[issue tracker]: https://youtrack.jetbrains.com/newIssue?project=IDEA&clearDraft=true&c=Subsystem%20Java.%20Decompiler.%20Engine

# Fernflower

Fernflower is the first actually working analytical decompiler for Java and probably for a high-level programming language in general.

Releases are published in the [Intellij Community repository](https://www.jetbrains.com/intellij-repository/releases).

Please send your bug reports and feature requests to the [issue tracker] (in subsystem `Java. Decompiler. Engine`).

### IntelliJ usage

Fernflower is bundles as in IntelliJ IDEA and is primarily used to show Java code when debugging or navigating through class files.

### Command line usage

Fernflower can be run from the command line in stand-alone mode using the following command:

```console
java -jar fernflower.jar [-<option>=<value>]* [<source>]+ <destination>
```

`*` means zero or more times

`+` means one or more times

`<source>`: 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 analyzing
relationships between classes or methods.
Especially renaming of identifiers (see the `ren` option) can benefit from information about external classes.

`<destination>`: destination directory to place the resulting Java source into

`<option>=<value>`: a command-line option with the corresponding value (see "Command-line options" below).

#### Examples

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

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

#### Command-line options

Except for `mpm` and `urc` the value of 1 means the option is activated, 0 - deactivated.
The 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` (0): 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
- `ump` (1): reconstruct parameter names from corresponding attributes, 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 a user-supplied class implementing `IIdentifierRenamer` interface. It is used to determine which class identifiers should be renamed and provides new identifier names (see "Renaming identifiers")
- `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 a 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)
- `crp` (0): use record patterns where it is possible
- `cps` (0): use switch with patterns where it is possible
- `log` (INFO): a logging level, possible values are TRACE, INFO, WARN, ERROR
- `iec` (0): include the entire classpath in context when decompiling
- `isl` (1): inline simple lambda expressions
- `ucrc` (1): hide unnecessary record constructor and getters
- `cci` (1): check if resource in try-with-resources actually implements `AutoCloseable` interface
- `jvn` (0): overwrite any local variable names with JAD style names
- `jpr` (0): include parameter names in JAD naming
- `pbo` (0): add clarifying parentheses when bitwise/shift operators are mixed with arithmetic

#### 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. The 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)_\<consecutive unique number>  
  You can overwrite these 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.IMemberIdentifierRenamer` in the option `urc`
  (e.g. -urc=com.example.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.

### Development

Build an executable start-up script:

```console
./gradlew :installDist
```

The startup script is generated in `build/install/engine/bin`.

### Contributing

Fernflower is part of the [IntelliJ Community](https://github.com/JetBrains/intellij-community) project, contributions are welcome, the 
preferred way of sending patches is by creating a pull request in the IntelliJ community repository. When submitting patches, please
follow the [contribution guidelines](https://github.com/JetBrains/intellij-community/blob/master/CONTRIBUTING.md).

### License

Fernflower is licensed under the [Apache License Version 2.0](http://www.apache.org/licenses/LICENSE-2.0).

### Credits

Fernflower was originally developed by [Stiver](https://blog.jetbrains.com/idea/2024/11/in-memory-of-stiver) but has been maintained by
JetBrains since 2014. Some patches from [ForgeFlower](https://github.com/MinecraftForge/ForgeFlower) were merged into this repository. The
mirror of Fernflower was also maintained by [Andrew McRae](https://github.com/fesh0r) for a very long time.

### Naming

The correct name is Fernflower, not FernFlower.