Decompiling an APK file allows you to convert an Android app package back to its original source code and resources. This lets you study the code, modify components, extract resources, and more.
While APK decompilation requires some technical skill, it can be a valuable tool for analysts, security researchers, developers, and advanced Android users.
In this comprehensive guide, we’ll cover different methods and tools to decompile APK files on Android.
What is an APK File?
APK stands for Android Application Package. It’s the package file format used to distribute and install mobile apps on Android.
APK files contain an app’s:
- DEX files – Compiled source code files
- Resources – Images, layouts, strings, etc.
- Manifest – App requirements and permissions
- Certificates – Security credentials
- Assets – Extra auxiliary files
By decompiling APKs, you can reverse engineer them back to human-readable source code and editable resource files.
Why Decompile APK Files?
Here are some of the main reasons to decompile Android APKs:
- Security research – Inspect code vulnerabilities, malware, spyware.
- Customization – Edit resources for UI mods and customization.
- Learning – Study source code of apps for educational purposes.
- Localization – Translate text strings in apps.
- Development – View code samples and frameworks.
- Backup – Retrieve lost app source code and assets.
As long as it’s for personal use, decompiling apps is generally legal under fair use exemptions. But redistributing modified code or assets may violate copyrights.
APK Decompilation Tools
There are various open source tools available to decompile APK files. Here are some of the most popular options:
Apktool is one of the leading open source APK decompilers for Android. It disassembles resources back into nearly original form and rebuilds them after making modifications.
Jadx converts DEX files back to Java source code for analysis. It produces clean and human-readable decompiled code.
3. CFR Decompiler
CFR (ClassFileReader) also decompiles DEX bytecode back into Java source files for inspection.
JD-GUI provides a graphical interface to view Java source code extracted from APK files. It also supports editing code.
AndroChef can decompile Android apps into Java classes as well as view smali assembly code.
6. Apk Studio
Apk Studio is an IDE with integrated APK decompiler capabilities for easy modding and analysis.
Dex2Jar converts Android .dex files into .class files that can then be decompiled to Java code.
Enjarify is a tool created by Google that converts Dalvik bytecode to equivalent Java bytecode. This allows Java decompilation.
How to Decompile an APK File – Step-by-Step
Let’s go through the general process for decompiling an APK file using free tools like Apktool, Jadx and JD-GUI:
Step 1: Download the APK File
First, you need to acquire the APK file for the app you want to decompile. Make sure the APK file is exactly the same as the live app installed from Google Play Store.
You can extract installed APKs from your own device using adb backup or find APK downloads on sites like APKMirror.
Step 2: Unzip the APK File
APK files are basically ZIP archives. Use any file archive utility like 7-Zip or WinRAR to unzip the APK.
This will extract the DEX file(s), resources, manifest and other assets contained in the APK.
Step 3: Decompile DEX to Java Code
Use a tool like Jadx or CFR to decompile the classes.dex file into readable Java code files.
You’ll also see folder structures recreated with the app packages and classes. This outputs the source code powering app behaviors.
Step 4: View Resources
Use Apktool to unpack resources into nearly original form.
This will reconstruct all the app graphics, layouts, strings, etc. stored in the res folder so you can view/edit them.
Step 5: Analyze Manifest
View the AndroidManifest.xml file to inspect app components, permissions, services declared and more.
This gives an overview of how the app is structured. The manifest also reveals allowed API usage declared.
Step 6: Modify and Recompile (Optional)
At this point, the APK is now decompiled. You can optionally make code changes and recompile with apktool using:
apktool b [folder]
This will rebuild a modded APK with your source modifications included.
And there you have it! With these steps, you can fully decompile an APK down to its original components for inspection and analysis on any Android app package.
Decompiling APK Files on Android Studio
For developers using Android Studio, you can also decompile APKs using built-in tools:
- Open Android Studio and go to File > New > Project from Version Control.
- Enter the APK url ending in .apk as the URL.
- Click Clone. Android Studio will now decompile the APK.
- View the extracted manifest, resources, sources etc.
This method integrates APK decompilation directly into your IDE for easier analysis during development.
APK Decompiling Considerations
When analyzing decompiled APK files, keep these important considerations in mind:
- Decompiled code may not perfectly match original source. Optimization and obfuscation can cause discrepancies.
- Review permissions and API usage declared in manifest before making assumptions about app actions.
- Look for evidence of malicious behavior in code like covert data access or transmission.
- Decompilation breaks app signatures. Recompiled APKs will not be signed and can’t install without patching.
- Making source code changes will likely heavily break app functionality when recompiled.
- Some elements like proprietary libraries may not fully decompile and analyze well.
- Certain app protections like obfuscation can deliberately deter decompilation through compiler changes.
- Resource modifications may violate copyrights if redistributed, even if source code editing falls under fair use exemptions.
Legalities of APK Decompilation
Reversing apps through decompilation raises some legal considerations:
- Decompiling APKs you created from your own code is completely legal.
- Decompiling third-party APKs for personal education/research is generally permitted under fair use.
- Publicly distributing modified or decompiled APK code/assets likely violates IP and copyrights.
- Decompiling apps you downloaded but didn’t pay for likely violates creator terms and rights.
In summary, decompiling APKs for non-commercial research or education purposes is legally defensible for private use under fair use rights. But redistributing decompiled IP publicly crosses into unprotected territory.
Decompiling APK files allows viewing an app’s internals for education, research, modding, and more. Use tools like apktool, jd-gui, jadx, cfr and others to unpack an APK’s manifest, resources, bytecode and assets.
Key highlights include:
- Use apktool to unpack manifests, resources, assets from APK archives
- Employ jadx, cfr and jd-gui to decompile DEX code back to Java source
- Analyze components carefully, as obfuscated code won’t perfectly match original
- Legally, decompiling third-party APKs for private research falls under fair use
- Editing resources may be permitted, but redistributing modified code likely violates IP
With these best practices, you can safely decompile APKs within reason. Just be thoughtful when handling third-party code and assets.