The most important property of software developers is their knowledge. The source code of Android apps can easily be recovered from a apk file. This is fundamentally different from platforms like iOS. Code obfuscation is common on Android, but doesn't solve the real problem.
ClassGuard for Android is a tool to prevent dex decompiling. Your dex archive is encrypted using a 128Bit AES encryption. The AES key is generated randomly every time you start the encryption tool. The dex file is decrypted transparently by a custom bootstrap activity. The main part of this bootstraper is written in C++ using the Android NDK to prevent decompiling and other tampering.
ClassGuard for Android is currently available as a supported beta for Android versions from Kitkat (4.4) to Oreo (8.1). All apps encrypted with this beta will expire on March, 31st 2019. A production version is planned for the end of February.