Java 对象的内存布局HotSpot 实现

发布时间：2023-04-15 11:23:14 所属栏目：教程来源：

导读：我们可以通过 jol 来查看一下某个对象的内存布局

引入依赖

<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.16</version>
</dependency>
示

我们可以通过 jol 来查看一下某个对象的内存布局

引入依赖

<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.16</version>
</dependency>
示例代码如下

import org.openjdk.jol.info.ClassLayout;
/**
* 对象的内存布局
* @author <a href="mailto:">Grey</a>
* @since
*/
// 配置VM参数 -XX:+UseCompressedClasspointers
public class ObjectModel {
public static void main(String[] args) {
System.out.println("======T1=======");
T1 o = new T1();
String s = ClassLayout.parseInstance(o).toPrintable();
System.out.println(s);
System.out.println("======T1=======");
System.out.println("======T2=======");
T2 o1 = new T2();
String s1 = ClassLayout.parseInstance(o1).toPrintable();
System.out.println(s1);
System.out.println("======T2=======");
}
static class T1{
public int a = 3;
}
static class T2{
public int a = 3;
public long b = 3L;
}
}
配置 JVM 参数，开启指针压缩

-XX:+UseCompressedClasspointers
可以看到结果是

======T1=======
# WARNING: Unable to get Instrumentation. Dynamic Attach Failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
git.snippets.juc.ObjectModel$T1 object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000005 (biasable; age: 0)
8 4 (object header: class) 0x00067248
12 4 int T1.a 3
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
======T1=======
======T2=======
git.snippets.juc.ObjectModel$T2 object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000005 (biasable; age: 0)
8 4 (object header: class) 0x00098490
12 4 int T2.a 3
16 8 long T2.b 3
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

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