JVM - 剖析Java对象头Object Header之对象大小

向右看齐 2023-02-20 08:15 13阅读 0赞

### 文章目录 ###

*  Pre
 *  总览
 *  对象头剖析
 *   *  查看对象内存的占用情况
 *  对象头C++源码 注释

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70]

# Pre #

[JVM - 写了这么多年代码，你知不道new对象背后的逻辑？][JVM - _new] 中大体介绍了Java中 new 对象背后的主要流程，其中对象头的部分，我们仅仅是点到为止，这里我们深入剖一下Object Header的奥秘 。

--------------------

# 总览 #

初始化默认值以后，JVM要对对象进行必要的设置，例如这个对象是哪个类的实例、如何才能找到类的元数据信息、对象的哈希码、对象的GC分代年龄等信息。这些信息存放在对象的对象头Object Header之中。 这部分数据的长度在32位和64位的虚拟机中分别为32个和64个bits，官方称它为“Mark Word”。

在HotSpot虚拟机中，对象在内存中存储的布局可以分为3块区域：对象头（Header）、 实例数据（Instance Data)和和对齐填充（Padding） 。

![在这里插入图片描述][2020062516001861.png]

HotSpot虚拟机的对象头包括两部分信息

*  第一部分用于**存储对象自身的运行时数据**， 如哈希码（HashCode）、GC分代年龄、锁状态标志、线程持有的锁、偏向线程ID、偏向时 间戳等。

我们以32位操作系统的组成来看一下，下面这个图也是从网上找的，感觉很清晰

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 1]

*  对象头的另外一部分是**类型指针**，即对象指向它的类元数据的指针，虚拟机通过这个指针来确定这个对象是哪个类的实例。

如下红色框框中的示意图  
![在这里插入图片描述][20200625161716297.png]

--------------------

# 对象头剖析 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 1]

--------------------

## 查看对象内存的占用情况 ##

推荐openjdk的jol工具， 可以查看对象内存的占用情况。

添加依赖

<dependency>
     <groupId>org.openjdk.jol</groupId>
        <artifactId>jol-core</artifactId>
         <version>0.10</version     
    </dependency>

示例

import org.openjdk.jol.info.ClassLayout;
    
    /** * @author 小工匠 * @version v1.0 * @create 2020-06-25 16:21 * @motto show me the code ,change the word * @blog https://artisan.blog.csdn.net/ * @description **/
    
    public class ObjectHeaderTest { 
    
        public static void main(String[] args) { 
            ClassLayout layout = ClassLayout.parseInstance(new Object());
            System.out.println(layout.toPrintable());
    
            System.out.println();
            ClassLayout layout1 = ClassLayout.parseInstance(new int[]{ });
            System.out.println(layout1.toPrintable());
    
            System.out.println();
            ClassLayout layout2 = ClassLayout.parseInstance(new ArtisanTest());
            System.out.println(layout2.toPrintable());
        }
    
        // -XX:+UseCompressedOops 默认开启的压缩所有指针
        // -XX:+UseCompressedClassPointers 默认开启的压缩对象头里的类型指针Klass Pointer
        // Oops : Ordinary Object Pointers
        public static class ArtisanTest { 
            //8B mark word
            //4B Klass Pointer 如果关闭压缩-XX:-UseCompressedClassPointers或-XX:-UseCompressedOops，则占用8B
            int id;        //4B
            String name;   //4B 如果关闭压缩-XX:-UseCompressedOops，则占用8B
            byte b;        //1B
            Object o;      //4B 如果关闭压缩-XX:-UseCompressedOops，则占用8B
        }
    }

【输出结果 】

java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
         12     4        (loss due to the next object alignment)
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
    
    
    [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)                           6d 01 00 f8 (01101101 00000001 00000000 11111000) (-134217363)
         12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
         16     0    int [I.<elements>                             N/A
    Instance size: 16 bytes
    Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
    
    
    com.gof.test.ObjectHeaderTest$ArtisanTest object internals:
     OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
          0     4                    (object header)                           05 00 00 00 (00000101 00000000 00000000 00000000) (5)
          4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
          8     4                    (object header)                           63 cc 00 f8 (01100011 11001100 00000000 11111000) (-134165405)
         12     4                int ArtisanTest.id                            0
         16     1               byte ArtisanTest.b                             0
         17     3                    (alignment/padding gap)                  
         20     4   java.lang.String ArtisanTest.name                          null
         24     4   java.lang.Object ArtisanTest.o                             null
         28     4                    (loss due to the next object alignment)
    Instance size: 32 bytes
    Space losses: 3 bytes internal + 4 bytes external = 7 bytes total
    
    
    Process finished with exit code 0

【结果说明】

3中类型 ： new Object() | new int\[\]\{\}数组 | new ArtisanTest()

我这个电脑是64位的操作系统，所以 mark word 占 8个字节。JDK8 klass point 默认开启了指针压缩，所以是4个字节 , 不足8的倍数的，使用padding对齐填充，其目的是为了计算机高效寻址。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 2]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 3]

最后一个，对于包含多个变量的对象的对象头

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 4]

--------------------

# 对象头C++源码 注释 #

Bit‐format of an object header (most significant first, big endian layout below):
    //
    // 32 bits:
    // ‐‐‐‐‐‐‐‐
    // hash:25 ‐‐‐‐‐‐‐‐‐‐‐‐>| age:4 biased_lock:1 lock:2 (normal object)
    // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
    // size:32 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐>| (CMS free block)
    // PromotedObject*:29 ‐‐‐‐‐‐‐‐‐‐>| promo_bits:3 ‐‐‐‐‐>| (CMS promoted object)
    //
    // 64 bits:
    // ‐‐‐‐‐‐‐‐
    // unused:25 hash:31 ‐‐>| unused:1 age:4 biased_lock:1 lock:2 (normal object)
    // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
    // PromotedObject*:61 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐>| promo_bits:3 ‐‐‐‐‐>| (CMS promoted object)
    // size:64 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐>| (CMS free block)
    //
    // unused:25 hash:31 ‐‐>| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
    // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
    // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ‐‐‐‐‐>| (COOPs && CMS promoted object)
    // unused:21 size:35 ‐‐>| cms_free:1 unused:7 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐>| (COOPs && CMS free block)

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/2020062515412388.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ==,size_16,color_FFFFFF,t_70
[JVM - _new]: https://artisan.blog.csdn.net/article/details/106912264
[2020062516001861.png]: https://img-blog.csdnimg.cn/2020062516001861.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/2020062517061545.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ==,size_16,color_FFFFFF,t_70
[20200625161716297.png]: https://img-blog.csdnimg.cn/20200625161716297.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20200625223143428.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ==,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 3]: https://img-blog.csdnimg.cn/2020062522294358.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ==,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70 4]: https://img-blog.csdnimg.cn/20200625225418638.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ==,size_16,color_FFFFFF,t_70