JVM - 結合代碼示例徹底搞懂Java內存區域_對象在堆-棧-方法區(元空間)之間的關係

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JVM - 結合代碼示例徹底搞懂Java內存區域_線程棧 | 本地方法棧 | 程序計數器

中我們探討了線程棧中的內部結構 ，大家有麼有想過main方法呢？ 我們繼續來看下main方法

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示例demo

package com.gof.test;


public class Artisan {
    public static final int FIVE = 5 ; // final靜態變
    public static  User user = new User();// 靜態變量

    public static void main(String[] args) {
        Artisan artisan = new Artisan();
        artisan.doSomething();
    }
    public int doSomething() {
        int a = 1 ;
        int b = 2 ;
        int c = (a + b) * 10 ;
        return c;
    }
}

final對象和static對象 ，我們知道都會存放在方法區（元空間）中的運行時常量池。 User 和 FIVE 會存放在該區域內。

方法區#運行時常量池 ，是方法區的一部分。 Class文件中的常量池表用於存放編譯期間生成的各種字面量和符號引用，這部分內容將在類加載後放到方法區的運行時常量池中。

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反彙編 ，附帶附件信息

 E:\Program Files\Java\jdk1.8.0_161\bin> ./javap -c -v D:\IdeaProjects\GOF23\target\classes\com\gof\test\Artisan.class > Artisan.txt
PS E:\Program Files\Java\jdk1.8.0_161\bin>

可以看到 Constant Pool

 Constant pool:
   #1 = Methodref          #8.#35         // java/lang/Object."<init>":()V
   #2 = Class              #36            // com/gof/test/Artisan
   #3 = Methodref          #2.#35         // com/gof/test/Artisan."<init>":()V
   #4 = Methodref          #2.#37         // com/gof/test/Artisan.doSomething:()I
   #5 = Class              #38            // com/gof/test/User
   #6 = Methodref          #5.#35         // com/gof/test/User."<init>":()V
   #7 = Fieldref           #2.#39         // com/gof/test/Artisan.user:Lcom/gof/test/User;
   #8 = Class              #40            // java/lang/Object
   #9 = Utf8               FIVE
  #10 = Utf8               I
  #11 = Utf8               ConstantValue
  #12 = Integer            5
  #13 = Utf8               user
  #14 = Utf8               Lcom/gof/test/User;
  #15 = Utf8               <init>
  #16 = Utf8               ()V
  #17 = Utf8               Code
  #18 = Utf8               LineNumberTable
  #19 = Utf8               LocalVariableTable
  #20 = Utf8               this
  #21 = Utf8               Lcom/gof/test/Artisan;
  #22 = Utf8               main
  #23 = Utf8               ([Ljava/lang/String;)V
  #24 = Utf8               args
  #25 = Utf8               [Ljava/lang/String;
  #26 = Utf8               artisan
  #27 = Utf8               doSomething
  #28 = Utf8               ()I
  #29 = Utf8               a
  #30 = Utf8               b
  #31 = Utf8               c
  #32 = Utf8               <clinit>
  #33 = Utf8               SourceFile
  #34 = Utf8               Artisan.java
  #35 = NameAndType        #15:#16        // "<init>":()V
  #36 = Utf8               com/gof/test/Artisan
  #37 = NameAndType        #27:#28        // doSomething:()I
  #38 = Utf8               com/gof/test/User
  #39 = NameAndType        #13:#14        // user:Lcom/gof/test/User;
  #40 = Utf8               java/lang/Object

找下User和 FIVE

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總體關係

Note： FIVE 是 final的，上圖少寫了

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代碼示例論證

當我們執行main方法

public static void main(String[] args) {
        Artisan artisan = new Artisan();
        artisan.doSomething();
    }

Artisan artisan = new Artisan(); 這個對象會在堆上分配一塊內存空間用來存儲該對象。

main也是個方法，也得有方法棧， 那方法棧中的局部變量表中存放的artisan是個啥呢？ 其實是 對象的引用 ，也就是對象的內存地址 。

這樣，線程棧和堆的關係就產生了。

當類在編譯階段，靜態類User , 會被分配到方法區 ， 那 new User(） 存放在 堆中，方法區的User 和 堆中的User 什麼關係呢？ 引用的關係，方法區中的User僅僅是個符號引用，指向真正的堆內存中的User對象。

