As I said, I'm back with more on the new JDK 1.5.

There is a new package called java.lang.instrument that allows you to intercept a class before being loaded and modify its bytecode, for example (can I hear standard entry point for AOP support? :-P). Well, let's use it for something different: measuring the size of some objects. Here is the code:

import java.lang.instrument.*;
import java.util.*;

public class InstrumentationTest {
   private static Instrumentation inst;

   public static void premain(String options, Instrumentation inst) {
      InstrumentationTest.inst = inst;

      System.out.println("options= " + options);

      // Get all classes currently loaded by VM
      Class[] loaded = inst.getAllLoadedClasses();

      // Sort them by name
      Arrays.sort(loaded, new Comparator() {
         public int compare(Class c1, Class c2) {
            return c1.getName().compareTo(c2.getName());
         }
      });

      //And print them!
      for (Class clazz : loaded) {
         System.out.println(clazz);
      }
   }

   public static long sizeOf(Object o) {
      assert inst != null;

      return inst.getObjectSize(o);
   }

   public static void main(String[] args) {
      System.out.println("Size of Object: " + sizeOf(new Object()));
      System.out.println("Size of direct subclass: " + sizeOf(
            new InstrumentationTest()));
      System.out.println("Size of Calendar: " + sizeOf(Calendar.getInstance()));

   }
}

Save it as InstrumentationTest.java and compile it as shown bellow:

javac -source 1.5 InstrumentationTest.java

To allow our class to be useful, we have to start the VM using this verbose command:

java -ea -javaagent:InstrumentationTest -cp . InstrumentationTest

Someone might be asking how it could be useful. As a friend of mine, Bruno Borges, suggested to me, it could give you a good idea if Prevayler is the right tool for your needs.

Hope you've enjoyed it. More to come!