Why fullMode hates Gson so much?

🙏 Disclaimer

Am starting to write this blog with a curious mind to understand why Gson started crashing after fullMode is enabled in a working release build. I can guess it could be because of Gson’s reflection usage and somehow fullMode modified or removed something, but let’s understand what’s the exact change. This analysis is not about “adding an extra rule should fix it”. Its more about “how an app which is running fine with fullMode = false breaks when fullMode set to true

📗 Context

A little bit of context if you’re hearing fullMode or r8 for the first time.

What’s r8?

R8 is a tool that shrinks, optimizes, and secures Android applications and libraries: Shrinking: R8 removes unused classes, members, and resources to reduce the size of your app. Optimizing: R8 rewrites code to improve performance.

What’s fullMode?

fullMode, also known as non-compat mode, is a feature in Android that allows R8 to perform more aggressive optimizations. This can help reduce the size of an app’s APK and improve its performance. fullMode is enabled by default from AGP 8 onwards

Source: Google

🧨 Reproducing the crash

First step of any analysis is reproducing the issue. Let’s do that.

Here’s a model called Car

package com.example.app

import com.google.gson.annotations.SerializedName

data class Car(
    @SerializedName("color")
    val color: String, // Blue
    @SerializedName("make")
    val make: String, // Toyota
    @SerializedName("model")
    val model: String, // Corolla
    @SerializedName("year")
    val year: Int, // 2022
)

and then activity where we convert our JSON to Car model

package com.example.app

class MainActivity : Activity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.main)

        val inputJson = """
            {
              "make": "Toyota",
              "model": "Corolla",
              "year": 2022,
              "color": "Blue"
            }
        """.trimIndent()

        val tvSample = findViewById<TextView>(R.id.tv_sample)
        tvSample.text = Gson().fromJson(inputJson, Car::class.java).toString()

    }
}

and my proguard rules have rules mentioned in the Gson example here except rules for the model Car.kt class. Now you may ask “that’s clearly the problem! why don’t you add it?“. You’re right. Adding model-specific rules will fix this, but as I mentioned above, this is about “why” it works without fullMode and crashes with fullMode 🤔, where obfuscation is enabled in both builds. This blog is just about that.

🎭 Behaviour

Alrighty, let’s go!

As you can see in the MainActivity, we’re parsing the JSON using Gson#fromJson and setting the toString() value of data class to a TextView (yeah, XML :P). Simple.

The release build with fullMode = false works fine

but, the app crashes when i run the release build with fullMode = true. Repro success 💥

The stacktrace looks like this (minified)

FATAL EXCEPTION: main
Process: com.example.app, PID: 27357
  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.app/com.example.app.MainActivity}: java.lang.ClassCastException
    ...
  Caused by: java.lang.ClassCastException
    at com.google.gson.stream.JsonToken$EnumUnboxingLocalUtility.m(SourceFile:1)
    at com.example.app.MainActivity.onCreate(SourceFile:218)
    ...

The above stacktrace says the crash happened due to a ClassCastException but that usually happens when you try to cast to a wrong class. Here, we have only one model class, and that too parsed with Car::class.java. Its not even “dynamic or generic”

👬 Compare

To clearly understand what exactly happened, we need to compare the two APKs: one with fullMode and one without fullMode. Luckily, I have a tiny tool created, called dex-diff to do just that (this is not a promotion… I’d use another tool if there’s a better one out there… oops, did I promote the tool again 🙊??).

Anyway, moving on, I’ve dex-diff already installed and also both APKs generated

Now I need to pass these two APKs to dex-diff and ask it to generate the diff. To do that, i need to execute this command

dex-diff without-fullMode.apk with-fullMode.apk com.example.app

After executing the command dex-diff will start decompile and diff for each class and will generate a html report.

⚔️ dex-diff v0.1.4
🧠 Heap size: 16384 MB
🚀 Initialising...
➡️ Decompiling APKs... (this may take some time)
✅ Decompile finished (7.61s)
➡️ Comparing before and after... (this may take some time)
✅ Comparing finished (0.24s)
➡️ Making report...
✅ Report ready (7.98s) -> report.html

Cool. The report is ready. Let’s look at it

📜 Report

As you can see in the “Removed app files” section, our Car class got completely removed, and it looks like MainActivity.java has had 64 lines removed and 12 lines added. Let’s see what happened there…

🤷‍♂️ Analysis : How it crashed?

So many interesting things happened in there, but there have been three major changes related to the crash where we’re focusing at the moment.

1. The Car.class is now SqlTypesSupport.AnonymousClass1.class 😮
2. Previously used .read(jsonReader) became .mo0read(jsonReader) 🤔
3. There’s no TextView and TextView#setText call in MainActivity and the area has been replaced with JsonToken$EnumUnboxingLocalUtility.m(cls.cast(obj)); 😕

Wait, is that the crash point for our crash.. JsonToken$EnumUnboxingLocalUtility 🤔.. yeah it is 🫠. So that means, failed at 2 and it went to the catch block and crashed at spot 3.

📦 What’s inside?

Let’s see what’s inside SqlTypesSupport.AnonymousClass1.class?

(To browse the decompiled code better, I’ll be using jadx)

Wow! Its an empty class! and what’s inside JsonToken$EnumUnboxingLocalUtility

Alright. Now the crash makes sense. I’ll tell you how it crashed.

1. At block 1, cls gets assigned to an empty class.
2. At block 2, the empty class tries to get converted to an object (parsing). It fails, and the EOFException gets thrown from the Gson library.

3. Because the parsing failed, the obj stays null and cls.cast(obj) throws ClassCastException. Because it was casting to null, and that’s why stack trace does not indicate which cast “to” failed. And this exception caused the app to crash.

🕵️ Analysis : Why it crashed?

Ultimately, the crash happened because Car class got replaced with an empty class called SqlTypesSupport.AnonymousClass1.

Why Car class got removed?

Because Gson uses reflection to create your model class, and not the class’s constructor. fullMode couldn’t find any constructor usage and it probably thought “Car constructor is not getting invoked, that means Car class objects are not getting created, that means Car class properties are not getting created, so since Car.class code is there, let’s replace the usage with an empty class that’s already present in the code, i.e., SqlTypesSupport.AnonymousClass1”.

To validate this theory, I created another Car object manually in MainActivity, and the app didn’t crash

NOTE: This is my best guess from the decompiled APK code. CMIIW

❤️‍🩹 How to fix it?

Fixing is simple. You just need to add the Car to your ProGuard rules, but a better fix would be to stop using Gson and migrate to a JSON library that doesn’t use reflection for parsing, like Moshi CodeGen (not moshi-reflect) or Kotlinx.serialisation

🤝 Thanks

Oh, wow. You made it to the end. That means I probably didn’t bore you or make you more confused. If that’s not the case, please use the box below to share that. I’d be happy to take the feedback and reflect in the next blog. Thanks for reading