初识Java内存结构

2023-11-20 16:44:22

我们的数据都是存放在内存中，Java内存大体可以分为寄存器、堆、栈、方法区、常量池。
程序=数据结构+算法，数据结构存在堆中，算法存在方法区中。

1、堆 heep

堆用于存储创建好的对象和数组(数组也是对象)；
JVM只有一个堆，被所有线程共享；
自动回收垃圾；
是一个不连续的内存空间，分配灵活，存取速度较慢；

2、方法区 method area

JVM只有一个方法区，被所有线程共享!
方法区实际也是堆，只是用于存储类、常量相关的信息!
用来存放程序中永远是不变或唯一的内容。(如类信息【Class对象】、静态变量、字符串常量等)

3、栈 stack

栈描述的是方法执行的内存模型。每个方法被调用都会创建一个栈帧(存储局部变量、操作数、方法出口等)；
JVM为每个线程创建一个栈，用于存放该线程执行方法的信息(实际参数、局部变量等)；
栈属于线程私有，不能实现线程间的共享!
栈的存储特性是“先进后出，后进先出”；
栈是由系统自动分配，速度快!栈是一个连续的内存空间!

4、寄存器 registers

程序计数寄存器：跟踪程序执行的准确性；
堆栈指针寄存器：指示操作栈顶；
框架寄存器：指向当前执行的环境；
变量寄存器：指向当前执行环境中第一个本地变量；

5、常量池 constant pool

用来存储某类型的常量信息；
存储较小的数值；
存储源文件中的字面量（文本字符串、被声明为final的常量值、基本数据类型的值等）；
提高存储效率；

6、本地方法栈 nativemethodstack

保存调用本地方法的地址；

【示例】Java内存分配分析

示例代码如下：

class Computer {
    // 品牌
    String brand;
}

public class Student {
	// 属性
    int id;
    String name;
    int age;
    Computer comp;
	
    // 方法
    void study() {
        System.out.println("我正在学习!使用我们的电脑:" + comp.brand);
    }

    public static void main(String[] args) {
        Student student = new Student();
        student.name = "白豆五";
        Computer computer = new Computer();
        computer.brand = "拯救者Y7000";
        student.comp = computer;
        student.study();
    }
}

UML类图如下：

内存分配图如下：

反编译内容如下：

// 反编译命令 ： javap -v Class文件
D:\Users\WorkSpace\Java\算法\ArraysDemo\out\production\ArraysDemo\com\baidou\hello>javap -v Student.class

