JVM虚拟机详解------javap概述及实战示例

柔情只为你懂 2022-10-25 11:28 1422阅读 0赞

# 1. javap相关 #

javap是官方提供的，用于反编译一个或多个.class文件的java工具，使用javap命令，我们可以查看类的具体信息，有助于我们分析代码的运行状态，更好的理解jvm工作原理。

[javap官方文档][javap]

# 2. javap使用格式 #

javap <options> <classes>

# 3. option集 #

-help  --help  -?        输出此用法消息
      -version                 版本信息
      -v  -verbose             输出附加信息
      -l                       输出行号和本地变量表
      -public                  仅显示公共类和成员
      -protected               显示受保护的/公共类和成员
      -package                 显示程序包/受保护的/公共类
                               和成员 (默认)
      -p  -private             显示所有类和成员
      -c                       对代码进行反汇编
      -s                       输出内部类型签名
      -sysinfo                 显示正在处理的类的
                               系统信息 (路径, 大小, 日期, MD5 散列)
      -constants               显示最终常量
      -classpath <path>        指定查找用户类文件的位置
      -cp <path>               指定查找用户类文件的位置
      -bootclasspath <path>    覆盖引导类文件的位置

# 4. javap命令展示 #

## 4.1. -version查看版本信息 ##

javap -version
    1.8.0_211

## 4.2. -v -verbose查看附加信息 ##

javap -v -verbose Main.class
    Classfile /D:/project/idea/doit/out/production/doit/cn/yzstu/jvm/Main.class
      Last modified 2021-1-30; size 673 bytes
      MD5 checksum 63f7245b740da5d47dae0b4216057254
      Compiled from "Main.java"
    public class cn.yzstu.jvm.Main
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref #10.#28 // java/lang/Object."<init>":()V
       #2 = Class #29 // java/lang/Thread
       #3 = Methodref #2.#28 // java/lang/Thread."<init>":()V
       #4 = Class #30 // cn/yzstu/jvm/Math
       #5 = Methodref #4.#28 // cn/yzstu/jvm/Math."<init>":()V
       #6 = Methodref #4.#31 // cn/yzstu/jvm/Math.compute:()I
       #7 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream;
       #8 = Methodref #34.#35 // java/io/PrintStream.println:(I)V
       #9 = Class #36 // cn/yzstu/jvm/Main
      #10 = Class #37 // java/lang/Object
      #11 = Utf8 <init>
      #12 = Utf8 ()V
      #13 = Utf8 Code
      #14 = Utf8 LineNumberTable
      #15 = Utf8 LocalVariableTable
      #16 = Utf8 this
      #17 = Utf8 Lcn/yzstu/jvm/Main;
      #18 = Utf8 main
      #19 = Utf8 ([Ljava/lang/String;)V
      #20 = Utf8 args
      #21 = Utf8 [Ljava/lang/String;
      #22 = Utf8 math
      #23 = Utf8 Lcn/yzstu/jvm/Math;
      #24 = Utf8 compute
      #25 = Utf8 I
      #26 = Utf8 SourceFile
      #27 = Utf8 Main.java
      #28 = NameAndType #11:#12 // "<init>":()V
      #29 = Utf8 java/lang/Thread
      #30 = Utf8 cn/yzstu/jvm/Math
      #31 = NameAndType #24:#38 // compute:()I
      #32 = Class #39 // java/lang/System
      #33 = NameAndType #40:#41 // out:Ljava/io/PrintStream;
      #34 = Class #42 // java/io/PrintStream
      #35 = NameAndType #43:#44 // println:(I)V
      #36 = Utf8 cn/yzstu/jvm/Main
      #37 = Utf8 java/lang/Object
      #38 = Utf8 ()I
      #39 = Utf8 java/lang/System
      #40 = Utf8 out
      #41 = Utf8 Ljava/io/PrintStream;
      #42 = Utf8 java/io/PrintStream
      #43 = Utf8 println
      #44 = Utf8 (I)V
    { 
      public cn.yzstu.jvm.Main();
        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 6: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcn/yzstu/jvm/Main;
    
