JavaAgent измерить размер объектов

java.lang.IllegalStateException: Agent not initialised

Exception in thread "main" java.lang.IllegalStateException: Agent not initialised
    at folder.AgentMemoryCounter.getSize(AgentMemoryCounter.java:25)
    at folder.AgentTester.printObjectSize(AgentTester.java:43)
    at folder.AgentTester.main(AgentTester.java:31)
Java Result: 1

public class AgentMemoryCounter {
    
    private static Instrumentation instrumentation;
    
    public static void premain(String args, Instrumentation instrumentation) {
        AgentMemoryCounter.instrumentation = instrumentation;
    }
    
    public static long getSize(Object obj) {
        if (instrumentation == null) {
            throw new IllegalStateException("Agent not initialised"); //вот это 25 строка
        }
        return instrumentation.getObjectSize(obj);
    }
}

        Serializable v = new Serializable() {
            int anInt;
        };
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(v);
        oos.flush();
        System.out.println(bos.toByteArray().length);