Fernflower, but with the things I hate about it fixed

About the decompiler

Fernflower is the first actually working analytical decompiler for Java and probably for a high-level programming language in general. Naturally it is still under development, please send your bug reports and improvement suggestions at fernflower.decompiler@gmail.com


Fernflower was originally created as closed source, obfuscated tool, but as of recently JetBrains acquired Fernflower for inclusion in their IDE, IntelliJ IDEA. Since this moment, JetBrains has opensourced this project, allowing me to take a bite of the code. After using FernFlower for several years, I have had a few things I found FernFlower did wrong (or not to my likings) which I will edit in this repository.

Changes include:

  • In for-loops, I made for (int i = 0; i < 2; ++i) become for (int i = 0; i < 2; i++) as I prefer that style.
  • Current FernFlower (well, at the time of writing) has a bug where some literals were output as 'é€' because the literal was printed as a char, which gave really messy code. I changed this bit to print the literal integer value.
  • In if-statements, FernFlower will format if (7 == obj) to if (obj == 7) (completely personal preference).
  • Added the option to order fields by their initialization order using the fdo argument.
  • Extract your decompiled jars using the -ext=1 argument (By default FernFlower does NOT extract).



Running from the command line

java -jar fernflower.jar [-

* means 0 or more times + means 1 or more times

: file or directory with files to be decompiled. Directories are recursively scanned. Allowed file extensions are class, zip and jar. Sources prefixed with -e= mean "library" files that won't be decompiled, but taken into account when analysing relationships between classes or methods. Especially renaming of identifiers (s. option 'ren') can benefit from information about external classes.
: destination directory : command line option with the corresponding value, see 4.


java -jar fernflower.jar -hes=0 -hdc=0 c:\Temp\binary\ -e=c:\Java\rt.jar c:\Temp\source\

java -jar fernflower.jar -dgs=1 c:\Temp\binary\library.jar c:\Temp\binary\Boot.class c:\Temp\source\

Command line options

With the exception of mpm and urc the value of 1 means the option is activated, 0 - deactivated. Default value, if any, is given between parentheses.

Typically, the following options will be changed by user, if any: hes, hdc, dgs, mpm, ren, urc The rest of options can be left as they are: they are aimed at professional reverse engineers.

  • fdo (1): order fields by their initialization order
  • ext (0): extract decompiled files
  • rbr (1): hide bridge methods
  • rsy (0): hide synthetic class members
  • din (1): decompile inner classes
  • dc4 (1): collapse 1.4 class references
  • das (1): decompile assertions
  • hes (1): hide empty super invocation
  • hdc (1): hide empty default constructor
  • dgs (0): decompile generic signatures
  • ner (1): assume return not throwing exceptions
  • den (1): decompile enumerations
  • rgn (1): remove getClass() invocation, when it is part of a qualified new statement
  • lit (0): output numeric literals "as-is"
  • asc (0): encode non-ASCII characters in string and character literals as Unicode escapes
  • bto (1): interpret int 1 as boolean true (workaround to a compiler bug)
  • nns (1): allow for not set synthetic attribute (workaround to a compiler bug)
  • uto (1): consider nameless types as java.lang.Object (workaround to a compiler architecture flaw)
  • udv (1): reconstruct variable names from debug information, if present
  • rer (1): remove empty exception ranges
  • fdi (1): de-inline finally structures
  • mpm (0): maximum allowed processing time per decompiled method, in seconds. 0 means no upper limit
  • ren (0): rename ambiguous (resp. obfuscated) classes and class elements
  • urc : full name of user-supplied class implementing IIdentifierRenamer. It is used to determine which class identifiers should be renamed and provides new identifier names. For more information see section 5 inn (1): check for IntelliJ IDEA-specific @NotNull annotation and remove inserted code if found lac (0): decompile lambda expressions to anonymous classes nls (0): define new line character to be used for output. 0 - '\r\n' (Windows), 1 - '\n' (Unix), default is OS-dependent ind : indentation string (default is " " (3 spaces))

The default logging level is INFO. This value can be overwritten by setting the option 'log' as follows: log (INFO): possible values TRACE, INFO, WARN, ERROR.

Renaming identifiers

Some obfuscators give classes and their member elements short, meaningless and above all ambiguous names. Recompiling of such code leads to a great number of conflicts. Therefore it is advisable to let the decompiler rename elements in its turn, ensuring uniqueness of each identifier.

Option 'ren' (i.e. -ren=1) activates renaming functionality. Default renaming strategy goes as follows:

  • rename an element if its name is a reserved word or is shorter than 3 characters
  • new names are built according to a simple pattern: (class|method|field)_

You can overwrite this rules by providing your own implementation of the 4 key methods invoked by the decompiler while renaming. Simply pass a class that implements org.jetbrains.java.decompiler.main.extern.IIdentifierRenamer in the option 'urc' (e.g. -urc=com.mypackage.MyRenamer) to Fernflower. The class must be available on the application classpath.

The meaning of each method should be clear from naming: toBeRenamed determine whether the element will be renamed, while the other three provide new names for classes, methods and fields respectively.

Please refer to errornoerror.com for more similar questions