How to use Fernflower from the command line to decompile a JAR file?

Run 'java -jar fernflower.jar .jar ' with optional flags like -e= for libraries and -ren=1 for obfuscated code. Refer to the README for examples and over 30 customization options to tweak output.

Fernflower vs CFR: which decompiler is better for Java?

Fernflower excels in producing recompilable code through analytical methods, making it ideal for serious reverse engineering. CFR is also accurate but may offer different trade-offs in speed or output style; choose Fernflower for integration with IntelliJ or when correctness is critical.

How to rename obfuscated class names in Fernflower?

Enable the -ren=1 option to auto-rename ambiguous identifiers based on length and reserved words. For custom rules, implement the IIdentifierRenamer interface and specify it with -urc=, as described in the 'Renaming identifiers' section of the README.

Does Fernflower support modern Java features like records and lambdas?

Yes, Fernflower supports records, lambdas, switch expressions, and other modern Java features, with options like -crp for record patterns and -lac for lambda handling. Always check the latest version for full compatibility with new Java releases.

Why does my decompiled code not compile even with Fernflower?

Ensure you've provided all necessary library references with -e= options and adjusted settings like -din for inner classes. Fernflower aims for recompilability, but heavily obfuscated code or missing debug info might require manual fixes or advanced renaming.

How to integrate Fernflower into a custom tool or script?

Invoke Fernflower programmatically via its JAR file in Java applications, or use the command-line interface in build tools like Gradle or Maven. Build from source with ./gradlew :installDist for a standalone script, as noted in the Development section.

Fernflower — Java Bytecode Decompiler

What is Fernflower?

Fernflower is a decompiler that converts Java bytecode back into Java source code. It solves the problem of recovering readable source from compiled .class files, enabling developers to debug, analyze, or understand third-party libraries when source isn't available. It uses analytical methods to reconstruct code structure rather than simple pattern matching.

Target Audience

Java developers, reverse engineers, and security researchers who need to examine compiled Java code for debugging, auditing, or educational purposes.

Value Proposition

Developers choose Fernflower because it produces semantically accurate, recompilable Java code and is the trusted engine behind IntelliJ IDEA's decompiler. Its analytical approach and extensive customization options make it superior for handling obfuscated or complex bytecode.

Decompiler from Java bytecode to Java, used in IntelliJ IDEA.

Use Cases

Best For

Debugging third-party Java libraries when source code is unavailable
Reverse engineering obfuscated Java applications for security analysis
Recovering lost source code from compiled JAR files
Understanding bytecode behavior in educational or research contexts
Integrating decompilation into custom tooling via its command-line interface
Auditing Java dependencies for license compliance or vulnerability assessment

Not Ideal For

Projects requiring instantly beautiful, formatted source code for presentation or documentation without post-processing
Teams needing a graphical interface for interactive decompilation without integrating IntelliJ IDEA
Situations where decompilation speed is paramount over semantic accuracy, such as quick exploratory analysis
Cases involving non-standard or experimental Java bytecode features that Fernflower's analytical engine may not yet fully support

Pros & Cons

Pros

Analytical Accuracy

Uses control and data flow analysis to reconstruct semantically correct Java code, ensuring it can be recompiled, unlike pattern-based decompilers that may produce syntactically invalid output.

Extensive Customization

Offers over 30 command-line options for fine-tuning output, including handling generics, lambdas, enums, and obfuscated code, as detailed in the README's option list.

Library Awareness

Accepts external library references via the -e= option to improve type analysis and identifier renaming, crucial for accurate decompilation of code with dependencies.

Debug Integration

Reconstructs variable and parameter names from debug attributes when available, enhancing the readability and usability of decompiled source code for debugging purposes.

Cons

Configuration Complexity

The multitude of command-line options and the need for custom IIdentifierRenamer implementations for advanced renaming can be overwhelming and time-consuming for users seeking quick results.

No Standalone GUI

Lacks a built-in graphical interface outside of IntelliJ IDEA, requiring command-line proficiency or additional tooling for those who prefer visual interaction with decompiled code.

Aesthetic Trade-offs

Prioritizes correctness over prettiness, so output may lack ideal formatting or require manual cleanup for human readability, especially with obfuscated or complex bytecode.

Frequently Asked Questions

What is Fernflower?

Target Audience

Java developers, reverse engineers, and security researchers who need to examine compiled Java code for debugging, auditing, or educational purposes.

Value Proposition

Use Cases

Best For

Debugging third-party Java libraries when source code is unavailable
Reverse engineering obfuscated Java applications for security analysis
Recovering lost source code from compiled JAR files
Understanding bytecode behavior in educational or research contexts
Integrating decompilation into custom tooling via its command-line interface
Auditing Java dependencies for license compliance or vulnerability assessment

Not Ideal For

Projects requiring instantly beautiful, formatted source code for presentation or documentation without post-processing
Teams needing a graphical interface for interactive decompilation without integrating IntelliJ IDEA
Situations where decompilation speed is paramount over semantic accuracy, such as quick exploratory analysis
Cases involving non-standard or experimental Java bytecode features that Fernflower's analytical engine may not yet fully support

Pros & Cons

Pros

Analytical Accuracy

Uses control and data flow analysis to reconstruct semantically correct Java code, ensuring it can be recompiled, unlike pattern-based decompilers that may produce syntactically invalid output.

Extensive Customization

Offers over 30 command-line options for fine-tuning output, including handling generics, lambdas, enums, and obfuscated code, as detailed in the README's option list.

Library Awareness

Accepts external library references via the -e= option to improve type analysis and identifier renaming, crucial for accurate decompilation of code with dependencies.

Debug Integration

Reconstructs variable and parameter names from debug attributes when available, enhancing the readability and usability of decompiled source code for debugging purposes.

Cons

Configuration Complexity

The multitude of command-line options and the need for custom IIdentifierRenamer implementations for advanced renaming can be overwhelming and time-consuming for users seeking quick results.

No Standalone GUI

Lacks a built-in graphical interface outside of IntelliJ IDEA, requiring command-line proficiency or additional tooling for those who prefer visual interaction with decompiled code.

Aesthetic Trade-offs

Prioritizes correctness over prettiness, so output may lack ideal formatting or require manual cleanup for human readability, especially with obfuscated or complex bytecode.

Frequently Asked Questions

Fernflower

What is Fernflower?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?

Fernflower

What is Fernflower?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?