Tuesday, June 12, 2012

Memory usage of Java objects

General formula for calculating memory usage

In general, the heap memory used by a Java object in Hotspot consists of:
  • an object header, consisting of a few bytes of "housekeeping" information;
  • memory for primitive fields, according to their size ;
  • memory for reference fields (4 bytes each);
  • padding: potentially a few "wasted" unused bytes after the object data, to make every object start at an address that is a convenient multiple of bytes and reduce the number of bits required to represent a pointer to an object.

    Java typeBytes required

    • a normal object requires 8 bytes of "housekeeping" space;
    • arrays require 12 bytes (the same as a normal object, plus 4 bytes for the array length).

Object size granularity

In Hotspot, every object occupies a number of bytes that is a multiple of 8. If the number of bytes required by an object for its header and fields is not a multiple 8, then you round up to the next multiple of 8.
This means, for example, that:
  • a bare Object takes up 8 bytes;
  • an instance of a class with a single boolean field takes up 16 bytes: 8 bytes of header, 1 byte for the boolean and 7 bytes of "padding" to make the size up to a multiple of 8;
  • an instance with eight boolean fields will also take up 16 bytes: 8 for the header, 8 for the booleans; since this is already a multiple of 8, no padding is needed;
  • an object with a two long fields, three int fields and a boolean will take up:
    • 8 bytes for the header;
    • 16 bytes for the 2 longs (8 each);
    • 12 bytes for the 3 ints (4 each);
    • 1 byte for the boolean;
    • a further 3 bytes of padding, to round the total up from 37 to 40, a multiple of 8.

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