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=3, args_size=1
             0: new           #2 // class java/lang/Thread
             3: dup
             4: invokespecial #3 // Method java/lang/Thread."<init>":()V
             7: pop
             8: new           #4 // class cn/yzstu/jvm/Math
            11: dup
            12: invokespecial #5 // Method cn/yzstu/jvm/Math."<init>":()V
            15: astore_1
            16: aload_1
            17: invokevirtual #6 // Method cn/yzstu/jvm/Math.compute:()I
            20: istore_2
            21: getstatic     #7 // Field java/lang/System.out:Ljava/io/PrintStream;
            24: iload_2
            25: invokevirtual #8 // Method java/io/PrintStream.println:(I)V
            28: return
          LineNumberTable:
            line 8: 0
            line 9: 8
            line 10: 16
            line 11: 21
            line 12: 28
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      29     0  args   [Ljava/lang/String;
               16      13     1  math   Lcn/yzstu/jvm/Math;
               21       8     2 compute   I
    }
    SourceFile: "Main.java"

这一部分信息，建议大家配合我的另一篇文章[jvm栈工作原理][jvm]，里面有分析这些信息的工具

## 4.3. -l ##

javap -l Main.class
    Compiled from "Main.java"
    public class cn.yzstu.jvm.Main { 
      public cn.yzstu.jvm.Main();
        LineNumberTable:
          line 6: 0
        LocalVariableTable:
          Start  Length  Slot  Name   Signature
              0       5     0  this   Lcn/yzstu/jvm/Main;
    
      public static void main(java.lang.String[]);
        LineNumberTable:
          line 8: 0
          line 9: 8
          line 10: 16
          line 11: 21
          line 12: 28
        LocalVariableTable:
          Start  Length  Slot  Name   Signature
              0      29     0  args   [Ljava/lang/String;
             16      13     1  math   Lcn/yzstu/jvm/Math;
             21       8     2 compute   I
    }

## 4.4. 其他 ##

剩下的懒得去展示了，自己去执行命令看吧(狗头)

# 5. javap实战示例 #

在[jvm栈工作原理][jvm]中其实有些许体现，但是有些地方没讲清楚，这里说一下我在使用javap命令时的思路

## 5.1. 反编译 ##

首先我们拿到的是.class文件，里面都是字节码，难以阅读，所以首先需要反编译.class字节码，获取指令码

javap -c Main.class > Main.txt

上面的命令可以将我们的Main.class字节码文件反汇编成指令码，并且保存到同路径下的Main.txt文件中

Compiled from "Main.java"
    public class cn.yzstu.jvm.Main {
      public cn.yzstu.jvm.Main();
        Code:
           0: aload_0
           1: invokespecial #1                  // Method java/lang/Object."<init>":()V
           4: return
    
      public static void main(java.lang.String[]);
        Code:
           0: new           #2                  // class java/lang/Thread
           3: dup
           4: invokespecial #3                  // Method java/lang/Thread."<init>":()V
           7: pop
           8: new           #4                  // class cn/yzstu/jvm/Math
          11: dup
          12: invokespecial #5                  // Method cn/yzstu/jvm/Math."<init>":()V
          15: astore_1
          16: aload_1
          17: invokevirtual #6                  // Method cn/yzstu/jvm/Math.compute:()I
          20: istore_2
          21: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
          24: iload_2
          25: invokevirtual #8                  // Method java/io/PrintStream.println:(I)V
          28: return
    }

