Calculating the Memory Usage of java Object with VisualVM Heap Dump does not match with theoretical approach

Question

All I come with a simple question. According to the java docs and many articles about java memory object layout if we have a class with one int variable the total memory consumption for that object will be:

• 8 byte for the heading
• 4 byte for the int
• 4 byte padding (to round the total up to be multiple of 8 bytes) = 16 bytes total

public class Ab {        
    int b;
}

public static void main(String args[]) throws InterruptedException {
    Ab ab = new AB();  
}  

My problem now is that when I used the Visual vm and look at the Heap dump to observe this the theoretical approach I noticed that the memory consumption for that object was 20 byte instead of 16? Why is this happens? Can someone explain to me?

Answer 1

Using the Java Object Layout tool I received the following output:

 OFFSET  SIZE   TYPE DESCRIPTION        VALUE
      0    12        (object header)    N/A
     12     4    int Ab.b               N/A

Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

And with the -XX:-UseCompressedOops VM option (disable compressed references):

 OFFSET  SIZE   TYPE DESCRIPTION                                VALUE
      0    16        (object header)                            N/A
     16     4    int Ab.b                                       N/A
     20     4        (loss due to the next object alignment)

Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

Java environment:

java version "11" 2018-09-25
Java(TM) SE Runtime Environment 18.9 (build 11+28)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11+28, mixed mode)