Learn about object overhead in JVM

Question

I am studying java, and I remember reading somewhere that java objects, had some overhead inside the JVM, which was used for administration reasons by the virtual machine. S

Answer 1

You can't get the overhead directly. The amount of overhead is implementation dependent, and can vary based on a number of factors (e.g. the precise JVM version, and whether you are on a 32 or 64bit JVM).

However it is reasonably safe to assume that in typical modern JVM implementations like HotSpot, the overhead per object is between 8 and 16 bytes. Arrays typically have an overhead that is 4 bytes larger than other objects (to contain the integer array length).

See also:

• In Java, what is the best way to determine the size of an object?
• Memory usage of Java objects: general guide

Answer 2

I found this article rather informative, although I had some doubts by some of the values mentioned in the tables

Answer 3

Here is a snippet for object header, object overhead, array header, object reference. Hope it helps someone, if not the OP as it is a quite old question.

    private static int OBJ_HEADER;
    private static int ARR_HEADER;
    private static int INT_FIELDS = 12;
    private static int OBJ_REF;
    private static int OBJ_OVERHEAD;
    private static boolean IS_64_BIT_JVM;

     static {
    String arch = System.getProperty("sun.arch.data.model");

    IS_64_BIT_JVM = (arch == null) || arch.contains("32");
    OBJ_HEADER = IS_64_BIT_JVM ? 16 : 8;
    ARR_HEADER = IS_64_BIT_JVM ? 24 : 12;
    OBJ_REF = IS_64_BIT_JVM ? 8 : 4;
    OBJ_OVERHEAD = OBJ_HEADER + INT_FIELDS + OBJ_REF + ARR_HEADER;
       }

I should say that I know only the solution, but I haven't yet figured out why this works. This is why people should leave comments in their code... Oh, well, when I do figure it out, I will share the logic behind it.