oracle官方的反解析工具：javap详解-编程知识

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.classLast modified 2020-9-6; size 1358 bytesMD5 checksum 526b4a845e4d98180438e4c5781b7e88Compiled from "JavapTest.java"
public class com.atguigu.java1.JavapTestminor version: 0major version: 52flags: 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: Iflags: ACC_PRIVATEboolean flag;descriptor: Zflags:protected char gender;descriptor: Cflags: ACC_PROTECTEDpublic java.lang.String info;descriptor: Ljava/lang/String;flags: ACC_PUBLICpublic static final int COUNTS;descriptor: Iflags: ACC_PUBLIC, ACC_STATIC, ACC_FINALConstantValue: int 1public com.atguigu.java1.JavapTest();descriptor: ()Vflags: ACC_PUBLICCode:stack=2, locals=1, args_size=10: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: aload_05: ldc           #2                  // String java7: putfield      #3                  // Field info:Ljava/lang/String;10: returnLineNumberTable:line 20: 0line 18: 4line 22: 10LocalVariableTable:Start  Length  Slot  Name   Signature0      11     0  this   Lcom/atguigu/java1/JavapTest;private com.atguigu.java1.JavapTest(boolean);descriptor: (Z)Vflags: ACC_PRIVATECode:stack=2, locals=2, args_size=20: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: aload_05: ldc           #2                  // String java7: putfield      #3                  // Field info:Ljava/lang/String;10: aload_011: iload_112: putfield      #4                  // Field flag:Z15: returnLineNumberTable:line 23: 0line 18: 4line 24: 10line 25: 15LocalVariableTable:Start  Length  Slot  Name   Signature0      16     0  this   Lcom/atguigu/java1/JavapTest;0      16     1  flag   Zprivate void methodPrivate();descriptor: ()Vflags: ACC_PRIVATECode:stack=0, locals=1, args_size=10: returnLineNumberTable:line 28: 0LocalVariableTable:Start  Length  Slot  Name   Signature0       1     0  this   Lcom/atguigu/java1/JavapTest;int getNum(int);descriptor: (I)Iflags:Code:stack=2, locals=2, args_size=20: aload_01: getfield      #5                  // Field num:I4: iload_15: iadd6: ireturnLineNumberTable:line 30: 0LocalVariableTable:Start  Length  Slot  Name   Signature0       7     0  this   Lcom/atguigu/java1/JavapTest;0       7     1     i   Iprotected char showGender();descriptor: ()Cflags: ACC_PROTECTEDCode:stack=1, locals=1, args_size=10: aload_01: getfield      #6                  // Field gender:C4: ireturnLineNumberTable:line 33: 0LocalVariableTable:Start  Length  Slot  Name   Signature0       5     0  this   Lcom/atguigu/java1/JavapTest;public void showInfo();descriptor: ()Vflags: ACC_PUBLICCode:stack=3, locals=2, args_size=10: bipush        102: istore_13: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;6: new           #8                  // class java/lang/StringBuilder9: dup10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V13: aload_014: getfield      #3                  // Field info:Ljava/lang/String;17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;20: iload_121: 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;)V30: returnLineNumberTable:line 36: 0line 37: 3line 38: 30LocalVariableTable:Start  Length  Slot  Name   Signature0      31     0  this   Lcom/atguigu/java1/JavapTest;3      28     1     i   Istatic {};descriptor: ()Vflags: ACC_STATICCode:stack=1, locals=1, args_size=00: ldc           #14                 // String www.atguigu.com2: astore_03: returnLineNumberTable:line 15: 0line 16: 3LocalVariableTable:Start  Length  Slot  Name   Signature
}
SourceFile: "JavapTest.java"