Java Object Size and Overhead

Let's look at the details few details of object header and calculate the memory size an object occupies inside JVM Heap.

Each Object contains following information.

• The Object Header.

• The memory for primitive types.


• The memory for reference types.


• Offset / alignment - in fact, these are a few unused bytes that are placed after the data object itself. This is done in order that an address in memory was always a multiple of machine word, to speed up the memory read + reduce the number of bits for a pointer to an object. It is also worth noting that in java a size of any object is multiple of 8 bytes!

•   Object Header : 

          In case of 32-bit system, the header size is 8 bytes, in the case of 64-bit system, respectively is 16 bytes. It contains following information.

1  Hash Code -

2  Garbage Collection Information - each java object contains the information needed for the memory management.

3  Type Information Block Pointer - contains information about the type of object. This block contains information about the virtual method table, a pointer to an object that represents the type and pointers to some additional structure for more efficient call interfaces, and dynamic type checking.

4  Lock - each object contains information about the locking. 

5  Array Length - if the object - an array, then the header extends by 4 bytes to store the length of the array.

Lets calculate the size of Integer and String on 32-bit HotSpot JVM.

We can Use JOL Tool to find the object's sizes.

Integer:

Header: 8 bytes

Field int: 4 bytes

Alignment for the multiplicity of 8 : 4 byte (padding)

Total: 16 bytes To store 4 bytes of Integer, we are using 16 bytes of space(12 bytes extra)

String:

new String ()

Header: 8 bytes

Fields int: 4 bytes * 3 == 12 bytes              (1. an integer offset into the array at which the string starts;                 2.the length of the string;                 3. another int for the cached calculation of the hash code.)

The reference variable to an object of array: 4 bytes

Total: 24 bytes

Other Objects : new char [1]

Header: 8 bytes + 4 bytes for the array length == 12 bytes

Primitives char: 2 bytes * 1 == 2 bytes

Alignment for the multiplicity of 8 : 2 bytes

Total: 16 bytes

Total, the new String ("a") = 24+16 = 40 bytes To store 1 bytes of Character, we are using 40 bytes of space(12 bytes extra) If you are dealing with 1 billion strings , it is going to occupy 40GB of memory.