How is the default max Java heap size determined?

If I omit the -Xmxn option from the Java command line then a default value will be used. According to Java documentation

“the default value is chosen at runtime based on system configuration”

What system configuration settings influence the default value?

On Windows, you can use the following command to find out the defaults on the system where your applications runs.

java -XX:+PrintFlagsFinal -version | findstr HeapSize

Look for the options MaxHeapSize (for -Xmx) and InitialHeapSize for -Xms.

On a Unix/Linux system, you can do

java -XX:+PrintFlagsFinal -version | grep HeapSize

I believe the resulting output is in bytes.