1、解析字节码的作用

  • 通过反编译生成的字节码文件，我们可以深入的了解java代码的工作机制。但是，自己分析类文件结构太麻烦了!除了使用第三方的jclasslib工具之外，oracle官方也提供了工具:javap。
  • javap是jdk自带的反解析工具。它的作用就是根据class字节码文件，反解析出当前类对应的code区（字节码指令)、局部变量表、异常表和代码行偏移量映射表、常量池等信息。
  • 通过局部变量表，我们可以查看局部变量的作用域范围、所在槽位等信息，甚至可以看到槽位复用等信息。

    2、javac -g操作

    解析字节码文件得到的信息中，有些信息（如局部变量表、指令和代码行偏移量映射表、常量池中方法的参数名称等等）需要在使用javac编译成class文件时，指定参数才能输出。
    比如，你直接javac xx.java，就不会在生成对应的局部变量表等信息，如果你使用javac -g xx.java就可以生成所有相关信息了。如果你使用的eclipse或IDEA，则默认情况下，eclipse、IDEA在编译时会帮你生成局部变量表、指令和代码行偏移量映射表等信息的。

    3、javap的使用

    javap的用法格式:
    javap
    其中，classes就是你要反编译的class文件。
    在命令行中直接输入javap或javap -help可以看到javap的options有如下选项:
    image.png
    image.png
    image.png

