BSKJ/java-decompiler-plugin/references/decompilers.md

Java Decompilers Reference

Recommended Decompilers

1. CFR (Class File Reader) - Recommended

Best for: Modern Java (Java 8+), lambda expressions, enums

• Download: https://github.com/leibnitz27/cfr/releases
• Version: 0.152
• Usage: java -jar cfr.jar MyClass.class

Key features:

• Excellent Java 8+ support
• Handles modern language features
• Active development
• Command-line interface

2. Procyon

Best for: Complex generics, enum switch maps

• Download: https://github.com/mstrobel/procyon/releases
• Usage: java -jar procyon-decompiler.jar MyClass.class

3. Fernflower

Best for: IntelliJ IDEA integration

• Built into: IntelliJ IDEA
• Standalone: https://github.com/fesh0r/fernflower
• Usage: java -jar fernflower.jar MyClass.class output/

4. JD-GUI

Best for: Quick GUI browsing

• Download: http://java-decompiler.github.io/
• Features: Graphical interface, export source
• Usage: Open .class or .jar file in GUI

5. JADX

Best for: Android DEX/Android bytecode

• Download: https://github.com/skylot/jadx
• Features: GUI and CLI, Android support

Decompiler Comparison

Feature	CFR	Procyon	Fernflower	JD-GUI
Java 8+	✅	✅	✅	⚠️
Lambda	✅	✅	✅	❌
Enum	✅	✅	✅	⚠️
Generics	✅	✅	✅	⚠️
Speed	Fast	Medium	Fast	Fast
CLI	✅	✅	✅	❌
GUI	❌	❌	❌	✅

CFR Usage Examples

Basic Decompilation

# Single file
java -jar cfr.jar MyClass.class

# Output to directory
java -jar cfr.jar MyClass.class --outputdir ./src

# Multiple files
java -jar cfr.jar *.class --outputdir ./src

Advanced Options

# Decode lambda expressions
java -jar cfr.jar MyClass.class --decodelambdas true

# Remove boilerplate
java -jar cfr.jar MyClass.class --removeboilerplate true

# Handle obfuscated code
java -jar cfr.jar MyClass.class --renamedupmembers true

# Add classpath for dependencies
java -jar cfr.jar MyClass.class --extraclasspath ./lib/*.jar

Batch Processing

# Decompile all classes in directory
find ./classes -name "*.class" -exec java -jar cfr.jar {} --outputdir ./src \;

# Using xargs for efficiency
find ./classes -name "*.class" | xargs -I {} java -jar cfr.jar {} --outputdir ./src

Troubleshooting

Common Errors

ClassFormatError

• Cause: Corrupted or invalid class file
• Solution: Verify file integrity, re-download if possible

UnsupportedClassVersionError

• Cause: Class compiled with newer Java version
• Solution: Use matching or newer Java version to decompile

NoClassDefFoundError

• Cause: Missing dependency classes
• Solution: Add dependencies to classpath with --extraclasspath

OutOfMemoryError

• Cause: Large class file or complex code
• Solution: Increase heap size: java -Xmx2g -jar cfr.jar ...

Performance Tips

1. Batch processing: Use xargs or scripts for multiple files
2. Memory: Set appropriate heap size for large projects
3. Parallel: Process multiple files in parallel
4. Filtering: Skip test classes if not needed

Post-Decompilation

After decompiling:

1. Verify: Check output file count matches input
2. Review: Spot-check complex files for accuracy
3. Organize: Move to proper Maven/Gradle structure
4. Fix: Address any decompilation artifacts
5. Compile: Test with mvn compile

Resources

• CFR GitHub: https://github.com/leibnitz27/cfr
• Procyon GitHub: https://github.com/mstrobel/procyon
• Fernflower GitHub: https://github.com/fesh0r/fernflower
• JD-GUI: http://java-decompiler.github.io/
• JADX GitHub: https://github.com/skylot/jadx