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ProGuard

The ProGuard tool shrinks, optimizes, and obfuscates your code by removing unused code and renaming classes, fields, and methods with semantically obscure names. The result is a smaller sized .apk file that is more difficult to reverse engineer. Because ProGuard makes your application harder to reverse engineer, it is important that you use it when your application utilizes features that are sensitive to security like when you are Licensing Your Applications.

ProGuard is integrated into the Android build system, so you do not have to invoke it manually. ProGuard runs only when you build your application in release mode, so you do not have to deal with obfuscated code when you build your application in debug mode. Having ProGuard run is completely optional, but highly recommended.

This document describes how to enable and configure ProGuard as well as use the retrace tool to decode obfuscated stack traces.

Enabling ProGuard (Gradle Builds)

When you create a project in Android Studio or with the Gradle build system, the minifyEnabled property in the build.gradle file enables and disables ProGuard for release builds. The minifyEnabled property is part of the buildTypes release block that controls the settings applied to release builds. Set the minifyEnabled property to true to enable ProGuard, as shown in this example.

  android {
   ...
 
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
            'proguard-rules.pro'
        }
    }
  }
  

The getDefaultProguardFile('proguard-android.txt') method obtains the default ProGuard settings from the Android SDK tools/proguard/ folder. The proguard-android-optimize.txt file is also available in this Android SDK folder with the same rules but with optimizations enabled. ProGuard optimizations perform analysis at the bytecode level, inside and across methods to help make your app smaller and run faster. Android Studio adds the proguard-rules.pro file at the root of the module, so you can also easily add custom ProGuard rules specific to the current module.

You can also add ProGuard files to the getDefaultProguardFile directive for all release builds or as part of the productFlavor settings in the build.gradle file to customize the settings applied to build variants. This example adds the proguard-rules-new.pro to the proguardFiles directive and the other-rules.pro file to the flavor2 product flavor.

    android {
   ...
 
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
            'proguard-rules.pro', 'proguard-rules-new.pro'
        }
    }
 
   productFlavors {
        flavor1 {
        }
        flavor2 {
            proguardFile 'other-rules.pro'
        }
    }
 }
  

Configuring ProGuard

For some situations, the default configurations in the ProGuard configuration file will suffice. However, many situations are hard for ProGuard to analyze correctly and it might remove code that it thinks is not used, but your application actually needs. Some examples include:

  • a class that is referenced only in the AndroidManifest.xml file
  • a method called from JNI
  • dynamically referenced fields and methods

The default ProGuard configuration file tries to cover general cases, but you might encounter exceptions such as ClassNotFoundException, which happens when ProGuard strips away an entire class that your application calls.

You can fix errors when ProGuard strips away your code by adding a -keep line in the ProGuard configuration file. For example:

-keep public class <MyClass>

There are many options and considerations when using the -keep option, so it is highly recommended that you read the ProGuard Manual for more information about customizing your configuration file. The Overview of Keep options and Examples sections are particularly helpful. The Troubleshooting section of the ProGuard Manual outlines other common problems you might encounter when your code gets stripped away.

Decoding Obfuscated Stack Traces

When your obfuscated code outputs a stack trace, the method names are obfuscated, which makes debugging hard, if not impossible. Fortunately, whenever ProGuard runs, it outputs a mapping.txt file, which shows you the original class, method, and field names mapped to their obfuscated names.

The retrace.bat script on Windows or the retrace.sh script on Linux or Mac OS X can convert an obfuscated stack trace to a readable one. It is located in the <sdk_root>/tools/proguard/ directory. The syntax for executing the retrace tool is:

retrace.bat|retrace.sh [-verbose] mapping.txt [<stacktrace_file>]

For example:

retrace.bat -verbose mapping.txt obfuscated_trace.txt

If you do not specify a value for <stacktrace_file>, the retrace tool reads from standard input.

Debugging considerations for published applications

Save the mapping.txt file for every release that you publish to your users. By retaining a copy of the mapping.txt file for each release build, you ensure that you can debug a problem if a user encounters a bug and submits an obfuscated stack trace. A project's mapping.txt file is overwritten every time you do a release build, so you must be careful about saving the versions that you need. The file is stored in the app build/outs/ folder.

For example, say you publish an application and continue developing new features of the application for a new version. You then do a release build using ProGuard soon after. The build overwrites the previous mapping.txt file. A user submits a bug report containing a stack trace from the application that is currently published. You no longer have a way of debugging the user's stack trace, because the mapping.txt file associated with the version on the user's device is gone. There are other situations where your mapping.txt file can be overwritten, so ensure that you save a copy for every release that you anticipate you have to debug.

How you save the mapping.txt files is your decision. For example, you can rename the files to include a version or build number, or you can version control them along with your source code.