Problem reported by one of our users:

import jdk.nashorn.internal.ir.debug.ObjectSizeCalculator;

public class GetObjectSizeDemo {

    public static void main(String[] args) {
        GetObjectSizeDemo demo = new GetObjectSizeDemo();
        try {
            long size = ObjectSizeCalculator.getObjectSize(demo);
            System.out.println("size = " + size);
        } catch (Exception e){
            System.out.println(e.getStackTrace());
        }
    }

}

$ java GetObjectSizeDemo

Exception in thread "main" java.lang.ExceptionInInitializerError
        at jdk.nashorn.internal.ir.debug.ObjectSizeCalculator.getObjectSize(ObjectSizeCalculator.java:122)
        at GetObjectSizeDemo.main(GetObjectSizeDemo.java:8)
Caused by: java.lang.UnsupportedOperationException: ObjectSizeCalculator only supported on HotSpot VM
        at jdk.nashorn.internal.ir.debug.ObjectSizeCalculator.getEffectiveMemoryLayoutSpecification(ObjectSizeCalculator.java:400)
        at jdk.nashorn.internal.ir.debug.ObjectSizeCalculator$CurrentLayout.<clinit>(ObjectSizeCalculator.java:102)
        ... 2 more

Trivial fix:

diff -r f5a3d8f60cf5 src/jdk/nashorn/internal/ir/debug/ObjectSizeCalculator.java
--- a/src/jdk/nashorn/internal/ir/debug/ObjectSizeCalculator.java Tue May 19 05:11:20 2020 +0100
+++ b/src/jdk/nashorn/internal/ir/debug/ObjectSizeCalculator.java Fri May 22 15:50:37 2020 +0800
@@ -396,7 +396,8 @@
      */
     public static MemoryLayoutSpecification getEffectiveMemoryLayoutSpecification() {
         final String vmName = System.getProperty("java.vm.name");
- if (vmName == null || !vmName.startsWith("Java HotSpot(TM) ")) {
+ if (vmName == null
+ || (!vmName.startsWith("OpenJDK ") && !vmName.startsWith("Java HotSpot(TM) "))) {
             throw new UnsupportedOperationException(
                     "ObjectSizeCalculator only supported on HotSpot VM");
         }