JarCastingAbout fernflower
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 [email protected]
Licence
Fernflower is licenced under the Apache Licence Version 2.0. It can be found here: http://www.apache.org/licenses/LICENSE-2.0
Running from command line
java -jar fernflower.jar [-=]* []+
- means 0 or more times
- means 1 or more times
Examples
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.
- 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.
