«It won't start» covers half a dozen unrelated causes. Working through them in this order matters, because each check rules out the ones below it and the first two are free.
In this order
- Read the console from the top
Not the bottom. Java prints the real cause first and then the chain of things that failed because of it. The last line on screen is usually the least informative one.
- Check the jar filename in the Startup tab
Unable to access jarfileat the very first line means the filename in the panel does not match a file that exists. This is the single most common cause after a version change. - Check the EULA
A server that starts, prints a line about the EULA and stops within two seconds has not been agreed to.
eula=trueineula.txt. - Check the port
Failed to bind to portmeans something already holds it — usually a previous instance that did not shut down. Stop the server properly and start it again. - Move the plugins folder aside
If it starts with no plugins, one of them is the problem, and bisection finds it in four restarts. If it still does not start, plugins are innocent and you have saved yourself an hour.
- Check the Java version against the Minecraft version
UnsupportedClassVersionErrormeans the Java is older than the server needs. Newer Minecraft versions require newer Java, and the error names both numbers.
First line of the failure → cause
| You see | It is |
|---|---|
Unable to access jarfile | Wrong filename in Startup |
You need to agree to the EULA | eula.txt |
Failed to bind to port | Port already in use |
UnsupportedClassVersionError | Java too old |
Could not load 'plugins/...' | One plugin, named in the line |
java.lang.OutOfMemoryError | Not enough memory for what you are loading |
Reinstalling replaces files and can lose configuration, and it fixes none of the six causes above except a genuinely corrupt jar. It is a last resort with a backup taken first, not a reset button.
Startup is where servers explain themselves. Sixty seconds of reading beats twenty minutes of changing settings and hoping — and it means that if you do need help, you can say what it said.