Question

以下是我想根据问题提出的两个示例（假设您在此处JOL）：

Layouter layout32Bits =  new HotSpotLayouter(new X86_32_DataModel());
Layouter layout64BitsComp = new HotSpotLayouter(new X86_64_COOPS_DataModel());

使用此示例：

int [] ints = new int[10];  

System.out.println(ClassLayout.parseInstance(ints, layout32Bits).toPrintable());
System.out.println(ClassLayout.parseInstance(ints, layout64BitsComp).toPrintable());

这是两个输出：

  [I object internals:
  OFFSET  SIZE   TYPE DESCRIPTION             VALUE
  0     4        (object header)     09 00 00 00 (00001001 00000000 00000000 00000000) (9)
  4     4        (object header)     00 00 00 00 (00000000 00000000 00000000 00000000) (0)
  8     4        (object header)     10 1b 0c 1a (00010000 00011011 00001100 00011010) (437000976)
 12    40    int [I.<elements>            N/A
 52    12        (loss due to the next object alignment)
 Instance size: 64 bytes
 Space losses: 0 bytes internal + 12 bytes external = 12 bytes total

  [I object internals:
  OFFSET  SIZE   TYPE DESCRIPTION             VALUE
  0     4        (object header)     09 00 00 00 (00001001 00000000 00000000 00000000) (9)
  4     4        (object header)     00 00 00 00 (00000000 00000000 00000000 00000000) (0)
  8     4        (object header)     10 1b 0c 1a (00010000 00011011 00001100 00011010) (437000976)
 12     4        (object header)     01 00 00 00 (00000001 00000000 00000000 00000000) (1)
 16    40    int [I.<elements>            N/A
 56     8        (loss due to the next object alignment)
 Instance size: 64 bytes
 Space losses: 0 bytes internal + 8 bytes external = 8 bytes total

我主要理解输出，我不知道这些是什么：

12 bytes external and 8 bytes external

一般情况下，对象8 bytes已对齐，为什么需要添加更多填充？为什么需要？

我知道一些奇怪的事情，the first one has to do with the API that JOL is using和第二个has to do with internal data, that needs to be hidden。

我也知道this，但它似乎没有关系，因为它意味着内部填充。

有人可以对此有所了解吗？

Answer 1

Instance size: 64 bytes是针对当前VM配置计算的，但您明确指定了不同的（不兼容的）Layouter。

实际尺寸（使用Instrumentation.getObjectSize计算）与预期尺寸（由Layouter计算）之间的差异将被视为loss due to the next object alignment。

请参阅ClassLayout.java

内存中的Java原始数组布局

1 个答案: