Day346&347&348&349

高版本虚拟机，可以解释运行低版本字节码文件

[](()3、常量池：存放所有常量

Class 文件是 JVM 的基石，常量池是 Class 文件的基石

[](()①常量池计数器

常量池计数器与常量池表数量的关系相差1；

当常量池计数器为 1 时，常量池表中没有常量项

16(十六进制)===>22(十进制) = = =>代表有 22-1 个常量项

[](()②常量池表

字面量和符号引用：↓

常量类型和结构：↓

在 class 字节码文件中所表示的是指向某索引的符号有引用，不代表真正的内存地址；

当被类加载器加载到内存中，所存放到运行时数据区中，此时的符号引用就编译成了真正指向真实内存地址的直接引用

[](()4、访问标识

[](()5、类索引、父类索引、接口索引集合

当前类叫什么、当前类的父类谁、当前类实现了几个接口

[](()6、字段表集合

平时我们说的类中的属性

[](()①字段计数器

[](()②字段表

字段（类成员变量）的描述符为：参数类型

[](()7、方法表集合

[](()①方法计数器

[](()②方法表

方法的描述符为：方法的入参列表、方法的返回值类型

![[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-szCFuePj-1628002213657)(C:/Users/PePe/AppData/Roaming/Typora/typora-user-images/image-20210803213729005.png)]](https://img-blog.csdnimg.cn 《一线大厂 Java 面试题解析+后端开发学习笔记+最新架构讲解视频+实战项目源码讲义》无偿开源 威信搜索公众号【编程进阶路】 /5f0da650dec4438ca04c588190795ca6.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzMjg0NDY5,size_16,color_FFFFFF,t_70)

[](()8、属性表集合

方法中可以有属性，字段中也可以有属性

[](()①属性计数器

[](()②属性表

[](()9、总结

[](()四、使用 javap 指令解析 Class 文件

[](()1、解析字节码的作用

[](()2、javac -g 操作

[](()3、javap 的用法

最全信息的写法：javap -v -p XXX.class

[](()4、使用举例

• 代码

public class JavapTest {

private int num;

boolean flag;

protected char gender;

public String info;

public static final int COUNTS = 1;

static {

String url = "www.atguigu.com";

}

{

info = "java";

}

public JavapTest() {

}

private JavapTest(boolean falg) {

this.flag = flag;

}

private void methodPrivate() {

}

int getNum(int i) {

return num + i;

}

protected char showGender() {

return gender;

}

public void showInfo() {

int i = 100;

System.out.println(info + i);

}

}

• javap 字节码文件分析

javap -v -p JavapTest.class

Classfile /C:/Users/achang/Desktop/JavapTest.class // 字节码文件所属的路径

Last modified 2021-2-24; size 1393 bytes // 最后修改时间，字节码文件的大小

MD5 checksum 2c764244fa3a95bfb346c9e416a7a3f6 // MD5 散列值

Compiled from "JavapTest.java" // 源文件的名称

public class io.renren.JavapTest

minor version: 0 // 副版本

major version: 52 // 主版本

flags: ACC_PUBLIC, ACC_SUPER // 访问标识

//*************************** 常量池↓********************************

Constant pool:

#1 = Methodref #16.#48 // java/lang/Object."":()V

#2 = String #49 // java

#3 = Fieldref #15.#50 // io/renren/JavapTest.info:Ljava/lang/String;

#4 = Fieldref #15.#51 // io/renren/JavapTest.flag:Z

#5 = Fieldref #15.#52 // io/renren/JavapTest.num:I

#6 = Fieldref #15.#53 // io/renren/JavapTest.gender:C

#7 = Fieldref #54.#55 // java/lang/System.out:Ljava/io/PrintStream;

#8 = Class #56 // java/lang/StringBuilder

#9 = Methodref #8.#48 // java/lang/StringBuilder."":()V

#10 = Methodref #8.#57 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

#11 = Methodref #8.#58 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;

#12 = Methodref #8.#59 // java/lang/StringBuilder.toString:()Ljava/lang/String;

#13 = Methodref #60.#61 // java/io/PrintStream.println:(Ljava/lang/String;)V

#14 = String #62 // www.atguigu.com

#15 = Class #63 // io/renren/JavapTest

#16 = Class #64 // java/lang/Object

#17 = Utf8 num

#18 = Utf8 I

#19 = Utf8 flag

#20 = Utf8 Z

#21 = Utf8 gender

#22 = Utf8 C

#23 = Utf8 info

#24 = Utf8 Ljava/lang/String;

#25 = Utf8 COUNTS

#26 = Utf8 ConstantValue

#27 = Integer 1

#28 = Utf8

#29 = Utf8 ()V

#30 = Utf8 Code

#31 = Utf8 LineNumberTable

#32 = Utf8 LocalVariableTable

#33 = Utf8 this

#34 = Utf8 Lio/renren/JavapTest;

#35 = Utf8 (Z)V

#36 = Utf8 falg

#37 = Utf8 MethodParameters

#38 = Utf8 methodPrivate

#39 = Utf8 getNum

#40 = Utf8 (I)I

#41 = Utf8 i

#42 = Utf8 showGender

#43 = Utf8 ()C

#44 = Utf8 showInfo

#45 = Utf8

#46 = Utf8 SourceFile

#47 = Utf8 JavapTest.java

#48 = NameAndType #28:#29 // "":()V

#49 = Utf8 java

#50 = NameAndType #23:#24 // info:Ljava/lang/String;

#51 = NameAndType #19:#20 // flag:Z

#52 = NameAndType #17:#18 // num:I

#53 = NameAndType #21:#22 // gender:C

#54 = Class #65 // java/lang/System

#55 = NameAndType #66:#67 // out:Ljava/io/PrintStream;

#56 = Utf8 java/lang/StringBuilder

#57 = NameAndType #68:#69 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

#58 = NameAndType #68:#70 // append:(I)Ljava/lang/StringBuilder;

#59 = NameAndType #71:#72 // toString:()Ljava/lang/String;

#60 = Class #73 // java/io/PrintStream

#61 = NameAndType #74:#75 // println:(Ljava/lang/String;)V

#62 = Utf8 www.atguigu.com

#63 = Utf8 io/renren/JavapTest

#64 = Utf8 java/lang/Object

#65 = Utf8 java/lang/System

#66 = Utf8 out

#67 = Utf8 Ljava/io/PrintStream;

#68 = Utf8 append

#69 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;

#70 = Utf8 (I)Ljava/lang/StringBuilder;

#71 = Utf8 toString

#72 = Utf8 ()Ljava/lang/String;

#73 = Utf8 java/io/PrintStream

#74 = Utf8 println

#75 = Utf8 (Ljava/lang/String;)V

//字段表集合的信息↓********

{

private int num; // 字段名

descriptor: I // 字段描述符，字段类型

flags: ACC_PRIVATE // 字段的访问标识

boolean flag;

descriptor: Z

flags:

protected char gender;

descriptor: C

flags: ACC_PROTECTED

public java.lang.String info;