Lack of ram

[ThisIsADumbUsernam]

Recently, I set up a server for my friend and I using his private world, and I have had absolutely no problems starting it up the first 2 times. But, after that, It would always get stuck on preparing spawn chunks and 

[03:31:02 ERROR]: Encountered an unexpected exception

java.lang.OutOfMemoryError: Java heap space

[03:31:29 WARN]: Failed retrieving info for group processor

java.lang.OutOfMemoryError: Java heap space

[03:32:11 FATAL]: Failed to read chunk data for task: Task for world: 'Prism_the_end' at 18,24 poi: false, hash: 1196562768

java.lang.OutOfMemoryError: Java heap space

Exception: java.lang.OutOfMemoryError thrown from the UncaughtExceptionHandler in thread "Server thread"

[03:32:17 WARN]: Failed retrieving info for group virtual memory

I looked it up and its a problem of lack of ram allocated to java heap space. 

Please fix this.