• The content is a blog post that explains how to replace files in an existing directory in BUNDLE-METADATA folder in Android App Bundles (.aab) file using bundletool and protoc.

Android App Bundles (AAB) are a publishing format for Android apps that allows developers to optimize the delivery of their apps to different devices. AAB files contain all the compiled code and resources of an app, but they defer the generation and signing of APKs to Google Play or other app stores. This way, users only download the code and resources that are needed for their specific device configuration, resulting in smaller and more efficient downloads.

However, AAB files also pose some challenges for developers who want to modify or replace some of the files inside them. For example, if you want to replace the Debug Symbols (.dbg) files that are generated during the build process with the ones you receive from other sources, you might encounter some difficulties. This is because AAB files are not meant to be directly edited or manipulated by developers. They are only meant to be uploaded to app stores and processed by them.

In this blog post, we will show you how to replace files in an existing directory in BUNDLE-METADATA folder in Android App Bundles (.aab) file. This is useful for scenarios where you need to update or replace some of the files that are included in your app bundle, such as debug symbols, assets, or configuration files. We will use a tool called bundletool, which is an official command-line tool that helps you manipulate and test AAB files. We will also use a tool called protoc, which is a protocol buffer compiler that helps you parse and edit the manifest file inside the AAB file.

What is BUNDLE-METADATA folder?

BUNDLE-METADATA folder is a special folder that contains additional information about your app bundle, such as debug symbols, assets packs, instant app metadata, and feature module metadata. These files are not part of your app’s code or resources, but they are used by app stores or other tools to optimize the delivery and installation of your app.

The BUNDLE-METADATA folder is located at the root of your AAB file, and it has the following structure:

BUNDLE-METADATA
├── com.android.tools.build.debugsymbols
│   ├── arm64-v8a
│   │   └── libnative-lib.dbg
│   ├── armeabi-v7a
│   │   └── libnative-lib.dbg
│   └── x86
│       └── libnative-lib.dbg
├── com.android.tools.build.libraries
│   └── dependencies.pb
├── com.android.tools.build.apkzlib
│   └── apkzlib.properties
└── com.google.android.instantapps.supervisor.apk
    └── supervisor.apk

As you can see, there are different subfolders for different types of metadata. The one we are interested in is the com.android.tools.build.debugsymbols folder, which contains the Debug Symbols Files for each native library and each supported ABI (Application Binary Interface). These files are used by app stores or other tools to symbolicate native crashes and provide more meaningful stack traces.

Why do we need to replace debug symbols files?

Debug Symbols Files are generated during the build process of your app, and they contain information that maps the binary code of your native libraries to the source code and line numbers. This information is useful for debugging purposes, as it helps you identify where and why a native crash occurred.

However, sometimes the debug symbols files that are generated by default are not sufficient or accurate for desymbolication. For example, if you use third-party libraries or tools that provide their own debug symbols files, or if you use obfuscation or optimization techniques that alter the binary code of your native libraries, you might need to replace the default debug symbols files with the ones that match your final binary code.

If you don’t replace the debug symbols files, you might encounter issues such as:

• Incomplete or incorrect stack traces for native crashes
• Missing or mismatched source code and line numbers for native crashes
• Inability to upload your app bundle to Google Play Console due to size limit (150 MB) or format error

Therefore, it is important to replace the debug symbols files with the ones that correspond to your final binary code before uploading your app bundle to app stores or other tools.

How to replace debug symbols files using bundletool?

To replace debug symbols files using bundletool, you need to follow these steps:

1. Download and install bundletool from here.
2. Download and install protoc from here.
3. Extract the manifest file from your AAB file using this command:

   unzip -p app.aab base/manifest/AndroidManifest.xml > AndroidManifest.pb
   

   This will extract the manifest file from the base module of your app bundle and save it as AndroidManifest.pb in your current directory. Note that the manifest file is in a protocol buffer format, not in an XML format.

4. Parse and edit the manifest file using protoc and your preferred text editor. You need to use the Resources.proto file from here as the definition of the protocol buffer. For example, you can use this command to parse the manifest file and open it in nano editor:

   protoc --decode_raw 

   This will decode the manifest file using the raw protocol buffer format and open it in nano editor. You can also use other editors such as vim or emacs.

