Brief object memory layout

Object memory layout

HotSpot In the virtual machine , The layout of objects stored in memory can be divided into three areas ： Object head （Header）、 The instance data （Instance Data） And align fill （Padding）.

1. Object head

Java The object header of an object consists of three parts ：

1. Mark Word
2. Type a pointer
3. The length of the array （ Only array objects have ）

1.1 Mark Word

MarkWord Used to store runtime data of the object itself , Such as hash code （HashCode）、GC Generational age 、 Lock information, etc .

Mark Word stay 32 position JVM The length in is 32bit, stay 64 position JVM The middle length is 64bit.

For example, in 32 Bit HotSpot A virtual machine in which objects are not locked ,Mark Word Of 32 individual bits In the space 25bits Used to store object hash codes （HashCode）,4bits Used to store object generational ages ,2bits Used to store lock flag bits ,1bit Fixed for 0, In other states （ Lightweight locking 、 Heavyweight lock 、GC Mark 、 Can be biased ） The storage contents of the following objects are shown in the following table .

1.2 Type a pointer

The pointer is in 32 position JVM The length in is 32bit, stay 64 position JVM The middle length is 64bit.

The type pointer points to the class metadata of the object , The virtual machine USES this pointer to determine which class the object is an instance of .

Java The class data of the object is stored in the method area .

1.3 The length of the array

If the object is an array , Then the object header needs extra space to store the length of the array .

The length of this data also increases with JVM Architecture is different ：32 Bit JVM On , The length is 32 position ;64 position JVM Then for 64 position .

64 position JVM If open +UseCompressedOops Options , The length of the area will also be determined by 64 Bit compression to 32 position .

2. The instance data

Object instance data is in java The attributes you can see in the code and their values .

3. Align fill bytes

because JVM requirement Java The amount of memory occupied by the object should be 8bit Multiple , So there are a few bytes after that to make up the size of the object to 8bit Multiple , No special function .

4. Use JOL To analyze java Object layout

JOL The full name is Java Object Layout. It's an analysis of JVM in Object Layout gadgets . Include Object Occupancy in memory , Instance object reference, etc .

JOL You can use... In your code , It can also be run independently from the command line . I will not introduce the command line in detail here , Today, I will mainly explain how to use in code JOL.

Use JOL Need to add maven rely on ：

<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.9</version>
</dependency>

adopt JOL see jvm Information :

public class Test01 {
    
    public static void main(String[] args) {
    
        Object o = new Object();
        System.out.println(ClassLayout.parseInstance(o).toPrintable());
    }
}

Output ：

First explain the meaning of each field

• OFFSET It's the offset , That is, the space occupied by the location of this field byte Count ,
• SIZE Is the size of the latter type ,
• TYPE yes Class Type defined in ,
• DESCRIPTION Is a description of the type ,
• VALUE yes TYPE Value in memory .

As can be seen from the above figure Object o = new Object(); In memory 16 Bytes .