oracle官方的反解析工具：javap详解

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的用法

javap的用法格式：javap <options> <classes>

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：

这里重组一下：

 -help  --help  -?      输出此用法消息
 -version               版本信息，其实是当前javap所在jdk的版本信息，不是class在哪个jdk下生成的。
 -public                仅显示公共类和成员
 -protected             显示受保护的/公共类和成员
 -p  -private           显示所有类和成员
 -package               显示程序包/受保护的/公共类 和成员 (默认)
 -sysinfo               显示正在处理的类的系统信息 (路径, 大小, 日期, MD5 散列,源文件名)
 -constants             显示静态最终常量
 
 -s                     输出内部类型签名
 -l                     输出行号和本地变量表
 -c                     对代码进行反汇编
 -v  -verbose           输出附加信息（包括行号、本地变量表，反汇编等详细信息）
 
 -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

一般常用的是-v -l -c三个选项。
javap -l 会输出行号和本地变量表信息。
javap -c 会对当前class字节码进行反编译生成汇编代码。
javap -v classxx 除了包含-c内容外，还会输出行号、局部变量表信息、常量池等信息。

4、使用举例

public class JavapTest {
    private int num;
    boolean flag;
    protected char gender;
    public String info;

    public static final int COUNTS = 1;
    static{
        String url = "www.atguigu.com";
    }
    {
        info = "java";
    }
    public JavapTest(){

    }
    private JavapTest(boolean flag){
        this.flag = flag;
    }
    private void methodPrivate(){

    }
    int getNum(int i){
        return num + i;
    }
    protected char showGender(){
        return gender;
    }
    public void showInfo(){
        int i = 10;
        System.out.println(info + i);
    }
}

希望输出的信息比较完整的话，使用如下操作：javap -v -p JavapTest.class

对应的内容信息为：
Classfile /C:/Users/songhk/Desktop/2/JavapTest.class
  Last modified 2020-9-6; size 1358 bytes
  MD5 checksum 526b4a845e4d98180438e4c5781b7e88
  Compiled from "JavapTest.java"
public class com.atguigu.java1.JavapTest
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #16.#46        // java/lang/Object."<init>":()V
   #2 = String             #47            // java
   #3 = Fieldref           #15.#48        // com/atguigu/java1/JavapTest.info:Ljava/lang/String;
   #4 = Fieldref           #15.#49        // com/atguigu/java1/JavapTest.flag:Z
   #5 = Fieldref           #15.#50        // com/atguigu/java1/JavapTest.num:I
   #6 = Fieldref           #15.#51        // com/atguigu/java1/JavapTest.gender:C
   #7 = Fieldref           #52.#53        // java/lang/System.out:Ljava/io/PrintStream;
   #8 = Class              #54            // java/lang/StringBuilder
   #9 = Methodref          #8.#46         // java/lang/StringBuilder."<init>":()V
  #10 = Methodref          #8.#55         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #11 = Methodref          #8.#56         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #12 = Methodref          #8.#57         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #13 = Methodref          #58.#59        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #14 = String             #60            // www.atguigu.com
  #15 = Class              #61            // com/atguigu/java1/JavapTest
  #16 = Class              #62            // java/lang/Object
  #17 = Utf8               num
  #18 = Utf8               I
  #19 = Utf8               flag
  #20 = Utf8               Z
  #21 = Utf8               gender
  #22 = Utf8               C
  #23 = Utf8               info
  #24 = Utf8               Ljava/lang/String;
  #25 = Utf8               COUNTS
  #26 = Utf8               ConstantValue
  #27 = Integer            1
  #28 = Utf8               <init>
  #29 = Utf8               ()V
  #30 = Utf8               Code
  #31 = Utf8               LineNumberTable
  #32 = Utf8               LocalVariableTable
  #33 = Utf8               this
  #34 = Utf8               Lcom/atguigu/java1/JavapTest;
  #35 = Utf8               (Z)V
  #36 = Utf8               methodPrivate
  #37 = Utf8               getNum
  #38 = Utf8               (I)I
  #39 = Utf8               i
  #40 = Utf8               showGender
  #41 = Utf8               ()C
  #42 = Utf8               showInfo
  #43 = Utf8               <clinit>
  #44 = Utf8               SourceFile
  #45 = Utf8               JavapTest.java
  #46 = NameAndType        #28:#29        // "<init>":()V
  #47 = Utf8               java
  #48 = NameAndType        #23:#24        // info:Ljava/lang/String;
  #49 = NameAndType        #19:#20        // flag:Z
  #50 = NameAndType        #17:#18        // num:I
  #51 = NameAndType        #21:#22        // gender:C
  #52 = Class              #63            // java/lang/System
  #53 = NameAndType        #64:#65        // out:Ljava/io/PrintStream;
  #54 = Utf8               java/lang/StringBuilder
  #55 = NameAndType        #66:#67        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #56 = NameAndType        #66:#68        // append:(I)Ljava/lang/StringBuilder;
  #57 = NameAndType        #69:#70        // toString:()Ljava/lang/String;
  #58 = Class              #71            // java/io/PrintStream
  #59 = NameAndType        #72:#73        // println:(Ljava/lang/String;)V
  #60 = Utf8               www.atguigu.com
  #61 = Utf8               com/atguigu/java1/JavapTest
  #62 = Utf8               java/lang/Object
  #63 = Utf8               java/lang/System
  #64 = Utf8               out
  #65 = Utf8               Ljava/io/PrintStream;
  #66 = Utf8               append
  #67 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #68 = Utf8               (I)Ljava/lang/StringBuilder;
  #69 = Utf8               toString
  #70 = Utf8               ()Ljava/lang/String;
  #71 = Utf8               java/io/PrintStream
  #72 = Utf8               println
  #73 = Utf8               (Ljava/lang/String;)V
{
  private int num;
    descriptor: I
    flags: ACC_PRIVATE
 
