Android Dalvik 中的對象大小

Size of data

• Size of reference

  In HotSpot, an object reference is 4 bytes in 32 bit JVM, 8 bytes in 64 bit JVM with -UseCompressedOops and 4 bytes with+UseCompressedOops. In Dalvik, reference is always 4 bytes.

• Size of primitive data type

  The size of the primitive data type is fixd as follows:

  Data type	32 bit JVM	64 bit +UseCompressedOops	64bit -UseCompressedOops
  Object reference	4	4	8
  boolean	1	1	1
  byte	1	1	1
  char	2	2	2
  short	2	2	2
  int	4	4	4
  float	4	4	4
  long	8	8	8
  double	8	8	8

  But the size of the primitive type data is very diffrent in Dalvik.

  The size of a primitive data type is not the same when it is a field of object or a variable from when it is an element in Array.

  Data type	Size as field / variable	Size in Array	32 bit JVM	64 bit +	64bit -
  Object reference	4	4	4	4	8
  boolean	4	1	1	1	1
  byte	4	1	1	1	1
  char	4	2	2	2	2
  short	4	2	2	2	2
  int	4	4	4	4	4
  float	4	4	4	4	4
  long	8	8	8	8	8
  double	8	8	8	8	8

Size of object

• Alignment

  In Dalvik, the boundary alignment of an object is also 8 bytes.

• Overhead of Object

  In HotSpot, as we know, the overhead of object is 8 bytes in 32 bit JVM, and 16 bytes in 64 bit JVM without UseCompressedOops and 12 bytes with +UseCompressedOops.

  In Dalvik, this is diffrent. The memory of an object looks like:

  +---------------------+----------------------+----------+
  |overheade of Object  | overhead of dlmalloc |   data   |
  +---------------------+----------------------+----------+
  |   8 bytes           |  4 or 8 bytes        |          |
  +---------------------+----------------------+----------+
  

  There is another overhead for dlmalloc, which will take 4 or 8 bytes.

  So an empty object will take 16bytes, 12 bytes for overhead, 4 bytes for padding.

  Here are some examples:

  class EmptyClass {
  }
  

  Total size: 8 (Object overhead) + 4 (dlmalloc) = 12 bytes. For 8 bytes alignment, the final total size is 16 bytes.

  class Integer {
      int value;  // 4 bytes
  }
  

  The total size is: 8 + 4 + 4 (int) = 16 bytes.

  static class HashMapEntry<K, V> {
      final K key;                // 4 bytes
      final int hash;             // 4 bytes
      V value;                    // 4 bytes
      HashMapEntry<K, V> next;    // 4 bytes
  }
  

  The total size: 8 + 4 + 4 * 4 = 28 bytes. Total aligned is 32 bytes.

Size of Array

The memory layout of Array looks like:

+---------------------+----------------------+----------+---------+------+
|overheade of Object  | overhead of dlmalloc |   size   | padding | data |
+---------------------+----------------------+----------+---------+------+
|   8 bytes           |  4 or 8 bytes        |  4 bytes | 4 bytes |      |
+---------------------+----------------------+----------+---------+------+

The alignment is also 8 bytes.

So byte[0] will take: 8 + 4 + 4 + 4 = 20 bytes. The final size after alignment is 24 bytes.

byte[0] ~ byte[4] are all 24 bytes.

char[0] will also take 24 bytes. And from char[0] to char[2], they are all 24 bytes.

Size of String

String is defined as follows:

class String {
    private final char[] value; // 4 bytes


private final int offset;   // 4 bytes

private final int count;    // 4 bytes

private int hashCode;       // 4 bytes

}
</code></pre> 
  
Total size: 8 + 4 + 4 * 4 = 28 bytes. Total aligned is 32 bytes, which excludes the retained memory of char array(at least 24 bytes).
 
  
So, even an empty String will still take at least 32 bytes of shadow heap and 24 bytes of retained heap.
 
  
References
 
  
http://stackoverflow.com/questions/14738786/how-are-java-objects-laid-out-in-memory-on-android
 
  
http://stackoverflow.com/questions/9009544/android-dalvik-get-the-size-of-an-object
 
  
https://speakerdeck.com/romainguy/android-memories
 
  
http://www.slideshare.net/SOURCEConference/forensic-memory-analysis-of-androids-dalvik-virtual-machine
 
  
http://stackoverflow.com/questions/10824677/is-dalvik-even-more-memory-hungry-than-hotspot-in-terms-of-object-sizes/
 
  
來源：http://www.liaohuqiu.net/posts/android-object-size-dalvik/