I have seen a few people argue that there’s no reason to let ProGuard run on the open-source libraries that they include in their app, because if the source is publicly available, there’s no point of obfuscating, right? Who are you fooling? Here’s an example from one library’s README file:

Their ProGuard rules file is just a big wildcard keep:

I think that’s exactly wrong, for two reasons — shrinking, and obfuscation.

Shrinking

The majority of the shrinking that ProGuard performs is by removing unused code from third-party libraries. You’re not writing a lot of code that you don’t use, but you’re probably not using all of the features in the third-party libraries that you include. If you use a

-keep com.example.** { *; }

-type rule on open source libraries, you won’t get the benefits from ProGuard’s shrinking. None of that unused code will be removed.

Obfuscation

I’m going to show this with an illustration. What does this class do?

The eagle-eyed among you may recognize a couple of clues and have a hunch, but I doubt you know everything that it’s doing. So let’s look at the same class, but without obfuscating the third-party libraries.

It’s a lot easier to figure out what this code does when the libraries aren’t obfuscated. Simply put, obfuscating the open-source code allows for better obfuscation of your own your code.
(I mention ProGuard by name, because it’s still the main tool for Android development, but these arguments apply just the same to R8.)

I’ve written several blog posts recently about technical aspects of using ProGuard to obfuscate your code in Android apps.  Then, in March, I gave a talk at DroidCon Boston walking through an overview of How ProGuard Works. Through all of those, I tried to keep the content very matter-of-fact and technical.  I got a lot of technical questions in response, but I also got a lot of questions about the organizational and software engineering aspects — how to think about maintenance, and testing, and debugging, and communicating with your team.  So what follows are the less technical, but possibly more practical, guidelines I’ve found for working with ProGuard.

You’ll find that most of these tips are focused on the fact that obfuscation can expose problems that are only seen at runtime, and then only when a specific code path is executed.  This means that most of these rules are focused on making sure you’re likely to discover those problems.

As early as possible

If possible, start using ProGuard from day 1 of a new project.  I’ve gone through the experience of enabling ProGuard on a project that’s been worked on for years without it, and it’s more difficult.  You’ll get a warning about some code, and you’ll find that nobody on the team remembers that code.

Enabling ProGuard from day 1 ensures that issues are encountered immediately, when the relevant code is fresh in your mind.

As many builds as possible

Don’t enable ProGuard in your day-to-day dev builds*, it will increase your build times a little and cause you extra pain in debugging.  But for every other build — QA, beta, release — turn ProGuard on. You want as many testers as possible exercising the code as many ways as possible to ensure that you notice any ProGuard bugs early on.

*There is an exception to this, which is when you get a report of a bug that you suspect might be triggered by obfuscation, you want to enable ProGuard in your dev environment so you can easily reproduce the bug.  This means that you want to have a single, easy, flag in your build.gradle that you can flip to true to enable ProGuard on your debug builds. Have all of the configuration files ready for that build type — the same configuration files you’re using for the rest of your builds — you want the only effort on your part to be flipping the one flag.

The Phased Approach

If you’ve inherited a codebase that is already in production, you can’t enable ProGuard from day 1, but you can try to fake it a bit.  Start by enabling ProGuard in your dev builds and fixing the compile-time errors. Then, run a ProGuard-ed build and exercise the app as much as possible.  Walk through each feature as well as you can, and try to find problems. Next, enable ProGuard for your QA builds, and wait for a bit. This is the real key.

You’ll give your QA team time to work through the app and find problems.  You want to make sure they’ve had time to exercise the app before you move on to the next phase, which is a beta build, if you have one.  A beta should hopefully expose your obfuscated app to a broad, but still small, set of users. Let that work for a bit, then you can enable for your public builds.

Make your Keep rules as specific as possible

I’ve mentioned this in my blog post and in the talk, but it bears repeating.  There’s no value in using ProGuard if you’re going to kneecap it with overly-broad keep rules.

Document every keep rule

This is deceptively important.  It’s tempting to just add the keep rule you need to a config file and be done, but you’re hurting yourself in the long run.  A year from now, you’ll look at the rule and wonder why it was there. It might be referring to code that’s long gone, or a library that’s no longer used.  Or it could be critically important. You don’t know, because the rule itself doesn’t tell you the reason it was added. And, perhaps more importantly, it doesn’t tell you how to test whether it’s still necessary.

Add a comment.  Explain what was broken and why that rule fixed it.

Conclusion

That’s it.  Try it out.  If you’ve got some different ideas, based on your experience, I would love to hear it.

Thanks for watching my talk, How ProGuard Works, at Droidcon Boston or Android Summit. Here are some of the links that I mentioned in my slides, as well as links to some of my blog posts including the diagrams I used.

