If you’ve been working on a large scene in the SDK, there is a good chance that you will need to increase the size of Java’s heap space (which is quite small by default). What is heap space?
To increase the amount of heap space allocated to the SDK, we must navigate to the SDK’s installation directory and edit the etc/jmonkeyplatform.conf file.
The installation directory for Mac and Windows is:
OS X: Applications/jmonkeyplatform.app/Contents/Resources/jmonkeyplatform/jmonkeyplatform
Windows: C:\Program Files\jmonkeyplatform
Tweaking the parameters to be right is quite a task because sometimes when specifing a too large value, the JRE might fall back to it’s default value of (for example) 32 MB. Also there are different values to increase. Take a look at your Error Message, it will show you whether you ran out of “Heap Space or “Direct Memory.
In the config file you look for default_options=“. If you ran out of Heap Space increase the Value of -Xmx=512m to like 1024m or even further. If you ran out of Direct Memory, increase -XX:MaxDirectMemorySize=256M
Note that Increasing the Heap Space might also fix problems where you ran out of DirectMemory.