Default Xmxsize in Java 8 (max heap size)

It varies on implementation and version, but usually it depends on the VM used (e.g. client or server, see -client and -server parameters) and on your system memory.

Often for client the default value is 1/4th of your physical memory or 1GB (whichever is smaller).

Also Java configuration options (command line parameters) can be "outsourced" to environment variables including the -Xmx, which can change the default (meaning specify a new default). Specifically the JAVA_TOOL_OPTIONS environment variable is checked by all Java tools and used if exists (more details here and here).

You can run the following command to see default values:

java -XX:+PrintFlagsFinal -version

It gives you a loooong list, -Xmx is in MaxHeapSize, -Xms is in InitialHeapSize. Filter your output (e.g. |grep on linux) or save it in a file so you can search in it.

Like you have mentioned, The default -Xmxsize (Maximum HeapSize) depends on your system configuration.

Java8 client takes Larger of 1/64th of your physical memory for your Xmssize (Minimum HeapSize) and Smaller of 1/4th of your physical memory for your -Xmxsize (Maximum HeapSize).

Which means if you have a physical memory of 8GB RAM, you will have Xmssize as Larger of 8*(1/6) and Smaller of -Xmxsizeas 8*(1/4).

You can Check your default HeapSize with

In Windows:

java -XX:+PrintFlagsFinal -version | findstr /i "HeapSize PermSize ThreadStackSize"

In Linux:

java -XX:+PrintFlagsFinal -version | grep -iE 'HeapSize|PermSize|ThreadStackSize'

These default values can also be overrided to your desired amount.

Surprisingly this question doesn't have a definitive documented answer. Perhaps another data point would provide value to others looking for an answer. On my systems running CentOS (6.8,7.3) and Java 8 (build 1.8.0_60-b27, 64-Bit Server):

default memory is 1/4 of physical memory, not limited by 1GB.

Also, -XX:+PrintFlagsFinal prints to STDERR so command to determine current default memory presented by others above should be tweaked to the following:

java -XX:+PrintFlagsFinal 2>&1 | grep MaxHeapSize

The following is returned on system with 64GB of physical RAM:

uintx MaxHeapSize                                  := 16873684992      {product}