Examining memory layout in Java

Question

I am experimenting with inheritance and for educational purposes want to examine the addresses allocated for various objects and the fields within the object. Is there a tool which will let me see what memory the JVM is using and what it is using it for.

For example if I have two classes:

class A { int i,j; int f { ...} }
class B extends A { int c; /* more methods, overriding f and declaring new ones as well */ }

and instantiate these classes in objects a and b.

Is there a tool I can use to profile the memory usage and see exactly what memory is allocated for these?

Thank you!

Answer 1

Since the original question was posted the situation changed a bit.

The jol tool ("java object layout", by Aleksey Shipilev), is now part of OpenJDK allows you to check actual memory layout and usage of classes. http://openjdk.java.net/projects/code-tools/jol/

Example output looks like this:

$ java -jar jol-cli/target/jol-internals.jar java.util.HashMap
  Running 64-bit HotSpot VM.
  Using compressed references with 3-bit shift.
  Objects are 8 bytes aligned.
  Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
  Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

  java.util.HashMap object internals:
   OFFSET  SIZE       TYPE DESCRIPTION                    VALUE
        0     4            (object header)                01 00 00 00 (00000001 00000000 00000000 00000000)
        4     4            (object header)                00 00 00 00 (00000000 00000000 00000000 00000000)
        8     4            (object header)                0f 0f 3e e0 (00001111 00001111 00111110 11100000)
       12     4        Set AbstractMap.keySet             null
       16     4 Collection AbstractMap.values             null
       20     4        int HashMap.size                   0
       24     4        int HashMap.threshold              16
       28     4      float HashMap.loadFactor             0.75
       32     4        int HashMap.modCount               0
       36     4        int HashMap.hashSeed               0
       40     4    Entry[] HashMap.table                  []
       44     4        Set HashMap.entrySet               null
  Instance size: 48 bytes (estimated, add this JAR via -javaagent: to get accurate result)
  Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

Answer 2

I guess is instructive first to have an image about how function JVM over some operating system, so take a look on The Java Virtual Machine. Also, a related question is https://softwareengineering.stackexchange.com/questions/151076/approaching-java-jvm-internals