UseCompressedOops UseCompressedClassPointers 在 jdk-13 和 jdk-15

Question

我偶然发现jdk-15 发生了我不知道的变化。假设我有一个非常简单的问题：由 3 个整数组成的数组的大小是多少？为此，我使用JOL。代码相当简单：

import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;

public class Array {
    public static void main(String [] args){
       int [] array = new int[3];
       System.out.println(ClassLayout.parseInstance(array).toPrintable());
    }
}

我用jdk-13 运行这个：

  java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar  Array.java

我得到了输出：

    [I 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)                           18 0e 07 00 (00011000 00001110 00000111 00000000) (462360)
     12     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     16    12    int [I.<elements>                             N/A
     28     4        (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

这很明显：

     12 bytes --> Object headers
     4  bytes --> size of array
     12 bytes --> elements of array themselves
     4  bytes --> padding to align by 8 bytes
     ----
     32 bytes total

使用jdk-15 运行此示例会产生相同的输出，相同的32 bytes。预计...

---

对于第二部分，我想禁用 JVM 优化：-XX:-UseCompressedOops。我用jdk-13 运行这个：

java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar -XX:-UseCompressedOops  Array.java


    [I object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           11 00 00 00 (00010001 00000000 00000000 00000000) (17)
      4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4        (object header)                           40 0c f0 33 (01000000 00001100 11110000 00110011) (871369792)
     12     4        (object header)                           02 00 00 00 (00000010 00000000 00000000 00000000) (2)
     16     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     20     4        (alignment/padding gap)
     24    12    int [I.<elements>                             N/A
     36     4        (loss due to the next object alignment)
Instance size: 40 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes total

嗯，有点期待：

     16 bytes --> object headers (I did -XX:-UseCompressedOops after all)
     4 bytes  --> array size
     4 bytes  --> alignment for array headers (AFAIK this is only done for arrays)
     12 bytes --> array elements themselves
     4 bytes  --> 4 bytes padding
     ----
     40 bytes total

现在让我们用jdk-15 运行相同的示例：

[I 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)                           0e 09 00 00 (00001110 00001001 00000000 00000000) (2318)
     12     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     16    12    int [I.<elements>                             N/A
     28     4        (loss due to the next object alignment)
Instance size: 32 bytes

为什么现在是32 bytes？怎么不是40，和jdk-13一样？

Answer 1

在jdk-13 和jdk-15 中，这两个选项都默认开启：

java -XX:+PrintFlagsFinal -version | grep Compressed

 bool UseCompressedClassPointers  = true                                 
 bool UseCompressedOops           = true

当-XX:-UseCompressedOops 被禁用时，意味着UseCompressedClassPointers 也被禁用。这就是为什么当UseCompressedOops 关闭时，标头大小会增加4 bytes，因为UseCompressedOops 会关闭UseCompressedClassPointers。至少jdk-13是这样的：

  java -XX:+PrintFlagsFinal -XX:-UseCompressedOops -version | grep Compressed

    bool UseCompressedClassPointers = false                                  
    bool UseCompressedOops          = false

jdk-15 的情况发生了变化：

    bool UseCompressedClassPointers = true                                 
    bool UseCompressedOops          = false

所以禁用UseCompressedOops并不意味着UseCompressedClassPointers也被禁用，所以它保持在4 bytes。

虽然，我自己回答了这个问题，如果有人找到相关的错误/更改会很好吗？到目前为止，我还没有成功。