5. Find the debug symbols files that you want to replace in the manifest file. They are located under the XmlNode message, which represents the root element of the manifest file. For example, if you want to replace the debug symbols file for the arm64-v8a ABI, you need to find this entry:

   1 {
     1: "com.android.tools.build.debugsymbols"
     2 {
       1: "arm64-v8a"
       2: "libnative-lib.dbg"
     }
   }
   

   The first number (1) represents the tag number of the XmlNode message, which is defined in the Resources.proto file. The second number (1) represents the tag number of the name field, which is the name of the metadata folder. The third number (2) represents the tag number of the value field, which is a repeated message that contains the subfolder name and the file name.

6. Replace the file name with the new one that you want to use. For example, if you want to use libnative-lib-new.dbg instead of libnative-lib.dbg, you need to change this entry:

   2: "libnative-lib.dbg"
   

   To this:

   2: "libnative-lib-new.dbg"
   

7. Save and close the manifest file after editing it.
8. Re-encode the manifest file using protoc and overwrite the original one in your AAB file. You need to use the same Resources.proto file as before. For example, you can use this command to re-encode the manifest file and overwrite it:

   protoc --encode_raw 

   This will encode the manifest file using the raw protocol buffer format and overwrite it in the base module of your app bundle.

9. Replace the debug symbols files in your AAB file with the new ones that you have. You can use any zip tool to do this, such as unzip, zip, or 7-zip. For example, you can use this command to replace the debug symbols file for the arm64-v8a ABI:

   zip -0 app.aab BUNDLE-METADATA/com.android.tools.build.debugsymbols/arm64-v8a/libnative-lib-new.dbg
   

   This will replace the debug symbols file in your app bundle with the new one that you have.

10. Test your app bundle using bundletool or other tools to make sure that everything works as expected.

Frequently Asked Questions (FAQ)

Here are some common questions and answers related to replacing files in BUNDLE-METADATA folder in Android App Bundles (.aab) file.

Question: What is bundletool and where can I get it?

Answer: Bundletool is an official command-line tool that helps you manipulate and test AAB files. You can download it from here.

Question: What is protoc and where can I get it?

Answer: Protoc is a protocol buffer compiler that helps you parse and edit protocol buffer files, such as the manifest file inside an AAB file. You can download it from here.

Question: What is a protocol buffer and where can I find its definition?

Answer: A protocol buffer is a binary format for serializing structured data, such as XML or JSON. It is used by Google and other companies for various purposes, such as communication, storage, or configuration. You can find its definition in a .proto file, which specifies how to encode and decode a message using a schema.

Question: How can I verify that the debug symbols files are replaced correctly?

Answer: You can use bundletool or other tools to test your app bundle and check the stack traces of native crashes. You can also use bundletool to extract the debug symbols files from your app bundle and compare them with the ones that you have. For example, you can use this command to extract the debug symbols file for the arm64-v8a ABI:

bundletool extract-apks --apks=app.apks --output-dir=extracted --device-spec=device-spec.json

This will extract the APKs from your app bundle using a device specification file (device-spec.json) and save them in a directory (extracted). You can then find the debug symbols file in the extracted directory under splits/base-arm64_v8a.apk/lib/arm64-v8a/libnative-lib.dbg.

Question: How can I reduce the size of my app bundle if the debug symbols files are too large?

Answer: You can use different techniques to reduce the size of your debug symbols files, such as stripping, compressing, or splitting them. You can also use tools such as [Crashlytics] or [Sentry] to upload your debug symbols files to their servers and avoid including them in your app bundle.

Conclusion

In this blog post, we have shown you how to replace files in an existing directory in BUNDLE-METADATA folder in Android App Bundles (.aab) file using bundletool and protoc. This is useful for scenarios where you need to update or replace some of the files that are included in your app bundle, such as debug symbols, assets, or configuration files.

We hope that this blog post has been helpful for you and that you have learned something new. If you have any questions or feedback, please feel free to leave a comment below.

Disclaimer: This blog post is for educational purposes only and does not constitute professional advice. The author is not responsible for any consequences that may arise from following or applying the information in this blog post. The user is solely responsible for verifying the accuracy and validity of the information in this blog post and for complying with any applicable laws and regulations. The user should always consult with a qualified expert before making any decisions or taking any actions related to the topic of this blog post. The author does not endorse or recommend any products, services, or tools that are mentioned or linked in this blog post. The author does not own or claim any rights to the content or images that are used or referenced in this blog post. All rights belong to their respective owners.

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