1、验证（Verify）

• 目的在于确保Class文件的字节流中包含信息符合当前虚拟机要求，保证被加载类的正确性，不会危害虚拟机自身安全。
• 主要包括四种验证，文件格式验证，元数据验证，字节码验证，符号引用验证。

2、准备（Prepare）

package com.jvm.java;public class HelloApp {//在prepare：a=0  ---->initial：a=1private static int a = 1;public static void main(String[] args) {System.out.println(a);}
}

• 为类变量分配内存并且设置该类变量的默认初始值，即零值。
• 这里不包括含有final修饰的static，因为final在编译的时候就会分配了，准备阶段会显式初始化；
• 这里不会为实例变量分配初始化，类变量会分配在方法区中，而实例变量是会随着对象一起分配到Java堆中。

3、解析（Resolve）

• 将常量池内的符号引用转换为直接引用的过程。
• 事实上，解析操作往往会伴随着JVM在执行完初始化之后再执行。
• 符号引用就是一组符号来描述所引用的目标。符号引用的字面量形式明确定义在《java虚拟机规范》的Class文件格式中。直接引用就是直接指向目标的指针、相对偏移量或一个间接定位到目标的句柄。
• 解析动作主要针对类或接口、字段、类名称、接口方法、方法类型等。对应常量池中的CONSTANT_Class_info、CONSTANT_Fieldref_info、CONSTANT_Methodref_info等。

3.1、反编译

 

PS F:\IdeaProjects\JavaSenior\out\production\Test\com\jvm\java> javap -v HelloApp.class
Classfile /F:/IdeaProjects/JavaSenior/out/production/Test/com/jvm/java/HelloApp.classLast modified 2023-8-24; size 600 bytesMD5 checksum 5ed26a0b8b074c9a78dfa95afc3ae540Compiled from "HelloApp.java"                
public class com.jvm.java.HelloApp             minor version: 0                             major version: 61                            flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref          #2.#3          // java/lang/Object."<init>":()V#2 = Class              #4             // java/lang/Object#3 = NameAndType        #5:#6          // "<init>":()V#4 = Utf8               java/lang/Object#5 = Utf8               <init>#6 = Utf8               ()V#7 = Fieldref           #8.#9          // java/lang/System.out:Ljava/io/PrintStream;#8 = Class              #10            // java/lang/System#9 = NameAndType        #11:#12        // out:Ljava/io/PrintStream;#10 = Utf8               java/lang/System#11 = Utf8               out#12 = Utf8               Ljava/io/PrintStream;#13 = Fieldref           #14.#15        // com/jvm/java/HelloApp.a:I#14 = Class              #16            // com/jvm/java/HelloApp#15 = NameAndType        #17:#18        // a:I#16 = Utf8               com/jvm/java/HelloApp#17 = Utf8               a#18 = Utf8               I#19 = Methodref          #20.#21        // java/io/PrintStream.println:(I)V#20 = Class              #22            // java/io/PrintStream#21 = NameAndType        #23:#24        // println:(I)V#22 = Utf8               java/io/PrintStream#23 = Utf8               println#24 = Utf8               (I)V#25 = Utf8               Code#26 = Utf8               LineNumberTable#27 = Utf8               LocalVariableTable#28 = Utf8               this#29 = Utf8               Lcom/jvm/java/HelloApp;#30 = Utf8               main#31 = Utf8               ([Ljava/lang/String;)V#32 = Utf8               args#33 = Utf8               [Ljava/lang/String;#34 = Utf8               <clinit>#35 = Utf8               SourceFile#36 = Utf8               HelloApp.java
{public com.jvm.java.HelloApp();descriptor: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 3: 0LocalVariableTable:Start  Length  Slot  Name   Signature0       5     0  this   Lcom/jvm/java/HelloApp;public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=2, locals=1, args_size=10: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;3: getstatic     #13                 // Field a:I6: invokevirtual #19                 // Method java/io/PrintStream.println:(I)V9: returnStart  Length  Slot  Name   Signature0      10     0  args   [Ljava/lang/String;static {};descriptor: ()Vflags: ACC_STATICCode:stack=1, locals=0, args_size=00: iconst_11: putstatic     #13                 // Field a:I4: returnLineNumberTable:line 6: 0
}
SourceFile: "HelloApp.java"
PS F:\IdeaProjects\JavaSenior\out\production\Test\com\jvm\java>