JVM对象内存规范

最新推荐文章于 2024-08-26 17:02:01 发布

原创最新推荐文章于 2024-08-26 17:02:01 发布 · 310 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#java

java 专栏收录该内容

3 篇文章

订阅专栏

本文探讨了Java对象在内存中的布局方式及其优化手段。通过合理的属性排序，可以减少内存填充，从而节省内存空间。例如，一个包含多种类型属性的类，在经过属性重排后，其内存占用可以从40字节降低到32字节。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

http://www.codeinstructions.com/2008/12/java-objects-memory-structure.html

This scheme allows for a good optimization of memory usage. For example, imagine you declared the following class:

class MyClass {
byte a;
int c;
boolean d;
long e;
Object f;
}

If the JVM didn't reorder the attributes, the object memory layout would be like this:

[HEADER: 8 bytes] 8
[a: 1 byte ] 9
[padding: 3 bytes] 12
[c: 4 bytes] 16
[d: 1 byte ] 17
[padding: 7 bytes] 24
[e: 8 bytes] 32
[f: 4 bytes] 36
[padding: 4 bytes] 40

Notice that 14 bytes would have been wasted with padding and the object would use 40 bytes of memory. By reordering the objects using the rules above, the in memory structure of the object becomes:

HEADER: 8 bytes] 8
[e: 8 bytes] 16
[c: 4 bytes] 20
[a: 1 byte ] 21
[d: 1 byte ] 22
[padding: 2 bytes] 24
[f: 4 bytes] 28
[padding: 4 bytes] 32

This time, only 6 bytes are used for padding and the object uses only 32 bytes of memory.

通读上面的链接原文得知： java中新建一个无属性对象花费 8byte。有属性时，object reference 占4byte.

所以大致上可以认为java中新建一个全string对象占用的内存是（8 + string属性*4 + string属性本来占用空间）byte