Fix Minecraft "java.lang.OutOfMemoryError: Java heap space"

The "OutOfMemoryError: Java heap space" crash means your Minecraft server tried to use more RAM than it was allowed and Java gave up. Here is what causes it and how to fix it — properly, not just by throwing RAM at it.

Cause 1: the RAM limit is too low

Your server starts with a maximum-memory flag, -Xmx. If it is too low for your modpack or player count, you will run out. Raise it in your start command (see how to allocate more RAM):

java -Xms4G -Xmx4G -jar server.jar nogui

A rough guide: vanilla/Paper with a few players is happy on 2–4 GB; medium modpacks want 4–6 GB; big modpacks (All the Mods, RLCraft) often need 8–10 GB or more.

Cause 2: you are giving Java more than the machine has

You cannot allocate more RAM than the machine physically has — and you must leave headroom for the OS. On a box with 8 GB, do not set -Xmx8G; leave 1–2 GB for the system. On shared hosting, your plan's RAM is the ceiling.

Cause 3: a memory leak from a mod or plugin

If memory climbs steadily until it crashes every few hours no matter how much you give it, a misbehaving plugin or mod is likely leaking. Narrow it down by reviewing recent additions and your crash report, then remove or update the culprit.

Cause 4: too much going on

Huge render/view distance, thousands of loaded entities, or massive redstone and farms all eat RAM. Reduce view-distance, limit entities and pre-generate your world — the same tuning that fixes low TPS and general lag.

The fix, in order

1. Raise -Xmx to a sensible value for your pack — but leave OS headroom.
2. Make sure the machine or plan actually has that much RAM.
3. Use a tuned flag set (Aikar's flags) for better garbage collection.
4. Find and remove any leaking mod or plugin.
5. Lower view-distance and entity counts; pre-generate the world.

More RAM buys headroom, but a leak or a runaway farm will eat any amount you give it. Fix the cause, not just the number.

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