這樣 方法區和堆的關係就產生了 。

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反彙編

Classfile /D:/IdeaProjects/GOF23/target/classes/com/gof/test/Artisan.class
  Last modified 2020-6-22; size 772 bytes
  MD5 checksum 89d38013e132ab14203f796617191dd9
  Compiled from "Artisan.java"
public class com.gof.test.Artisan
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#35         // java/lang/Object."<init>":()V
   #2 = Class              #36            // com/gof/test/Artisan
   #3 = Methodref          #2.#35         // com/gof/test/Artisan."<init>":()V
   #4 = Methodref          #2.#37         // com/gof/test/Artisan.doSomething:()I
   #5 = Class              #38            // com/gof/test/User
   #6 = Methodref          #5.#35         // com/gof/test/User."<init>":()V
   #7 = Fieldref           #2.#39         // com/gof/test/Artisan.user:Lcom/gof/test/User;
   #8 = Class              #40            // java/lang/Object
   #9 = Utf8               FIVE
  #10 = Utf8               I
  #11 = Utf8               ConstantValue
  #12 = Integer            5
  #13 = Utf8               user
  #14 = Utf8               Lcom/gof/test/User;
  #15 = Utf8               <init>
  #16 = Utf8               ()V
  #17 = Utf8               Code
  #18 = Utf8               LineNumberTable
  #19 = Utf8               LocalVariableTable
  #20 = Utf8               this
  #21 = Utf8               Lcom/gof/test/Artisan;
  #22 = Utf8               main
  #23 = Utf8               ([Ljava/lang/String;)V
  #24 = Utf8               args
  #25 = Utf8               [Ljava/lang/String;
  #26 = Utf8               artisan
  #27 = Utf8               doSomething
  #28 = Utf8               ()I
  #29 = Utf8               a
  #30 = Utf8               b
  #31 = Utf8               c
  #32 = Utf8               <clinit>
  #33 = Utf8               SourceFile
  #34 = Utf8               Artisan.java
  #35 = NameAndType        #15:#16        // "<init>":()V
  #36 = Utf8               com/gof/test/Artisan
  #37 = NameAndType        #27:#28        // doSomething:()I
  #38 = Utf8               com/gof/test/User
  #39 = NameAndType        #13:#14        // user:Lcom/gof/test/User;
  #40 = Utf8               java/lang/Object
{
  public static final int FIVE;
    descriptor: I
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    ConstantValue: int 5

  public static com.gof.test.User user;
    descriptor: Lcom/gof/test/User;
    flags: ACC_PUBLIC, ACC_STATIC

  public com.gof.test.Artisan();
    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 4: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/gof/test/Artisan;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: new           #2                  // class com/gof/test/Artisan
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: astore_1
         8: aload_1
         9: invokevirtual #4                  // Method doSomething:()I
        12: pop
        13: return
      LineNumberTable:
        line 9: 0
        line 10: 8
        line 11: 13
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      14     0  args   [Ljava/lang/String;
            8       6     1 artisan   Lcom/gof/test/Artisan;

  public int doSomething();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=4, args_size=1
         0: iconst_1
         1: istore_1
         2: iconst_2
         3: istore_2
         4: iload_1
         5: iload_2
         6: iadd
         7: bipush        10
         9: imul
        10: istore_3
        11: iload_3
        12: ireturn
      LineNumberTable:
        line 13: 0
        line 14: 2
        line 15: 4
        line 16: 11
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      13     0  this   Lcom/gof/test/Artisan;
            2      11     1     a   I
            4       9     2     b   I
           11       2     3     c   I

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=2, locals=0, args_size=0
         0: new           #5                  // class com/gof/test/User
         3: dup
         4: invokespecial #6                  // Method com/gof/test/User."<init>":()V
         7: putstatic     #7                  // Field user:Lcom/gof/test/User;
        10: return
      LineNumberTable:
        line 6: 0
}
SourceFile: "Artisan.java"