4、使用举例

  1. public class JavapTest{
  3.     private int num;
  4.     boolean flag;
  5.     protected char gender;
  6.     public String info;
  8.     public static final int COUNTS = 1;
  9.      static{
  10.          String url = "www.atguigu.com";
  11.      }
  12.     {
  13.         info = "java";
  14.     }
  16.     public JavapTest(){}
  18.     private JavapTest(boolean flag){
  19.         this.flag = flag;
  20.     }
  21.     private void methodPrivate(){
  23.     }
  24.     int getNum(int i){
  25.         return num + i;
  26.     }
  27.     protected char shwoGender(){
  28.         return gender;
  29.     }
  30.     public void shwoInfo(){
  31.         int i = 10;
  32.         System.out.println(info + i);
  33.     }
  34. }
  1. Classfile /F:/home/idea/JavapTest.class           //字节码文件所属路径
  2.   Last modified 2021-4-26; size 1130 bytes        //最后修改时间，字节码文件的大小
  3.   MD5 checksum 206819f6dea876b5ebbd2afa2aaa3fe1   //MD5散列值
  4.   Compiled from "JavapTest.java"                  //源文件的名称
  5. public class JavapTest
  6.   minor version: 0                                //副版本
  7.   major version: 52                               //主版本
  8.   flags: ACC_PUBLIC, ACC_SUPER                    //访问标识
  9. Constant pool:                                    //常量池
  10.    #1 = Methodref          #16.#42        // java/lang/Object."<init>":()V
  11.    #2 = String             #43            // java
  12.    #3 = Fieldref           #15.#44        // JavapTest.info:Ljava/lang/String;
  13.    #4 = Fieldref           #15.#45        // JavapTest.flag:Z
  14.    #5 = Fieldref           #15.#46        // JavapTest.num:I
  15.    #6 = Fieldref           #15.#47        // JavapTest.gender:C
  16.    #7 = Fieldref           #48.#49        // java/lang/System.out:Ljava/io/PrintStream;
  17.    #8 = Class              #50            // java/lang/StringBuilder
  18.    #9 = Methodref          #8.#42         // java/lang/StringBuilder."<init>":()V
  19.   #10 = Methodref          #8.#51         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  20.   #11 = Methodref          #8.#52         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  21.   #12 = Methodref          #8.#53         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  22.   #13 = Methodref          #54.#55        // java/io/PrintStream.println:(Ljava/lang/String;)V
  23.   #14 = String             #56            // www.atguigu.com
  24.   #15 = Class              #57            // JavapTest
  25.   #16 = Class              #58            // java/lang/Object
  26.   #17 = Utf8               num
  27.   #18 = Utf8               I
  28.   #19 = Utf8               flag
  29.   #20 = Utf8               Z
  30.   #21 = Utf8               gender
  31.   #22 = Utf8               C
  32.   #23 = Utf8               info
  33.   #24 = Utf8               Ljava/lang/String;
  34.   #25 = Utf8               COUNTS
  35.   #26 = Utf8               ConstantValue
  36.   #27 = Integer            1
  37.   #28 = Utf8               <init>
  38.   #29 = Utf8               ()V
  39.   #30 = Utf8               Code
  40.   #31 = Utf8               LineNumberTable
  41.   #32 = Utf8               (Z)V
  42.   #33 = Utf8               methodPrivate
  43.   #34 = Utf8               getNum
  44.   #35 = Utf8               (I)I
  45.   #36 = Utf8               shwoGender
  46.   #37 = Utf8               ()C
  47.   #38 = Utf8               shwoInfo
  48.   #39 = Utf8               <clinit>
  49.   #40 = Utf8               SourceFile
  50.   #41 = Utf8               JavapTest.java
  51.   #42 = NameAndType        #28:#29        // "<init>":()V
  52.   #43 = Utf8               java
  53.   #44 = NameAndType        #23:#24        // info:Ljava/lang/String;
  54.   #45 = NameAndType        #19:#20        // flag:Z
  55.   #46 = NameAndType        #17:#18        // num:I
  56.   #47 = NameAndType        #21:#22        // gender:C
  57.   #48 = Class              #59            // java/lang/System
  58.   #49 = NameAndType        #60:#61        // out:Ljava/io/PrintStream;
  59.   #50 = Utf8               java/lang/StringBuilder
  60.   #51 = NameAndType        #62:#63        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  61.   #52 = NameAndType        #62:#64        // append:(I)Ljava/lang/StringBuilder;
  62.   #53 = NameAndType        #65:#66        // toString:()Ljava/lang/String;
  63.   #54 = Class              #67            // java/io/PrintStream
  64.   #55 = NameAndType        #68:#69        // println:(Ljava/lang/String;)V
  65.   #56 = Utf8               www.atguigu.com
  66.   #57 = Utf8               JavapTest
  67.   #58 = Utf8               java/lang/Object
  68.   #59 = Utf8               java/lang/System
  69.   #60 = Utf8               out
  70.   #61 = Utf8               Ljava/io/PrintStream;
  71.   #62 = Utf8               append
  72.   #63 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  73.   #64 = Utf8               (I)Ljava/lang/StringBuilder;
  74.   #65 = Utf8               toString
  75.   #66 = Utf8               ()Ljava/lang/String;
  76.   #67 = Utf8               java/io/PrintStream
  77.   #68 = Utf8               println
  78.   #69 = Utf8               (Ljava/lang/String;)V
  80.  ##########################字段表集合的信息########################################
  81. {
  82.   private int num;               //字段名
  83.     descriptor: I                //字段描述符：字段的类型
