参考：https://tech.meituan.com/2019/09/05/java-bytecode-enhancement.html
Class File
image.png
这些信息是给虚拟机用的，虚拟机为了处理咱们写的这些类而需要的信息。
Classfile /Users/xixi/workspace6/JVMStudy/target/classes/com/xixi/jvm/classdef/Hello.class
Last modified 2020年9月12日; size 1123 bytes
MD5 checksum d558027a518219013be49f4bef4d0dd5
Compiled from "Hello.java"
public class com.xixi.jvm.classdef.Hello
minor version: 0
major version: 55
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #7 // com/xixi/jvm/classdef/Hello
super_class: #8 // java/lang/Object
interfaces: 0, fields: 1, methods: 3, attributes: 3
Constant pool:
#1 = Methodref #8.#25 // java/lang/Object."":()V
#2 = Fieldref #26.#27 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Fieldref #7.#28 // com/xixi/jvm/classdef/Hello.msg:Ljava/lang/String;
#4 = InvokeDynamic #0:#32 // #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
#5 = Methodref #33.#34 // java/io/PrintStream.println:(Ljava/lang/String;)V
#6 = String #35 // Good morning
#7 = Class #36 // com/xixi/jvm/classdef/Hello
#8 = Class #37 // java/lang/Object
#9 = Utf8 msg
#10 = Utf8 Ljava/lang/String;
#11 = Utf8
#12 = Utf8 ()V
#13 = Utf8 Code
#14 = Utf8 LineNumberTable
#15 = Utf8 LocalVariableTable
#16 = Utf8 this
#17 = Utf8 Lcom/xixi/jvm/classdef/Hello;
#18 = Utf8 main
#19 = Utf8 ([Ljava/lang/String;)V
#20 = Utf8 args
#21 = Utf8 [Ljava/lang/String;
#22 = Utf8
#23 = Utf8 SourceFile
#24 = Utf8 Hello.java
#25 = NameAndType #11:#12 // "":()V
#26 = Class #38 // java/lang/System
#27 = NameAndType #39:#40 // out:Ljava/io/PrintStream;
#28 = NameAndType #9:#10 // msg:Ljava/lang/String;
#29 = Utf8 BootstrapMethods
#30 = MethodHandle 6:#41 // REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#31 = String #42 // msg == \u0001
#32 = NameAndType #43:#44 // makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
#33 = Class #45 // java/io/PrintStream
#34 = NameAndType #46:#47 // println:(Ljava/lang/String;)V
#35 = Utf8 Good morning
#36 = Utf8 com/xixi/jvm/classdef/Hello
#37 = Utf8 java/lang/Object
#38 = Utf8 java/lang/System
#39 = Utf8 out
#40 = Utf8 Ljava/io/PrintStream;
#41 = Methodref #48.#49 // java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#42 = Utf8 msg == \u0001
#43 = Utf8 makeConcatWithConstants
#44 = Utf8 (Ljava/lang/String;)Ljava/lang/String;
#45 = Utf8 java/io/PrintStream
#46 = Utf8 println
#47 = Utf8 (Ljava/lang/String;)V
#48 = Class #50 // java/lang/invoke/StringConcatFactory
#49 = NameAndType #43:#54 // makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#50 = Utf8 java/lang/invoke/StringConcatFactory
#51 = Class #56 // java/lang/invoke/MethodHandles$Lookup
#52 = Utf8 Lookup
#53 = Utf8 InnerClasses
#54 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#55 = Class #57 // java/lang/invoke/MethodHandles
#56 = Utf8 java/lang/invoke/MethodHandles$Lookup
#57 = Utf8 java/lang/invoke/MethodHandles
{
public com.xixi.jvm.classdef.Hello();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/xixi/jvm/classdef/Hello;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: getstatic #3 // Field msg:Ljava/lang/String;
6: invokedynamic #4, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;)Ljava/lang/String;
11: invokevirtual #5 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: return
LineNumberTable:
line 7: 0
line 8: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 args [Ljava/lang/String;
static {};
descriptor: ()V
flags: (0x0008) ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: ldc #6 // String Good morning
2: putstatic #3 // Field msg:Ljava/lang/String;
5: return
LineNumberTable:
line 4: 0
}
SourceFile: "Hello.java"
InnerClasses:
public static final #52= #51 of #55; // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #30 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
Method arguments:
#31 msg == \u0001
常量池中存储两类常量：字面量和符号引用。
字面量：就是代码中声明为Final的常量值，
符号引用：如类和接口的全局限定名，字段的名称和描述符，方法的名称和描述符，
image.png
常量计数器：由于常量的数量不固定，所以需要防止两个字节俩存储常量池的数量值。
image.png