IT科技类资讯2025-10-03 21:08:3666185

如何得到一个对象真实的内存大小

如何得到一个对象真实的到个对象的内内存大小

介绍一款工具(memory-measurer)可方便的测量一个对象真实占用内存大小如有这么一个User对象

public class User { private Integer id; private String mobile; private Date createTime; }

先看一个空User对象的内存占用量

User u = new User(); System.out.println(MemoryMeasurer.measureBytes(u)); //24 System.out.println(ObjectGraphMeasurer.measure(u)); //Footprint{ Objects=1, References=3, Primitives=[]}

可知一个对象三个引用共占了24字节

逐个赋值后占用内存是多少呢?

// 给id赋值 Integer id = new Integer(1); System.out.println(MemoryMeasurer.measureBytes(id)); // 16 u.setId(id); System.out.println(MemoryMeasurer.measureBytes(u)); // 40 System.out.println(ObjectGraphMeasurer.measure(u)); //Footprint{ Objects=2, References=3, Primitives=[int]}

一个Integer对象占用16字节于是给id赋值后 user对象变成了24+16=40字节了。

// 给mobile赋值 String mobile = "13600000001"; System.out.println(MemoryMeasurer.measureBytes(mobile)); // 64 u.setMobile(mobile); System.out.println(MemoryMeasurer.measureBytes(u)); // 104 System.out.println(ObjectGraphMeasurer.measure(u)); //Footprint{ Objects=4,真实 References=4, Primitives=[int x 2, char x 11]}

一个11位长的mobile字符串对象占用了64字节，于是到个对象的内user对象变成了40+64=104字节

// 给createTime赋值 Date createTime = new Date(); System.out.println(MemoryMeasurer.measureBytes(createTime)); // 24字节 u.setCreateTime(createTime); System.out.println(MemoryMeasurer.measureBytes(u)); // 128 System.out.println(ObjectGraphMeasurer.measure(u)); //Footprint{ Objects=5, References=5, Primitives=[int x 2, long, char x 11]}

可知一个Date对象占用了24字节，于是服务器租用真实全部属性不为空的一个User对象占用内存为128字节。

另外还可以通过另外一个工具--JOL (Java Object Layout)--可知更详细的到个对象的内Footprint信息

通过上面的工具我们只是知道一个空User对象占用了24字节以及简单的

Footprint{ Objects=1, References=3, Primitives=[]}

通过此工具可知这24个字节是怎么分配的了

System.out.println(ClassLayout.parseClass(User.class).toPrintable()); memorymeasurer.User object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 4 Integer User.id N/A 16 4 String User.mobile N/A 20 4 Date User.createTime N/A Instance size: 24 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

上面我们知道一个Integer对象占用了16字节看这16个字节是怎么分配

System.out.println(ClassLayout.parseClass(Integer.class).toPrintable()); java.lang.Integer object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 4 int Integer.value N/A Instance size: 16 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

上面我们知道一个11位长的String对象占用了64字节看其是网站模板怎么分配的

System.out.println(ClassLayout.parseClass(String.class).toPrintable()); java.lang.String object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 4 char[] String.value N/A 16 4 int String.hash N/A 20 4 (loss due to the next object alignment) Instance size: 24 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

即一个空String对象占用了24字节

System.out.println(ClassLayout.parseClass(char[].class).toPrintable()); [C object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 16 (object header) N/A 16 0 char [C.<elements> N/A Instance size: 16 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

一个长度为0的char数组占了16字节于是11位长的char数组占用字节为： 16+2*11=38 因为需要按8字节对齐于是还得加上2字节的填充符于是变成了40字节。所以一个11位长的真实字符串的占用字节为24+40=64

补充

memory-measurer如何使用

git clone https://github.com/msteindorfer/memory-measurer cd memory-measurer mvn clean install

pom文件中添加依赖

<dependency> <groupId>com.github.msteindorfer</groupId> <artifactId>memory-measurer</artifactId> <version>0.1.0-SNAPSHOT</version> </dependency>

运行时时显式添加vm参数如

-javaagent:/Users/zhugw/workspace/memory-measurer/target/memory-measurer-0.1.0-SNAPSHOT.jar

jol使用说明

只需添加依赖

<dependency> <groupId>org.openjdk.jol</groupId> <artifactId>jol-core</artifactId> <version>0.6</version> </dependency>

相关参考文档

https://github.com/msteindorf...

http://openjdk.java.net/proje...

http://blog.omalley.id.au/201...

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