如何获取类的jvm字节码

本是古典何须时尚 2024-04-18 19:12 45阅读 0赞

### 一、为什么要获取jvm字节码 ###

1.一些代码的执行在源码层面是看不出来的，因为jvm执行的是字节码，只有通过字节码才能清除的知道代码的执行。

### 二、javap ###

1.javap是jdk自带的一款工具，可以通过该命令获取字节码

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1MTUxMzQ2_size_16_color_FFFFFF_t_70][]

### 三、实战 ###

1.java源代码

package com.jvm;
    
    public class Test2 {
    	
    	public static void main(String[] args) {
    		int a=1;
    		int b=2;
    		int c=a+b;
    		System.out.println(c);
    	}
    }

2.获取jvm字节码

通过javac命令将Test2.java编译过后获取.class文件

![20190910215443622.png][]

通过javap命令获取Test2的jvm字节码

3.字节码文件

//最后修改时间，大小408bytes
      Last modified 2019-9-10; size 408 bytes
    
      MD5 checksum 31369d53bfd4e15c9dcfda94450c431d
      Compiled from "Test2.java"
    
    //包名和类名
    public class com.jvm.Test2
    
    //jdk版本52为jdk1.8
      minor version: 0
      major version: 52
    
    //类的访问修饰符public
      flags: ACC_PUBLIC, ACC_SUPER
    
    //常量池
    Constant pool:
    
    //Methodref（方法的引用） 引用了#5.#14；#5引用了#20，#20=java/lang/Object；
    //#14=#6:#7；即=<init>：()V ，所以#1=java/lang/Object.<init>：()V
    //Object为对象，init构造方法，（）表示无参，V代表返回值，V=void，即返回值为void
       #1 = Methodref          #5.#14         // java/lang/Object."<init>":()V
       #2 = Fieldref           #15.#16        // java/lang/System.out:Ljava/io/PrintStream;
       #3 = Methodref          #17.#18        // java/io/PrintStream.println:(I)V
       #4 = Class              #19            // com/jvm/Test2
       #5 = Class              #20            // java/lang/Object
    
    //utf8位字符串，<init>为构造方法
       #6 = Utf8               <init>
       #7 = Utf8               ()V
       #8 = Utf8               Code
       #9 = Utf8               LineNumberTable
      #10 = Utf8               main
      #11 = Utf8               ([Ljava/lang/String;)V
      #12 = Utf8               SourceFile
      #13 = Utf8               Test2.java
      #14 = NameAndType        #6:#7          // "<init>":()V
      #15 = Class              #21            // java/lang/System
      #16 = NameAndType        #22:#23        // out:Ljava/io/PrintStream;
      #17 = Class              #24            // java/io/PrintStream
      #18 = NameAndType        #25:#26        // println:(I)V
      #19 = Utf8               com/jvm/Test2
      #20 = Utf8               java/lang/Object
      #21 = Utf8               java/lang/System
      #22 = Utf8               out
      #23 = Utf8               Ljava/io/PrintStream;
      #24 = Utf8               java/io/PrintStream
      #25 = Utf8               println
      #26 = Utf8               (I)V
    {
    
      public com.jvm.Test2();
        //方法描述符无参，无返回值
        descriptor: ()V
    
        //方法访问修饰符为public
        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 3: 0
    
        //main函数
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
    
        //stack代表操作数栈的一个深度，一个方法代表一个栈帧，
        //locals本地变量表的最大长度，如果是static方法，索引从0开始，如果是非static，则0表示this
        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: istore_3
             8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            11: iload_3
            12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            15: return
    
     
          LineNumberTable:
            line 6: 0
            line 7: 2
            line 8: 4
            line 9: 8
            line 10: 15
    }
    SourceFile: "Test2.java"

4.常量池的类型有很多，具体可以看官网[https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html\#jvms-4.4.1][https_docs.oracle.com_javase_specs_jvms_se8_html_jvms-4.html_jvms-4.4.1]

\#1=utf8  \#6

\#2=class  \#5

这些都是符号引用，初始化之后，都会转换为直接引用。

5.jvm中Field description

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1MTUxMzQ2_size_16_color_FFFFFF_t_70 1][]

6.![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1MTUxMzQ2_size_16_color_FFFFFF_t_70 2][]

java里面，参数为int， double，Thread。返回则值为Object

则jvm字节码为I，D，L java/lang/Thread; 返回值为L java/lang/Object;

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[20190910215443622.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/18/1cc7003d72f64ec3856d2537e611fd10.png
[https_docs.oracle.com_javase_specs_jvms_se8_html_jvms-4.html_jvms-4.4.1]: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4.1
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1MTUxMzQ2_size_16_color_FFFFFF_t_70 1]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/18/2f5efdf6e30c4f168266e96091f10661.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1MTUxMzQ2_size_16_color_FFFFFF_t_70 2]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/18/a9ca585202544162a4669aae0865013c.png
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