Classfile /D:/Users/WorkSpace/Java/算法/ArraysDemo/out/production/ArraysDemo/com/baidou/hello/Student.class
  Last modified 2022-1-6; size 1156 bytes
  MD5 checksum faf4c49627d987c6f35393b6998953bd
  Compiled from "Student.java"
public class com.baidou.hello.Student
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
// 常量池
Constant pool:
   #1 = Methodref          #19.#43        // java/lang/Object."<init>":()V
   #2 = Fieldref           #44.#45        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Class              #46            // java/lang/StringBuilder
   #4 = Methodref          #3.#43         // java/lang/StringBuilder."<init>":()V
   #5 = String             #47            // 我正在学习!使用我们的电脑:
   #6 = Methodref          #3.#48         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #7 = Fieldref           #11.#49        // com/baidou/hello/Student.comp:Lcom/baidou/hello/Computer;
   #8 = Fieldref           #15.#50        // com/baidou/hello/Computer.brand:Ljava/lang/String;
   #9 = Methodref          #3.#51         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #10 = Methodref          #52.#53        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #11 = Class              #54            // com/baidou/hello/Student
  #12 = Methodref          #11.#43        // com/baidou/hello/Student."<init>":()V
  #13 = String             #55            // 白豆五
  #14 = Fieldref           #11.#56        // com/baidou/hello/Student.name:Ljava/lang/String;
  #15 = Class              #57            // com/baidou/hello/Computer
  #16 = Methodref          #15.#43        // com/baidou/hello/Computer."<init>":()V
  #17 = String             #58            // 拯救者Y7000
  #18 = Methodref          #11.#59        // com/baidou/hello/Student.study:()V
  #19 = Class              #60            // java/lang/Object
  #20 = Utf8               id
  #21 = Utf8               I
  #22 = Utf8               name
  #23 = Utf8               Ljava/lang/String;
  #24 = Utf8               age
  #25 = Utf8               comp
  #26 = Utf8               Lcom/baidou/hello/Computer;
  #27 = Utf8               <init>
  #28 = Utf8               ()V
  #29 = Utf8               Code
  #30 = Utf8               LineNumberTable
  #31 = Utf8               LocalVariableTable
  #32 = Utf8               this
  #33 = Utf8               Lcom/baidou/hello/Student;
  #34 = Utf8               study
  #35 = Utf8               main
  #36 = Utf8               ([Ljava/lang/String;)V
  #37 = Utf8               args
  #38 = Utf8               [Ljava/lang/String;
  #39 = Utf8               student
  #40 = Utf8               computer
  #41 = Utf8               SourceFile
  #42 = Utf8               Student.java
  #43 = NameAndType        #27:#28        // "<init>":()V
  #44 = Class              #61            // java/lang/System
  #45 = NameAndType        #62:#63        // out:Ljava/io/PrintStream;
  #46 = Utf8               java/lang/StringBuilder
  #47 = Utf8               我正在学习!使用我们的电脑:
  #48 = NameAndType        #64:#65        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #49 = NameAndType        #25:#26        // comp:Lcom/baidou/hello/Computer;
  #50 = NameAndType        #66:#23        // brand:Ljava/lang/String;
  #51 = NameAndType        #67:#68        // toString:()Ljava/lang/String;
  #52 = Class              #69            // java/io/PrintStream
  #53 = NameAndType        #70:#71        // println:(Ljava/lang/String;)V
  #54 = Utf8               com/baidou/hello/Student
  #55 = Utf8               白豆五
  #56 = NameAndType        #22:#23        // name:Ljava/lang/String;
  #57 = Utf8               com/baidou/hello/Computer
  #58 = Utf8               拯救者Y7000
  #59 = NameAndType        #34:#28        // study:()V
  #60 = Utf8               java/lang/Object
  #61 = Utf8               java/lang/System
  #62 = Utf8               out
  #63 = Utf8               Ljava/io/PrintStream;
  #64 = Utf8               append
  #65 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #66 = Utf8               brand
  #67 = Utf8               toString
  #68 = Utf8               ()Ljava/lang/String;
  #69 = Utf8               java/io/PrintStream
  #70 = Utf8               println
  #71 = Utf8               (Ljava/lang/String;)V
{
  int id;
    descriptor: I
    flags:

  java.lang.String name;
    descriptor: Ljava/lang/String;
    flags:

  int age;
    descriptor: I
    flags:

  com.baidou.hello.Computer comp;
    descriptor: Lcom/baidou/hello/Computer;
    flags:

  public com.baidou.hello.Student();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/baidou/hello/Student;

  void study();
    descriptor: ()V
    flags:
    Code:
      stack=3, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: new           #3                  // class java/lang/StringBuilder
         6: dup
         7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
        10: ldc           #5                  // String 我正在学习!使用我们的电脑:
        12: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        15: aload_0
        16: getfield      #7                  // Field comp:Lcom/baidou/hello/Computer;
        19: getfield      #8                  // Field com/baidou/hello/Computer.brand:Ljava/lang/String;
        22: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        25: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        28: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        31: return
      LineNumberTable:
        line 16: 0
        line 17: 31
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      32     0  this   Lcom/baidou/hello/Student;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: new           #11                 // class com/baidou/hello/Student
         3: dup
         4: invokespecial #12                 // Method "<init>":()V
         7: astore_1
         8: aload_1
         9: ldc           #13                 // String 白豆五
        11: putfield      #14                 // Field name:Ljava/lang/String;
        14: new           #15                 // class com/baidou/hello/Computer
        17: dup
        18: invokespecial #16                 // Method com/baidou/hello/Computer."<init>":()V
        21: astore_2
        22: aload_2
        23: ldc           #17                 // String 拯救者Y7000
        25: putfield      #8                  // Field com/baidou/hello/Computer.brand:Ljava/lang/String;
        28: aload_1
        29: aload_2
        30: putfield      #7                  // Field comp:Lcom/baidou/hello/Computer;
        33: aload_1
        34: invokevirtual #18                 // Method study:()V
        37: return
      LineNumberTable:
        line 20: 0
        line 21: 8
        line 22: 14
        line 23: 22
        line 24: 28
        line 25: 33
        line 26: 37
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      38     0  args   [Ljava/lang/String;
            8      30     1 student   Lcom/baidou/hello/Student;
           22      16     2 computer   Lcom/baidou/hello/Computer;
}
SourceFile: "Student.java"

码农公寓

1、堆 heep

2、方法区 method area

3、栈 stack

4、寄存器 registers

5、常量池 constant pool

6、本地方法栈 nativemethodstack

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