Java Decompiler
Purpose
Java bytecode (.class) retains nearly all source-level information, making
decompilation highly effective. This skill extracts, decompiles, and analyses
Java applications across all packaging formats: JAR, WAR, EAR, APK, and raw
class files.
Workflow
┌──────────────────┐ ┌──────────────────┐ ┌──────────────────────┐
│ target.jar / .apk│────▶│ decompile_java │────▶│ java-decompiled/ │
└──────────────────┘ └──────────────────┘ │ ├─ src/ │
│ │ ├─ manifest.json │
│ │ └─ obfuscation.json │
▼ └──────────────────────┘
┌──────────────────┐ ┌──────────────────────┐
│ analyze_java │────▶│ java_analysis.json │
└──────────────────┘ └──────────────────────┘
Usage
# Decompile a JAR
python java-decompiler/scripts/decompile_java.py app.jar --out output/java-decompiled
# Decompile an Android APK
python java-decompiler/scripts/decompile_java.py app.apk --out output/java-decompiled
# Decompile with specific tool
python java-decompiler/scripts/decompile_java.py app.jar --out output/java-decompiled --decompiler cfr
# Analyse decompiled source
python java-decompiler/scripts/analyze_java.py output/java-decompiled --out output/java-analysis
Decompiler Priority
| Tool | Best for | Install |
|---|
| CFR | General JAR/class (best output quality) | Download cfr.jar |
| Procyon | Complex generics, lambdas | Download procyon-decompiler.jar |
| FernFlower | IntelliJ-style output | Download fernflower.jar |
| JADX | Android APK (dex → java directly) | pip install jadx or download |
| javap | Fallback bytecode listing | Included with JDK |
Outputs
| File | Content |
|---|
src/ | Decompiled .java source files |
manifest.json | Parsed MANIFEST.MF + entry points |
obfuscation.json | Detected obfuscator + evidence |
java_analysis.json | License logic, secrets, endpoints, dependencies |
When to use
binary-identifier reports Java language- Target is a
.jar, .war, .ear, .apk, or .class file
- Target contains
META-INF/MANIFEST.MF or classes.dex
Anti-patterns
| Mistake | Fix |
|---|
| Using only one decompiler | CFR fails on some patterns → fallback to Procyon/FernFlower |
| Ignoring MANIFEST.MF | Contains Main-Class, classpath, version — always parse |
| Skipping obfuscator detection | Deobfuscate first (string decryption) for readable output |
| Treating APK as JAR | APK uses DEX bytecode — needs dex2jar or JADX, not cfr directly |
| Missing inner classes | Inner classes are separate .class files — decompile the whole JAR |
