup page in the FREAKHOSTING Game Panel with a read-only Startup Command using Aikar's flags, a Docker Image drop-down with Java versions and variables for Minecraft Version, Server Jar File and Build Number — The Startup page of a Minecraft server

Item	What it is	Can you change it?
Startup Command	The Java command that starts your server, with Aikar’s flags and your plan’s memory.	No, it’s read-only.
Docker Image	The Java version your server runs on: Java 25, 21, 18, 17, 16, 11 or 8.	Yes, if your version needs another Java.
Minecraft Version, Build Number	Used when the server is first installed or reinstalled.	Leave them. Use Version Changer to change version.
Server Jar File	The jar that starts (`server.jar`).	Only if you upload your own jar with another name.

What do Aikar’s flags do?

The full set, as published by PaperMC, looks like this (memory values are examples):

java -Xms10G -Xmx10G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200
-XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch
-XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M
-XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4
-XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90
-XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:+PerfDisableSharedMem
-XX:MaxTenuringThreshold=1 -Dusing.aikars.flags=https://mcflags.emc.gs -Daikars.new.flags=true
-jar paper.jar --nogui

Part	What it does
`-Xms` / `-Xmx`	Start and maximum memory for Java. Setting both the same avoids resizing. On our servers this comes from your plan.
`-XX:+UseG1GC`	Uses the G1 garbage collector, which suits Minecraft.
`-XX:MaxGCPauseMillis=200`	Aims to keep each garbage-collection pause short.
`-XX:+DisableExplicitGC`	Ignores plugins that try to force garbage collection.
`-XX:+AlwaysPreTouch`	Reserves all memory at start-up.
The `G1…` settings	Tune how G1 handles Minecraft’s many short-lived objects.

PaperMC also advises leaving about 1,000–1,500 MB of memory for Java’s own overhead. That’s why the memory graph can sit near the limit on a healthy server.

Which Java version does my Minecraft version need?

Version Changer sets the right Java version for you when you install a version (for example Java 25 for 26.x). If you change version another way, check this table:

Minecraft version	Java version
26.1 and newer	Java 25
1.20.5 to 1.21.11	Java 21
1.18 to 1.20.4	Java 17
1.17 to 1.17.1	Java 16
1.12 to 1.16.5	Java 8 (some older modpacks need exactly Java 8)

How to change the Java version

Open Startup

Click Startup in the sidebar.

Pick the Docker image

Under Docker Image, choose the Java version from the table above.

Restart

Restart the server so it starts on the new Java version.

If the console shows UnsupportedClassVersionError, the Java version is too old. See UnsupportedClassVersionError.

Why is the startup command read-only?

The startup command contains your plan’s memory and flags that suit our hardware. Keeping it read-only stops a typo from making the server fail to start, and stops -Xmx being set above your plan. Everything you normally need is still in your hands: the Java version (Docker Image), the server type and version (Version Changer) and your server’s settings. If you need a special Java option for a plugin or mod, open a ticket and tell us which option and why.

Frequently asked questions

Can I change -Xmx on my server?

No. The memory in the startup command comes from your plan and is read-only. To get more memory, upgrade your plan in the client area.

Do I need to add Aikar’s flags myself?

No. They are already in the Startup Command of every FREAKHOSTING Minecraft server.

Do Aikar’s flags work on Fabric and NeoForge?

Yes. They are Java settings, so they help any Minecraft server, not only Paper.

Should I use a newer Java than my version needs?

Usually the version in the table is best. Newer Minecraft versions need newer Java, and some very old modpacks only start on Java 8.

Why does the Startup page show an old Minecraft Version?

The Minecraft Version variable is only used for a fresh install. Version Changer shows what is installed now, in its status line.

Minecraft server guide

Every Minecraft guide in one place.

UnsupportedClassVersionError

Fix the wrong Java version.

RAM for modded servers

How much memory you need.

Startup variables

How the Startup page works.

bukkit.yml settings

Tune spawning for less lag.

Reviewed by the FREAKHOSTING team on 29 October 2026. Game Panel steps were checked on our panel (Minecraft 26.1, Paper and Fabric). Flags checked against the PaperMC Aikar’s flags guide; game and plugin details follow their official documentation. Steps may differ slightly if the game change.

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Email: support@freakhosting.com

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Last Updated: October 2026

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Aikar's Flags: Minecraft Startup Flags and Java Version

What does the Startup page show?

What do Aikar’s flags do?

Which Java version does my Minecraft version need?

How to change the Java version

Why is the startup command read-only?

Frequently asked questions

Can I change -Xmx on my server?

Do I need to add Aikar’s flags myself?

Do Aikar’s flags work on Fabric and NeoForge?

Should I use a newer Java than my version needs?

Why does the Startup page show an old Minecraft Version?

Minecraft server guide

UnsupportedClassVersionError

RAM for modded servers

Startup variables

bukkit.yml settings

Need Extra Help?

Save on Your Hosting

​What does the Startup page show?

​What do Aikar’s flags do?

​Which Java version does my Minecraft version need?

​How to change the Java version

​Why is the startup command read-only?

​Frequently asked questions

​Can I change -Xmx on my server?

​Do I need to add Aikar’s flags myself?

​Do Aikar’s flags work on Fabric and NeoForge?

​Should I use a newer Java than my version needs?

​Why does the Startup page show an old Minecraft Version?

​Related guides

Minecraft server guide

UnsupportedClassVersionError

RAM for modded servers

Startup variables

bukkit.yml settings

​Need Extra Help?

​Save on Your Hosting

What does the Startup page show?

What do Aikar’s flags do?

Which Java version does my Minecraft version need?

How to change the Java version

Why is the startup command read-only?

Frequently asked questions

Can I change -Xmx on my server?

Do I need to add Aikar’s flags myself?

Do Aikar’s flags work on Fabric and NeoForge?

Should I use a newer Java than my version needs?

Why does the Startup page show an old Minecraft Version?

Related guides

Need Extra Help?

Save on Your Hosting