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.

java 1.6.0_21 or later, or so...

$ java -XX:+PrintFlagsFinal -version 2>&1 | grep MaxHeapSize
uintx MaxHeapSize                         := 12660904960      {product}

It looks like the min(1G) has been removed.

Or on Windows using findstr

C:\>java -XX:+PrintFlagsFinal -version 2>&1 | findstr MaxHeapSize

The answer was in your question only in the first line itself -

default maximum heap size should be "Smaller of 1/4th" of the physical memory. In your case 1/4th of main memory is 24GB but heap size is 21GB, which is satisfying your first line statement.

To make it more clear run below code to get the actual main memory size

public class SizeOfMainMemory {

    public static void main(String[] args) {
        com.sun.management.OperatingSystemMXBean mxbean = (com.sun.management.OperatingSystemMXBean) ManagementFactory
                .getOperatingSystemMXBean();
        System.out.println(mxbean.getTotalPhysicalMemorySize()/1024/1024);
    }

}

You will find your HEAP SIZE is 1/4th of your main memory or may be little less.

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.

This dates back to JDK 5, which introduced JVM ergonomics. Prior to this, the JVM would set very small defaults for the heap space. JDK 1.1 had a default of 16Mb for both Xms and Xmx, JDK 1.2 changed this to Xms of 1Mb and Xmx of 64Mb by default. In JDK 1.3, Xms default increased to 2Mb.

Since Java was proving more popular on servers and memory capacities were increasing significantly, Sun introduced the concept of a server-class machine in JDK 5. This is one that has 2 or more physical processors and 2 or more Gb of memory (if I remember rightly, in JDK 5, the machine also had to not be running Windows to count as a server).

On server-class machines by default, the following parameters were set

• Throughput garbage collector (i.e. the parallel collector)
• initial heap size of 1/64 of physical memory up to 1Gbyte
• maximum heap size of 1/4 of physical memory up to 1Gbyte
• Server runtime compiler

Ergonomics provided two command-line flags that allowed a user to set a performance goal for the JVM; the idea being that the JVM would then figure out internally how to achieve this goal by modifying its parameters. The ultimate goal was to eliminate a lot of the -XX flags that were being used to tune JVM performance manually.

The parameters are:

-XX:MaxGCPauseMillis=nnn which sets the maximum pause time you want for GC in milliseconds.

-XX:GCTimeRatio= which sets the ratio of garbage collection time to application time being 1 / (1 + nnn). This was referred to as the throughput goal.

You can specify either of these goals or both. If the JVM manages to achieve both of these goals it then attempts to reduce the memory being used (the footprint goal).

There's more detail here:

https://www.oracle.com/technetwork/java/ergo5-140223.html