  boolean flag;
    descriptor: Z
    flags:
 
  protected char gender;
    descriptor: C
    flags: ACC_PROTECTED
 
  public java.lang.String info;
    descriptor: Ljava/lang/String;
    flags: ACC_PUBLIC
 
  public static final int COUNTS;
    descriptor: I
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    ConstantValue: int 1
 
  public com.atguigu.java1.JavapTest();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: ldc           #2                  // String java
         7: putfield      #3                  // Field info:Ljava/lang/String;
        10: return
      LineNumberTable:
        line 20: 0
        line 18: 4
        line 22: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/atguigu/java1/JavapTest;
 
  private com.atguigu.java1.JavapTest(boolean);
    descriptor: (Z)V
    flags: ACC_PRIVATE
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: ldc           #2                  // String java
         7: putfield      #3                  // Field info:Ljava/lang/String;
        10: aload_0
        11: iload_1
        12: putfield      #4                  // Field flag:Z
        15: return
      LineNumberTable:
        line 23: 0
        line 18: 4
        line 24: 10
        line 25: 15
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      16     0  this   Lcom/atguigu/java1/JavapTest;
            0      16     1  flag   Z
 
  private void methodPrivate();
    descriptor: ()V
    flags: ACC_PRIVATE
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 28: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0  this   Lcom/atguigu/java1/JavapTest;
 
  int getNum(int);
    descriptor: (I)I
    flags:
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: getfield      #5                  // Field num:I
         4: iload_1
         5: iadd
         6: ireturn
      LineNumberTable:
        line 30: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/atguigu/java1/JavapTest;
            0       7     1     i   I
 
  protected char showGender();
    descriptor: ()C
    flags: ACC_PROTECTED
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #6                  // Field gender:C
         4: ireturn
      LineNumberTable:
        line 33: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/atguigu/java1/JavapTest;
 
  public void showInfo();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=2, args_size=1
         0: bipush        10
         2: istore_1
         3: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
         6: new           #8                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
        13: aload_0
        14: getfield      #3                  // Field info:Ljava/lang/String;
        17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        20: iload_1
        21: invokevirtual #11                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        24: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        27: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        30: return
      LineNumberTable:
        line 36: 0
        line 37: 3
        line 38: 30
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      31     0  this   Lcom/atguigu/java1/JavapTest;
            3      28     1     i   I
 
  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=1, args_size=0
         0: ldc           #14                 // String www.atguigu.com
         2: astore_0
         3: return
      LineNumberTable:
        line 15: 0
        line 16: 3
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
}
SourceFile: "JavapTest.java"