Understanding objects in Java

Question

I'm trying to understand about objects in Java and how they reside in memory. I'm experimenting with the following application:

private static final Unsafe U = getUnsafe();

public static void main( String[] args )
{
    offsets(Test.class);
}

private static <T> void offsets(Class<T> clazz){
    Arrays.stream(clazz.getFields())
       .forEach(f -> 
            System.out.println(f.getName() + " offset:" + U.objectFieldOffset(f)
       )
    );
}

public static class Test{
    public boolean b;
    public byte b1;
    public short b2;
    public int b3;
}

The output:

b offset:18
b1 offset:19
b2 offset:16
b3 offset:12

QUESTION 1: I experimented with many kind of object and was surprised that fields offsets never less then 12. What is that 12? Some reserved object metadat?

QUESTION 2: Moreover I thought that memory alignement of the data structure should be equal to 4 or 8 bytes. But why offset of b1 is 19, for instance? How to interpret this Unsafe::fieldOffset output?

P.S. This is for educational purpose only, not intended for using in production.

Louis Wasserman · Accepted Answer · 2017-10-20 20:03:24Z

4

"Some reserved object metadata?" Yes.

"memory alignement of the data structure should be equal to 4 or 8 bytes." For the object as a whole, yes, but not necessarily for individual fields.

answered Oct 20, 2017 at 20:03

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Саша Данилин Over a year ago

Is there a way to estimate the alignment of Test in that case?

Саша Данилин Over a year ago

Btw, very interesting... Why is the offset of b field 18? Even if it put as the first one?

Саша Данилин Over a year ago

Could you advise some tool for monitoring object layout (with all metadata)?

Collectives™ on Stack Overflow

Understanding objects in Java

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related