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Using ProGuard Rules with Redex

The code and artifacts for this example are on GitHub.

Currently there is limited support for specifying ProGuard rules which Redex will try to honor when it considers deleting (shrinking) classes, methods and fields.

The Need To Control Shrinking​

One of the optimizations that Redex performs is to remove interfaces that have only one implementation. However, when there is a use of that interface through reflection or constructs like instanceof then this is an unsafe removal which should be prohibited by using a ProGuard rule.

Example​

Consider the following interface:

package com.facebook.redex.examples.proguardexample;

public interface Greek {
    int doubleWombat();
}

which only has one use:

package com.facebook.redex.examples.proguardexample;

public class Alpha implements Greek {

    private int wombat;

    public Alpha () {
        wombat = 21;
    }

    public int doubleWombat() {
        return 2 * wombat;
    }
}

and is instantiated in a main activity as follows:

package com.facebook.redex.examples.proguardexample;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = (TextView) findViewById(R.id.message);

        Alpha alphaObject = new Alpha();
        int ltuae = alphaObject.doubleWombat();
        textView.setText("The answer is " + ltuae + "\n");

        try {
            Class<?> greek = Class.forName("com.facebook.redex.examples.proguardexample.Greek");
            if (greek.isInstance(alphaObject)) {
                textView.append("Alpha is an instance of Greek");
            } else {
                textView.append("Alpha is not an instance of Greek");
            }
        } catch (ClassNotFoundException e) {
            textView.append("ERROR: Greek interface not found");
        }
    }
}

When you make a release build of this application and then process it with Redex you will get a crash because the Greek class will be removed because it only has a single implementation, but Redex did not notice that Greek is used as part of an instanceof check (or there could have been some use of reflection that mentioned the Greek class). Running the app gives the following output on the display of the device:

Missing Greek interface

You can instruct Redex to prevent a class or interface from begin deleted by providing a ProGuard rule. In this case we want to ensure the Greek interface is not deleted:

-keep interface com.facebook.redex.examples.proguardexample.Greek

When you run Redex you can specify a single ProGuard file containing simple keep rules for classes and interfaces. For example:

$ redex -o myfasterapp.apk myapp.apk -P proguard-rules.pro --sign -s ~/.android/debug.keystore -p android

Now when you run the post-Redex APK you will notice that the Greek class has not been stripped away:

With Greek interface

Limitations​

Right now we support only simple keep annotations for classes and interfaces. Shortly we will provide support a richer subset of the ProGuard configuration language.

Source for Example​

The source code for this example can be found in this directory.