import java.lang.instrument.*;
import java.io.File;
public class MyAgent {
    static long size;    
    public static void main(String[] args) {
    File f = new File("C:\\README.txt");
    Filewalker fn = new Filewalker();
    Filewalker.Item odyn = fn.new Item (f.length(), f.getName(), f.getPath()); //Item – вложженый файл
    Instrumentation inst;
    size = inst.getObjectSize(odyn);
    System.out.println("Bytes used by object: " + size);      
  }
  
}

import java.lang.instrument.*;
import java.io.File;

public class MyAgent1 {

  public static void premain(String args, Instrumentation inst) {
    File f = new File("C:\\README.txt");
    Filewalker fn = new Filewalker();
    Filewalker.Item odyn = fn.new Item (f.length(), f.getName(), f.getPath());
    long size = inst.getObjectSize(odyn);
    System.out.println("Bytes used by object: " + size);
  }
  
  public static void main(String[] args) {
    MyAgent1 mm = new MyAgent1();
    mm.premain();    
  }  
}

public class MyAgent {
  private static volatile Instrumentation globalInstr;
  public static void premain(String args, Instrumentation inst) {
    globalInstr = inst; // Что сам по себе вызывается -?
  }
  public static long getObjectSize(Object obj) {
    if (globalInstr == null)
      throw new IllegalStateException("Agent not initted");
    return globalInstr.getObjectSize(obj);
  }
}