偶然地,我偶然发现了我不知道的 jdk-15
变化。假设我有一个非常简单的问题:3 个整数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
运行它: java -Djdk.attach.allowAttachSelf -Djol.tryWithSudo=true -cp jol-cli.jar Array.java
我得到输出: [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
这非常明显: 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
。预期的...对于第二部分,我想禁用 JVM 优化:
-XX:-UseCompressedOops
。我用 jdk-13
运行它: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
好吧,也有点期待: 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
现在让我们使用 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
?怎么没有 40
,就像 jdk-13
一样?
最佳答案
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, header 大小会增加 4 bytes
,因为 UseCompressedOops
关闭 UseCompressedClassPointers
。至少在 jdk-13
中是这样的: java -XX:+PrintFlagsFinal -XX:-UseCompressedOops -version | grep Compressed
bool UseCompressedClassPointers = false
bool UseCompressedOops = false
jdk-15
中的事情发生了变化: 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,我在这方面还没有成功。
关于java - 在 jdk-13 和 jdk-15 中 UseCompressedOops UseCompressedClassPointers,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/62879157/