java对象数组大小

Question

When calculate memory size of an object array, following code gives "used 24 bytes" as expected, which, as far as I know, consists of:

4bytes(element pointer)+16bytes(object header)+4bytes(element space) = 24bytes

// with JVM argument -XX:-UseTLAB
public static void main(String[] args) {
    long size = memoryUsed();
    Object[] o = new Object[]{1};
    //Object[] o = new Object[]{1L};
    size = memoryUsed() - size;
    System.out.printf("used %,d bytes%n", size);
    //Output: used 24 bytes
}

public static long memoryUsed() {
    return Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
}

But when element type changed to Long (1L), the result is confusing, most of the time it's "used 9,264 bytes" , anyone can help enlighten me? what is the difference in memory allocation between this two element type?

// with JVM argument -XX:-UseTLAB
public static void main(String[] args) {
    long size = memoryUsed();
    //Object[] o = new Object[]{1};
    Object[] o = new Object[]{1L};
    size = memoryUsed() - size;
    System.out.printf("used %,d bytes%n", size);
    //Output: used 9,264 bytes
}

Answer 1

There is a better way to calculate an object size in general, as there is a specialized tool for that, called JOL .

It's not entirely correct how you think about it. The total size of that object is going to be 40 bytes . Let's see where this space is coming from:

12 bytes headers (8 bytes + 4 bytes, since there are two headers)

You thought it's 16 bytes ( 8 + 8 ), but there is the compressed oops option that is on by default. You can disable it via -XX:-UseCompressedOops and indeed in this case the size of the headers is going to be 16 bytes .

4 bytes size of the array (arrays have an int size that is stored in headers)
4 bytes is the reference size of the Integer (1)
4 bytes alignment (since objects are 8 bytes aligned - this is the trick behind CompressedOops btw)

So far you have 24 bytes for the array.

Now, you store an Integer inside it, that is an Object too, thus:

12 bytes headers
4 bytes for the actual int value inside Integer

Thus the total size is 40 bytes .