看不懂指令码的朋友可以扫描文章最下方我的公众号二维码，关注后回复“jvm指令手册”获取文档

## 5.2. 查看附加信息 ##

有些指令码后面带有类似“\#5”之类的参数，这些参数代表的意义可以在附加信息中查看，执行一下命令

javap -v Main.class

在执行结果中的常量池中可以看到其对应的信息

Constant pool:
       #1 = Methodref          #10.#28        // java/lang/Object."<init>":()V
       #2 = Class              #29            // java/lang/Thread
       #3 = Methodref          #2.#28         // java/lang/Thread."<init>":()V
       #4 = Class              #30            // cn/yzstu/jvm/Math
       #5 = Methodref          #4.#28         // cn/yzstu/jvm/Math."<init>":()V
       #6 = Methodref          #4.#31         // cn/yzstu/jvm/Math.compute:()I
       #7 = Fieldref           #32.#33        // java/lang/System.out:Ljava/io/PrintStream;
       #8 = Methodref          #34.#35        // java/io/PrintStream.println:(I)V
       #9 = Class              #36            // cn/yzstu/jvm/Main
      #10 = Class              #37            // java/lang/Object
      #11 = Utf8               <init>
      #12 = Utf8               ()V
      #13 = Utf8               Code
      #14 = Utf8               LineNumberTable
      #15 = Utf8               LocalVariableTable
      #16 = Utf8               this
      #17 = Utf8               Lcn/yzstu/jvm/Main;
      #18 = Utf8               main
      #19 = Utf8               ([Ljava/lang/String;)V
      #20 = Utf8               args
      #21 = Utf8               [Ljava/lang/String;
      #22 = Utf8               math
      #23 = Utf8               Lcn/yzstu/jvm/Math;
      #24 = Utf8               compute
      #25 = Utf8               I
      #26 = Utf8               SourceFile
      #27 = Utf8               Main.java
      #28 = NameAndType        #11:#12        // "<init>":()V
      #29 = Utf8               java/lang/Thread
      #30 = Utf8               cn/yzstu/jvm/Math
      #31 = NameAndType        #24:#38        // compute:()I
      #32 = Class              #39            // java/lang/System
      #33 = NameAndType        #40:#41        // out:Ljava/io/PrintStream;
      #34 = Class              #42            // java/io/PrintStream
      #35 = NameAndType        #43:#44        // println:(I)V
      #36 = Utf8               cn/yzstu/jvm/Main
      #37 = Utf8               java/lang/Object
      #38 = Utf8               ()I
      #39 = Utf8               java/lang/System
      #40 = Utf8               out
      #41 = Utf8               Ljava/io/PrintStream;
      #42 = Utf8               java/io/PrintStream
      #43 = Utf8               println
      #44 = Utf8               (I)V

这就是阅读指令码的简单实战

# 6. 总结 #

javap是一个强大工具类，熟练使用后能够帮助你更好的理解java运行机制。

我是Baldwin，一个25岁的程序员，致力于让学习变得更有趣！  
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往期好文：

[用Python每天给女神发一句手机短信情话][Python]

[MySQL优化之explain][MySQL_explain]

[Spring源码分析-MVC初始化][Spring_-MVC]

[春风得意马蹄疾，一文看尽（JVM）虚拟机][JVM]

[造轮子的艺术][Link 1]

[源码阅读技巧][Link 2]

[Java注解详解][Java]

[教你自建SpringBoot服务器][SpringBoot]

[更多文章请点击][Link 3]

[javap]: https://docs.oracle.com/javase/8/docs/technotes/tools/windows/javap.html
[jvm]: https://blog.csdn.net/shouchenchuan5253/article/details/113449227
[20210131163408612.gif_pic_center]: /images/20221024/b5c9107f03f44a8da120c0fa596acc73.png
[Python]: https://blog.csdn.net/shouchenchuan5253/article/details/105753596
[MySQL_explain]: https://blog.csdn.net/shouchenchuan5253/article/details/105722148
[Spring_-MVC]: https://blog.csdn.net/shouchenchuan5253/article/details/105625890
[JVM]: https://yzstu.blog.csdn.net/article/details/105462458
[Link 1]: https://blog.csdn.net/shouchenchuan5253/article/details/105256723
[Link 2]: https://blog.csdn.net/shouchenchuan5253/article/details/105196154
[Java]: https://blog.csdn.net/shouchenchuan5253/article/details/105145725
[SpringBoot]: https://blog.csdn.net/shouchenchuan5253/article/details/104773702
[Link 3]: https://blog.csdn.net/shouchenchuan5253/article/details/105020803