内存中“长”的大小 - Java

Question

Reviewed

1. size-of-a-byte-in-memory-java
2. byte-in-java-takes-4-bytes-by-default

I am still trying to figure out why long and double in Java consuming 12 bytes instead of 8 bytes in memory

Sure there is wrong assumptions somewhere; please guide me..

I have analyzed it using two ways,

1. Java VisualVM
2. Java Instrumentation based memory-measurer

Following is my logic/assumption/source info,

1. Define a single float or long property in a class and check its memory footprint
2. Object will carry its own Meta Data worth 12 bytes = [CLASS INFO OF 4 BYTES] + [FLAGS INFO OF 4 BYTES] + [LOCK INFO OF 4 BYTES]
3. Class size will be Total Memory - Meta Data
4. Result received is 12 bytes => 24 bytes (Total Memory) - 12 bytes (Meta Data)
5. JDK: 1.8.0_65; Java HotSpot(TM) 64-Bit Server VM
6. OS: Windows 8.1
7. Test program

[ ANSWER ] Difference of 4 bytes is owing to Padding Applied by JVM. Thanks Andy Turner...

Answer 1

Quoting this answer :

In a modern 64-bit JDK, an object has a 12-byte header, padded to a multiple of 8 bytes

The extra 4 bytes is padding to get to a multiple of 8.

Answer 2

you can check the size of double:

double numDouble=2;
long size=(long)(numDouble*Double.SIZE) / Byte.SIZE;
System.out.println(size);

output: 16