使用的工具
<dependency>
<groupId>org.openjdk.jol
</groupId>
<artifactId>jol-core
</artifactId>
<version>0.10
</version>
</dependency>
代码
public class Test {
public static void main(String
[] args
) throws IOException
{
System
.out
.println(ClassLayout
.parseInstance(new byte[0]).toPrintable());
System
.out
.println("-------------------");
System
.out
.println(ClassLayout
.parseInstance(new Object()).toPrintable());
}
}
输出结果
32位虚拟机
[B object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 09 00 00 00 (00001001 00000000 00000000 00000000) (9)
4 4 (object header) 78 04 96 06 (01111000 00000100 10010110 00000110) (110494840)
8 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
12 0 byte [B.<elements> N/A
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
-------------------
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 70 0d 96 06 (01110000 00001101 10010110 00000110) (110497136)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
可以看到，在32位虚拟机下，byte数组对象头总共12个字节，前4个字节存储的是mark word，接下来4个字节是指向对象所属的类的指针，也就是指向byte[].class的指针，最后4个字节保存数组的长度。 接下来是数组元素列表，由于这里是数组长度是0，所以，这里没有元素，占用0个字节，最后4个字节是对齐填充（为了加快访问速度，java对象的大小必然是8的整数倍），总共占用了16个字节。 object对像只有对象头，mark word 4个字节，class指针4个字节。总共占用8个字节。
64位虚拟机
[B object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 27 42 07 00 (00100111 01000010 00000111 00000000) (475687)
12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
16 0 byte [B.<elements> N/A
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
-------------------
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 96 20 00 00 (10010110 00100000 00000000 00000000) (8342)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
64位虚拟机默认开启了指针压缩，因此，class指针只占用4个字节，如果不开启指针压缩，指针将会占用8个字节。所以byte数组对象头 mark word是8个字节，class指针是4个字节，数组大小4个字节，因此byte数组不需要对齐，总共16字节，object对象 mark word 8字节，class直接4个字节，填充4个字节，总共16字节。
