Si necesitamos descompilar rapidamente un .class de Java para ver el código podemos utilizar una herramienta denominada “Jad”.

http://www.kpdus.com/jad.html

Accedemos al sitio web y nos descargamos el .zip correspondiente a nuestra version de Sistema operativo. Lo descomprimimos y le damos permisos de ejecución:

chmod +x ./jad

Si a la hora de ejecutar sale un error. nos aseguramos de que tenemos todas las dependencias instaladas con el comando “ldd”.

./jad
./jad: error while loading shared libraries: libstdc++-libc6.2-2.so.3: cannot open shared object file: No such file or directory
jose@soledad:~/Desktop$ ldd jad
linux-gate.so.1 => (0xffffe000)
libstdc++-libc6.2-2.so.3 => not found
libm.so.6 => /lib/tls/i686/cmov/libm.so.6 (0xb7f9b000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7e66000)
/lib/ld-linux.so.2 (0xb7fd3000)

En este ejemplo nos falta la libreria “libstdc++-lib6.2-2.so.3″. Tiramos de Google y nos bajamos el RPM correspondiente a esa libreria:

libstdc++2.10-2.96-0.83mdk.i586.rpm

Convertimos el rpm a .deb con el programa “alien”. Si no lo tienes instalado en tu maquina descargate el .deb correspondiente e instalalo manualmente o ejecuta:

sudo apt-get install alien

Para convertir el paquete rpm al formato debian ejecutamos:

alien -d libstdc++2.10-2.96-0.83mdk.i586.rpm

Se genera el fichero: libstdc++2.10_2.96-1.83_i386.deb que ya podemos instalar en nuestra maquina.

Resueltos los problemas de dependencias, ejecutamos Jad de la forma:

./jad <fichero.class>

Si lo ejecutamos sin parametros nos muestra la ayuda:

jose@soledad:~/Desktop$ ./jad
Jad v1.5.8e. Copyright 2001 Pavel Kouznetsov (kpdus@yahoo.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

Al descompilar el .class se genera un fichero con extension .jad en el mismo directorio con el contenido de la clase.

A descompilar!!!.