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Compile using Java 17

javac --add-exports java.base/jdk.internal.misc=ALL-UNNAMED Main.java

Run using java 17

java --add-opens java.base/jdk.internal.misc=ALL-UNNAMED -cp . Main

maxDirectMemory defaults to default max memory

Set Xmx but not MaxDirectMemorySize

maxDirectMemory defaults to Xmx in this case

java --add-opens java.base/jdk.internal.misc=ALL-UNNAMED -cp . -Xmx200m Main
sun.misc.VM.maxDirectMemory(): 209715200

Set Xmx and MaxDirectMemorySize

java --add-opens java.base/jdk.internal.misc=ALL-UNNAMED -cp . -Xmx200m -XX:MaxDirectMemorySize=60m Main
sun.misc.VM.maxDirectMemory(): 62914560

MaxDirectMemorySize takes precedence in this case

Main.java

import jdk.internal.misc.SharedSecrets;
import jdk.internal.misc.VM;

public class Main
{
    public static void main(String args[])
    {
       System.out.println("sun.misc.VM.maxDirectMemory(): " + VM.maxDirectMemory());
    }
}