Decompile Android APKs with Dex2jar & JD-GUI

If you’re into Android reverse engineering or learning how to analyze Android APKs, one of the first steps is to decompile the app and see the code behind it. 😎 In this guide, you’ll learn how to do it easily using open-source tools like Dex2jar and JD-GUI.

Download & Set Up Dex2jar 📥​

The first thing you need is Dex2jar:

• Download: Dex2jar.zip
• Extract it anywhere on your computer.

You’ll also need a sample app to practice on:

• Sample APK: yeahhub.apk
• Move it inside the dex2jar folder after extraction.

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About yeahhub.apk 🕵️‍♂️​

The yeahhub.apk app asks for a secret number. Your job is to decompile the app and find that number.
As we know, Java executables can be decompiled. Similarly, you can convert an APK into a JAR file.

Decompile APK to JAR 🛠️​

Open Command Prompt and run this command: d2j-dex2jar.bat yeahhub.apk
If everything works, you’ll get a file called: yeahhub-dex2jar.jar in the same folder. 🎉

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Open the JAR with JD-GUI 👀​

Next, download JD-GUI from the official site. Then:

1. Open JD-GUI.
2. Open the yeahhub-dex2jar.jar file inside the tool.

Now you can see the full source code of the APK! 😲

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Extract AndroidManifest.xml 📝​

After decompiling, you might want to extract AndroidManifest.xml which contains all the app settings. There are two ways:

1. Using Android Studio (more advanced).
2. Using WinRAR / WinZip (easy for beginners).

We’ll use WinRAR because it’s simple and fast.

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Rebuild & Install APK 🔧​

After making changes or extracting what you need, you can rebuild the APK and install it on any Android device using ADB Installer.

• We’ll cover detailed ADB installation steps in another guide.

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Quick Code Example 💻​

Here’s an example of Java decompiled code you might see inside an APK:

public class MainActivity extends AppCompatActivity {
    private String secretNumber = "12345"; // The secret number to find

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        System.out.println("Secret Number: " + secretNumber);
    }
}

Summary:
Using Dex2jar and JD-GUI, you can decompile any Android APK, explore its code, extract AndroidManifest.xml, and learn important concepts in Android reverse engineering and ethical hacking. 🔐