  84.     flags: ACC_PRIVATE           //字段的访问标识
  86.   boolean flag;
  87.     descriptor: Z
  88.     flags:
  90.   protected char gender;
  91.     descriptor: C
  92.     flags: ACC_PROTECTED
  94.   public java.lang.String info;
  95.     descriptor: Ljava/lang/String;
  96.     flags: ACC_PUBLIC
  98.   public static final int COUNTS;
  99.     descriptor: I
  100.     flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
  101.     ConstantValue: int 1                       //常量字段的属性：ConstantValue
  103. ##############################方法表集合的信息######################################
  104.   public JavapTest();                         //构造器1的信息
  105.     descriptor: ()V
  106.     flags: ACC_PUBLIC
  107.     Code:
  108.       stack=2, locals=1, args_size=1
  109.          0: aload_0
  110.          1: invokespecial #1                  // Method java/lang/Object."<init>":()V
  111.          4: aload_0
  112.          5: ldc           #2                  // String java
  113.          7: putfield      #3                  // Field info:Ljava/lang/String;
  114.         10: return
  115.       LineNumberTable:
  116.         line 16: 0
  117.         line 13: 4
  118.         line 16: 10
  120.   private JavapTest(boolean);                //构造器2的信息      
  121.     descriptor: (Z)V
  122.     flags: ACC_PRIVATE
  123.     Code:
  124.       stack=2, locals=2, args_size=2
  125.          0: aload_0
  126.          1: invokespecial #1                  // Method java/lang/Object."<init>":()V
  127.          4: aload_0
  128.          5: ldc           #2                  // String java
  129.          7: putfield      #3                  // Field info:Ljava/lang/String;
  130.         10: aload_0
  131.         11: iload_1
  132.         12: putfield      #4                  // Field flag:Z
  133.         15: return
  134.       LineNumberTable:
  135.         line 18: 0
  136.         line 13: 4
  137.         line 19: 10
  138.         line 20: 15
  140.   private void methodPrivate();
  141.     descriptor: ()V
  142.     flags: ACC_PRIVATE
  143.     Code:
  144.       stack=0, locals=1, args_size=1
  145.          0: return
  146.       LineNumberTable:
  147.         line 23: 0
  149.   int getNum(int);
  150.     descriptor: (I)I
  151.     flags:
  152.     Code:
  153.       stack=2, locals=2, args_size=2
  154.          0: aload_0
  155.          1: getfield      #5                  // Field num:I
  156.          4: iload_1
  157.          5: iadd
  158.          6: ireturn
  159.       LineNumberTable:
  160.         line 25: 0
  162.   protected char shwoGender();
  163.     descriptor: ()C
  164.     flags: ACC_PROTECTED
  165.     Code:
  166.       stack=1, locals=1, args_size=1
  167.          0: aload_0
  168.          1: getfield      #6                  // Field gender:C
  169.          4: ireturn
  170.       LineNumberTable:
  171.         line 28: 0
  173.   public void shwoInfo();
  174.     descriptor: ()V                          //方法描述符：方法的形参列表、返回值类型
  175.     flags: ACC_PUBLIC                        //方法的访问标识
  176.     Code:                                    //方法的Code属性
  177.       stack=3, locals=2, args_size=1         //stack:操作数栈的最大深度，locals:局部变量表的长度，args_size:方法接收参数的个数
  178.    //偏移量 操作码        操作数             //操作码占一个字节，操作数占两个字节 
  179.          0: bipush        10
  180.          2: istore_1
  181.          3: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
  182.          6: new           #8                  // class java/lang/StringBuilder
  183.          9: dup
  184.         10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
  185.         13: aload_0
  186.         14: getfield      #3                  // Field info:Ljava/lang/String;
  187.         17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  188.         20: iload_1
  189.         21: invokevirtual #11                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  190.         24: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  191.         27: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  192.         30: return
  193.       //行号表：指明字节码指令的偏移量与java源程序中代码的行号的一一对应关系
  194.       LineNumberTable:
  195.         line 31: 0
  196.         line 32: 3
  197.         line 33: 30
  198.       LocalVariableTable:
  199.         Start   Length   Slot    Name    Signature
  200.             0       31      0    this     JavapTest;
  201.             3       28      1      i        I
  203.   static {};
  204.     descriptor: ()V
  205.     flags: ACC_STATIC
  206.     Code:
  207.       stack=1, locals=1, args_size=0
  208.          0: ldc           #14                 // String www.atguigu.com
  209.          2: astore_0
  210.          3: return
  211.       LineNumberTable:
  212.         line 10: 0
  213.         line 11: 3
  214. }
  215. SourceFile: "JavapTest.java"  //附加属性：指明当前字节码文件对应的源程序文件名

在java文件所在目录用cmd执行 javap -v -p JavaTest.class>javaptest.txt，将反编译的字节码文件放入到javaptest.txt文件中
image.png
image.png
image.png
image.png
args_size：方法接收参数的个数
image.png
image.png
image.png

5、总结

1、通过javap命令可以查看一个java类反汇编得到的Class文件版本号、常量池、访问标识、变量表、指令代码行号表等等信息。不显示类索引、父类索引、接口索引集合、()、()等结构
2、通过对前面例子代码反汇编文件的简单分析，可以发现，一个方法的执行通常会涉及下面几块内存的操作:
(1) java栈中:局部变量表、操作数栈。
(2) java堆。通过对象的地址引用去操作。
(3）常量池。
(4）其他如帧数据区、方法区的剩余部分等情况，测试中没有显示出来，这里说明一下。
3、平常，我们比较关注的是java类中每个方法的反汇编中的指令操作过程，这些指令都是顺序执行的，可以参考官方文档查看每个指令的含义，很简单:
https : //docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html