Using JavaAgent to test the size of objects

Author: Ma Bingbing http://www.mashibing.com

Object size (64 bit machine)

Observe virtual machine configuration

java -XX:+PrintCommandLineFlags -version

Common object

1. Object header: markword 8
2. ClassPointer: - XX:+UseCompressedClassPointers is 4 bytes, not open is 8 bytes
3. Instance data
   1. Reference type: - XX:+UseCompressedOops is 4 bytes, not open is 8 bytes
      Oops Ordinary Object Pointers
4. Padding alignment, multiple of 8

Array object

1. Object header: markword 8
2. The ClassPointer is the same as above
3. Array length: 4 bytes
4. Array data
5. Align multiples of 8

experiment

1. New project ObjectSize (1.8)

2. Create file ObjectSizeAgent

   package com.mashibing.jvm.agent;
   
   import java.lang.instrument.Instrumentation;
   
   public class ObjectSizeAgent {
       private static Instrumentation inst;
   
       public static void premain(String agentArgs, Instrumentation _inst) {
           inst = _inst;
       }
   
       public static long sizeOf(Object o) {
           return inst.getObjectSize(o);
       }
   }
   

3. Create META-INF/MANIFEST.MF under src directory

   Manifest-Version: 1.0
   Created-By: mashibing.com
   Premain-Class: com.mashibing.jvm.agent.ObjectSizeAgent
   

   Note that the premain class line must be a new line (enter + line feed). Confirm that the idea cannot have any error prompt

4. Package jar files

5. Introduce the Jar package into projects that need to use the Agent Jar
   project structure - project settings - library add the jar package

6. The class of the Agent Jar is required at runtime. Add parameters:

   -javaagent:C:\work\ijprojects\ObjectSize\out\artifacts\ObjectSize_jar\ObjectSize.jar
   

7. How to use this class:

   ​```java
      package com.mashibing.jvm.c3_jmm;
      
      import com.mashibing.jvm.agent.ObjectSizeAgent;
      
      public class T03_SizeOfAnObject {
          public static void main(String[] args) {
              System.out.println(ObjectSizeAgent.sizeOf(new Object()));
              System.out.println(ObjectSizeAgent.sizeOf(new int[] {}));
              System.out.println(ObjectSizeAgent.sizeOf(new P()));
          }
      
          private static class P {
                              //8 _markword
                              //4 _oop pointer
              int id;         //4
              String name;    //4
              int age;        //4
      
              byte b1;        //1
              byte b2;        //1
      
              Object o;       //4
              byte b3;        //1
      
          }
      }
   

Hotspot turns on the rule of memory compression (64 bit machine)

1. Below 4G, directly cut off the high 32 bits
2. 4G - 32G, memory compression ClassPointers Oops is enabled by default
3. 32G, invalid compression, 64 bit
   The larger the memory, the better (-)

Problems with IdentityHashCode

Answer the question that white horse is not a horse:

After an object has calculated the identityHashCode, it cannot enter the bias lock state

https://cloud.tencent.com/developer/article/1480590
https://cloud.tencent.com/developer/article/1484167

https://cloud.tencent.com/developer/article/1485795

https://cloud.tencent.com/developer/article/1482500

Object positioning

•https://blog.csdn.net/clover_lily/article/details/80095580

1. Handle pool
2. Direct pointer

remarks

1. Object creation process

   Load class - > linking - > initialization - > allocate memory - > assign default value - > attribute assignment - > construction method

2. Object memory layout (compressed)

   General object

   Object header 8 bytes

   Type pointer 4 bytes

   Attribute instance

   panding

   Array object

   Object header 8 bytes

   Type pointer 4 bytes

   Array length

   Array contents

   panding

3. Object header content

   According to different states,

   state	Other information	Lock information
   ordinary	hash gc age	Lock flag: 01 bias bit: 0
   Lightweight Locking	Pointer on stack	Lock mark: 00
   Heavyweight lock	Pointer	Lock mark: 10
   gc	empty	Lock mark: 11
   Bias lock	Thread, epoch, gc age	Lock mark: 01 bias position: 1

4. Object size