What is the structure of Java objects in memory

1. Object header is divided into MarkWord and KlassPoint.

MarkWord (mark field): The HashCode of the object is stored by default, instead of age and locked mark position information. It will reuse its own storage space according to the state of the object, that is, the data stored in MarkWord will change as the lock mark position changes.

KlassPoint (type pointer): The pointer of an object to its class metadata. The virtual machine uses this pointer to determine which class the object belongs to.

2. Instance data. This part mainly stores data information and parent class information.

3. Alignment filling. Because the virtual machine requires the starting address of the object to be an integer multiple of 8 bytes, the padding data does not necessarily exist, just for byte alignment.

An empty object occupies 8 bytes because of alignment and padding. Alignment and padding of less than 8 bytes will help us complete it automatically.

Example

public class TestStudent {
public static void main(String[] args) {
int n = 10;  //栈存储
int m = 10;  //栈存储
Student stu = new Student();
//属性赋值
stu.name="张三";
System.out.println(stu.name);  //null   张三
System.out.println(stu.age);   //0  
//调用
String name = "ls";   //局部变量(常量池)
String aaaa="ls";
String pwd = "123";
stu.login(name,pwd);
System.out.println("name:"+name);   //ls
System.out.println("执行");
}
}
 
class Student {
// 属性
String name;
int age;
//用户登录
// String userName;
// String pwd;
public void login(String userName,String pwd){
System.out.println(userName+"===="+pwd);   //ls  123
}
public void show() {
System.out.println("姓名：" + name + ",年龄：" + age);
}
}

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