Exploring the depth of objects in JVM series

Start writing

The spirit of the recent period of time is a little depressed , As a result, the first two articles are not satisfactory , So I took a break to update the article .

Reference books ：“ In depth understanding of java virtual machine ”

personal java Knowledge Sharing Project gitee Address ：Cornucopia

personal java Knowledge Sharing Project github Address ：Cornucopia

Memory layout of objects

stay HotSpot In the virtual machine , The memory layout of the object is divided into the following 3 area ：

• Object head （Header）
• The instance data （Instance Data）
• Alignment filling （Padding）

The object header contains a type pointer （Class Pointer）, By changing the pointer, you can determine which class the object belongs to . If the object is an array object, the object header will also contain the length of the array .

Object head （Header）

stay HostSpot The object header of a virtual machine object contains two types of information ：

• Used to store runtime data of the object itself , Such as hash code 、GC Generational age 、 Lock status flag 、 A lock held by a thread 、 To the thread ID、 Biased timestamps and so on , The length of this part of data is 32 Bit and 64 The virtual machines of bit are respectively 32bit and 64bit（8byte）, It's officially called “Mark World”

• The other part of the object header is the type pointer , That is, the pointer of the object to its type metadata （Class Pointer）,Java The virtual machine uses this pointer to determine what kind of instance the object is . Besides , Because virtual machines can pass the general Java Object metadata information determination Java Size of object , But if the length of the array is uncertain , The size of the array cannot be inferred from the metadata information . therefore , If the object is an array , There must also be an array in the object header to record the length of the array , It's the one in the picture above Data Length.

The instance data （Instance Data）

Instance data is the real effective information stored by objects , That is, we define various types of field contents in the program code , Whether inherited from the parent class , Fields defined in subclasses must be recorded .

Alignment filling （Padding）

Alignment filling is not inevitable , It doesn't mean anything , It just acts as a placeholder . Why is there such a piece of data because HotSpot The automatic memory management system of virtual machine requires that the starting address of object must be 8byte Integer multiple , In other words, the size of any object must be 8 Integer multiples of bytes . The head of the object has been carefully designed to be exactly 8byte Multiple （1 Times or 2 times ）, therefore , If the data part of the object instance is not aligned , It needs to be filled by aligning the padding .

Analyze the case

We already have theoretical knowledge of object memory layout , Now? , Let's use a case to deepen our impression ：

Case study ：

new user(); Total number of bytes in memory ？

public class user {
    
   private String name;
   private int age;
}

analysis

Before we start , We need to create one maven project , Then introduce the JOL tool kit

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

Execute code ：

public class ObjectTest {
    
    public static void main(String[] args) {
    
        System.out.println(ClassLayout.parseInstance(new user()).toPrintable());
    }
    static class user {
    
        private String name;
        private int age;
    }
}

JVM Parameters ：

Print the set parameters ： -XX:+PrintCommandLineFlags

result ：

JVM Parameter printing ：

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 

Terminal output content ：

Let's analyze , First of all to see jvm In the parameter XX:+UseCompressedClassPointers , We know that it turns on pointer compression （ That is, it will compress ClassPointer The memory size of , from 8byte->4byte）, that , According to the theoretical knowledge we have learned above , An object memory contains three parts , The object header is fixed 8byte,ClassPointer If pointer compression is enabled, it is 4byte,Instance Data here user Contains one int Object occupation 4byte,String Is a reference object that occupies a fixed amount of 4byte, That is to say Instance Data Take up a total of 8byte, because Padding It's a placeholder , Let's not analyze , Here, the object header and object data account for 8+4+8=20, here 20 No 8 Multiple , because jvm Memory management limitations , here jvm Will padding Find distance by 20byte The least 8 To fill in , That is to say 24byte,padding Namely 4byte（ That is... In the picture (object alignment gap) Occupied byte size ）.