What happens to java process if the physical memory is very low on system

Question

I have a Java process running doing some tasks, after a couple of hours there are multiple other applications opened on the system causing a very low physical memory available on the system.

So, if the system has no physical memory/very less memory left, how would my java process respond to such a situation? Would it throw a 'out of memory' exception?

Answer 1

When RAM is exhausted the OS will usually use swap or pagefile to provide virtual memory:

RAM is a limited resource, whereas for most practical purposes, virtual memory is unlimited. There can be many processes, and each process has its own 2 GB of private virtual address space. When the memory being used by all the existing processes exceeds the available RAM, the operating system moves pages (4-KB pieces) of one or more virtual address spaces to the computer’s hard disk. This frees that RAM frame for other uses. In Windows systems, these “paged out” pages are stored in one or more files (Pagefile.sys files) in the root of a partition.

Paging usually results in a severe performance penalty because even modern SSD storage is not as fast as RAM. If the memory scarcity continues, the system may start thrashing.

Assuming that the JVM does not require more memory, e.g. it is already constrained by -Xmx and allocated all allowed memory, it will continue to run. Usually when the memory is exhausted OS will not allow new processes to start e.g. attempting to start new JVM process will result in following error:

Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine. 

At the end of the day it depends on the OS configuration. Usually this situation is not something you want to spend time investigating because RAM is much cheaper than developer's time.