Memory alignment of Java classes

Question

Suppose that I am in a 64-bit machine compiling a C program with gcc. I'm assuming that sizeof(int) is 8 bytes, and sizeof(char) is 1 byte.

Because of memory alignment, the following struct:

struct example{
    int a;
    char c;
}

does not actually have a size of 9 bytes, but 16 (twice sizeof(int)), so that both its beginning and ending addresses can be at multiples of the word size (assumed 8 bytes here).

I was wondering how large the following class would be in Java 8:

class Node {
    int val;
    Node left, right;
    boolean flag;
 }

I'm basically not certain whether we would align at multiples of 8 or 4 bytes.

Answer 1

You can use jol to know the exact layout of objects. This is the output of the program for your Node class (on Oracle JDK 1.8.0_121 64 bit):

# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass 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]

org.example.Node object internals:
OFFSET  SIZE    TYPE DESCRIPTION                    VALUE
     0     4         (object header)                01 00 00 00 (00000001 00000000 00000000 00000000) (1)
     4     4         (object header)                00 00 00 00 (00000000 00000000 00000000 00000000) (0)
     8     4         (object header)                70 22 01 f8 (01110000 00100010 00000001 11111000) (-134143376)
    12     4     int Node.val                       0
    16     1 boolean Node.flag                      false
    17     3         (alignment/padding gap)        N/A
    20     4    Node Node.left                      null
    24     4    Node Node.right                     null
    28     4         (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 3 bytes internal + 4 bytes external = 7 bytes total

So, the alignment is 8 bytes.

Note, this is platform-specific. You shouldn't rely much on this information.