反编译是有些应用场景的。
方法也不只一种。
1、jad.exe实现反编译。
2、阿里arthas监控工具带。
3、idea直接打开.class文件就是反编译(是不是很方便)。

这里只说jad.exe这种。

下载

官网下载地址(这是jad158g.win版本)：
https://varaneckas.com/jad/jad158g.win.zip

解压，放到文件夹中，可以放到java同目录，便于寻找。

然后配置环境变量。path中添加jad.exe所在的文件路径即可。
如： D:\Program Files\jad\jad158g.win

反编译

打开cmd，输入：jad 看是否有正确返回

反编译：jad ./demo.class
会在同目录生成demo.jad(注：扩展名默认是.jad)
可以通过-s(-s - output file extension (default: .jad))指定扩展名。

jad -sjava ./demo.class

注： 不要用-sJava(网上看有写作大写的Java的，是不对的)。

其他

官网地址

https://varaneckas.com/jad/

选最上面的版本就行，个人选的是这个。
Jad 1.5.8g for Windows 9x/NT/2000 on Intel platform (238600 bytes).

命令参数

直接复制一份了，也不翻译，就是为了看着方便。

PS C:\Users\PC\Desktop> jad
Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov (jad@kpdus.com).
Usage:    jad [option(s)] <filename(s)>
Options: -a       - generate JVM instructions as comments (annotate)-af      - output fully qualified names when annotating-b       - generate redundant braces (braces)-clear   - clear all prefixes, including the default ones-d <dir> - directory for output files-dead    - try to decompile dead parts of code (if there are any)-dis     - disassembler only (disassembler)-f       - generate fully qualified names (fullnames)-ff      - output fields before methods (fieldsfirst)-i       - print default initializers for fields (definits)-l<num>  - split strings into pieces of max <num> chars (splitstr)-lnc     - output original line numbers as comments (lnc)-lradix<num>- display long integers using the specified radix-nl      - split strings on newline characters (splitstr)-noconv  - don't convert Java identifiers into valid ones (noconv)-nocast  - don't generate auxiliary casts-noclass - don't convert .class operators-nocode  - don't generate the source code for methods-noctor  - suppress the empty constructors-nodos   - turn off check for class files written in DOS mode-nofd    - don't disambiguate fields with the same names (nofldis)-noinner - turn off the support of inner classes-nolvt   - ignore Local Variable Table entries (nolvt)-nonlb   - don't insert a newline before opening brace (nonlb)-o       - overwrite output files without confirmation-p       - send all output to STDOUT (for piping)-pa <pfx>- prefix for all packages in generated source files-pc <pfx>- prefix for classes with numerical names (default: _cls)-pe <pfx>- prefix for unused exception names (default: _ex)-pf <pfx>- prefix for fields with numerical names (default: _fld)-pi<num> - pack imports into one line using .* (packimports)-pl <pfx>- prefix for locals with numerical names (default: _lcl)-pm <pfx>- prefix for methods with numerical names (default: _mth)-pp <pfx>- prefix for method parms with numerical names (default:_prm)-pv<num> - pack fields with the same types into one line (packfields)-r       - restore package directory structure-radix<num>- display integers using the specified radix (8, 10, or 16)-s <ext> - output file extension (default: .jad)-safe    - generate additional casts to disambiguate methods/fields-space   - output space between keyword (if, while, etc) and expression-stat    - show the total number of processed classes/methods/fields-t<num>  - use <num> spaces for indentation (default: 4)-t       - use tabs instead of spaces for indentation-v       - show method names while decompiling-8       - convert Unicode strings into ANSI strings (ansi)-&       - redirect STDERR to STDOUT