Decompile Java Class File Info

We’ve all been there. You inherit a legacy project with no source code, a critical dependency throws an unexplainable error, or a vendor goes out of business taking their documentation with you.

public class UserService private String apiKey = "secret"; public String greetUser(String name) if (name == null decompile java class file

Have a war story about decompiling a nasty legacy system? Share it in the comments below. We’ve all been there

// Original public void process(int userId, String userName) ... // Decompiled (without debug symbols) public void process(int n, String s) ... Here’s a practical script to decompile a whole JAR using CFR: Share it in the comments below

public class UserService private String apiKey = "secret"; public String greetUser(String name) if (name == null

#!/bin/bash # decompile.sh JAR_FILE=$1 OUTPUT_DIR="./decompiled_src" unzip $JAR_FILE -d ./temp_classes Decompile all .class files find ./temp_classes -name "*.class" -type f | while read class; do java -jar cfr.jar "$class" --outputdir "$OUTPUT_DIR" done Clean up rm -rf ./temp_classes

java -jar procyon-decompiler.jar -o ./output MyClass.class If you use IntelliJ IDEA, you already have a decompiler. Open any .class file inside a JAR dependency. IDEA uses FernFlower under the hood. It’s decent for Java 11 but falls behind CFR for bleeding-edge syntax. Real-World Example: Decompiling a Simple Class Let's say you have this compiled UserService.class :