JVM - 结合代码示例彻底搞懂Java内存区域_对象在堆-栈-方法区(元空间)之间的关系
【摘要】
文章目录
Pre示例demo总体关系代码示例论证反汇编
Pre
JVM - 结合代码示例彻底搞懂Java内存区域_线程栈 | 本地方法栈 | 程序计数器
中我们探讨了线程栈中的内部结...
Pre
JVM - 结合代码示例彻底搞懂Java内存区域_线程栈 | 本地方法栈 | 程序计数器
中我们探讨了线程栈中的内部结构 ，大家有么有想过main方法呢？ 我们继续来看下main方法
示例demo
package com.gof.test;
public class Artisan {
public static final int FIVE = 5 ; // final静态变
public static User user = new User();// 静态变量
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;
}
}
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final对象和static对象 ，我们知道都会存放在方法区（元空间）中的运行时常量池。 User 和 FIVE 会存放在该区域内。
方法区#运行时常量池 ，是方法区的一部分。 Class文件中的常量池表用于存放编译期间生成的各种字面量和符号引用，这部分内容将在类加载后放到方法区的运行时常量池中。
反汇编 ，附带附件信息
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>
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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
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找下User和 FIVE
总体关系
Note： FIVE 是 final的，上图少写了
代码示例论证
当我们执行main方法
public static void main(String[] args) {
Artisan artisan = new Artisan();
artisan.doSomething();
}
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Artisan artisan = new Artisan(); 这个对象会在堆上分配一块内存空间用来存储该对象。
main也是个方法，也得有方法栈， 那方法栈中的局部变量表中存放的artisan是个啥呢？ 其实是 对象的引用 ，也就是对象的内存地址 。
这样，线程栈和堆的关系就产生了。
当类在编译阶段，静态类User , 会被分配到方法区 ， 那 new User(） 存放在 堆中，方法区的User 和 堆中的User 什么关系呢？ 引用的关系，方法区中的User仅仅是个符号引用，指向真正的堆内存中的User对象。
这样 方法区和堆的关系就产生了 。
反汇编
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"
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文章来源: artisan.blog.csdn.net，作者：小小工匠，版权归原作者所有，如需转载，请联系作者。
原文链接：artisan.blog.csdn.net/article/details/106893464
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