• ProGuard Manual
• ProGuard source code
• ProGuard v6 Release Notes (new options like -addconfigurationdebugging)
• Jake Wharton – Sinking Your Teeth into Bytecode at Droidcon UK 2017

R8:

• D8 / R8 – initial announcement
• R8 project source code
• R8 issue tracker

My posts:

• Most of the Apps on My Phone Aren’t Obfuscated
• Reading ProGuard’s Outputs
• Distinguishing Between the Different ProGuard Keep Directives
• Feeding ProGuard’s inputs: where are all of these rules coming from?
• ProGuard pro-tip: Don’t use ProGuard’s “-include” with Gradle builds
• Best Practices for Enabling ProGuard

Gradle snippet for forcing a particular version of ProGuard:

buildscript {
  ...
  configurations.all {
    resolutionStrategy {
      force 'net.sf.proguard:proguard-base:6.0.1'
    }
  }
}

When ProGuard runs on your Android app, it’s not only using the rules that you’ve explicitly added, but rules from a couple of other sources as well.  Below are four sources to watch out for:

Rules explicitly added by the developer

These are the rules in a file that you add with `proguardFile` in your build.gradle.  If you’ve used ProGuard at all, you’re probably accustomed to adding configuration rules here.

Default ProGuard file

A base set of widely-applicable rules get added by that “getDefaultProguardFile” line above, which you’ll see in the template build.gradle file.  It protects things like Parcelables, view getters and setters (to enable animations on view properties) and anything with the @Keep annotation.  You should walk through this file to see what ProGuard rules get added by default.  It’s also a great source of example rules when you’re trying to make your own.

AAPT generated rules

When AAPT processes the resources in your app, it generates rules to protect, for example, View classes that are referenced from layout files, and Activities referenced in your manifest. Each rule comes with one or more comments saying where the class was referenced, so you can track down the offending resource if need be.  You can find these rules in:

build/intermediates/proguard-rules/{buildType}/aapt_rules.txt.

Note that this source isn’t explicitly added in your build.gradle, the Android Gradle Plugin takes care of including these rules for you.

Rules from libraries

If an Android library has ProGuard configs that apps should use when they include that library, the library can declare those rules with the consumerProguardFile method in its build.gradle.  For example, here’s a snippet from Butterknife’s build.gradle.

 

Bringing it all together

Rather than trying to track down every single source of ProGuard config that’s getting added to your app, you can look at them all together.  You can specify

-printconfiguration proguard-config.txt

in your rules file and ProGuard will print out all of the configuration rules to the specified file.  Note that they’re not in the same order that they were originally specified, so it can be hard to figure out where a particular rule is coming from.  

When ProGuard processes an Android app, it generates a few output files to show what happened in each step. These files can be really helpful if you’re trying to figure out what ProGuard changed, or why your code ended up the way it did. But those files aren’t self-documenting, so I’m going to walk you through why each of those files is created and what it shows you.

These files are in the build directory, something like:

app/build/outputs/mapping/{buildType}/

Here’s a diagram I made with a really high-level overview of the steps ProGuard takes when it’s analyzing your app, because the output files line up nicely with these steps.  Refer back to this for some context around the following steps.

seeds.txt

The first things ProGuard does is to read all of your configuration files, and then read in all of the Java bytecode (.class files) to create what it calls the class pool.  ProGuard then looks through the class pool and prints to seeds.txt a list of every class and member that matches any of your keep rules.  This is useful for debugging if the keep rule you wrote actually matches the class you’re trying to keep.

If it’s a class that matches, there will be a line with just the fully-qualified name of the class.  For a member, it will be the fully-qualified class name, followed by a colon, followed by the member’s signature.

usage.txt

Knowing what code it has to keep, ProGuard then goes through the class pool and finds code that it doesn’t need to keep.  This is the shrinking phase, where ProGuard strips out unused code from the app.  As it’s doing this, it prints out unused code — code that’s being removed — to usage.txt.  Now this name seems backwards to me; I think it should be unused.txt or shrinkage.txt or something, but that’s just me.  

This is useful if you’re trying to figure out why a class doesn’t exist at runtime.  You can check whether it got removed here, or got renamed in the next step.

If an entire class is removed, you’ll get a line with the fully-qualified class name.  If only certain members of a class are removed, you get the class name followed by a colon, and then a line (indented with four spaces) for each member that was removed.

mapping.txt

The next thing ProGuard needs to do is obfuscate as much code as possible — that is, it’s going to rename classes and members to meaningless names like “a”, “b”, etc.  As it’s doing this, ProGuard prints the old name and new name for every class and member to mapping.txt.  Not all code is renamed, but all code is listed in mapping.txt.

This is the file you need if you’re trying to de-obfuscate a stacktrace.  It allows you to work backwards from the obfuscated names to the original names of code.

Each line is of the form “{old name} -> {new name}”.  You get a line for the class name, then a line for each member of the class.  Note that constructors are shown as “<init>()”.  

dump.txt

After ProGuard has done all of its magic (shrinking and obfuscating), it prints out one last file which is essentially a full listing of all the code after processing.  That is, everything that’s left in the class files, but in a less optimized format, so it’s a huge file.  I have a demo app that I use for testing ProGuard stuff, and the final app is about 1 MB, but the dump.txt is almost 18 MB.  It’s enormous. Here’s the output for a trivial class:

This can be really useful, though, if you want to see what’s in your class files but don’t want to decompile the .class or .dex files.

Archiving

One last note is that these files are important artifacts of your build — especially mapping.txt.  If this is a build you’re going to distribute (say on the Play Store, or even internally for testing), you’re going to need your mapping.txt to de-obfuscate stacktraces.