classesdex

classes.dex is the primary Dalvik Executable file within an Android APK. It contains the compiled bytecode for the app’s Java and Kotlin classes, and is packaged alongside resources, the manifest, and native libraries. The DEX format, developed for the Dalvik and later ART runtimes, is designed to be compact and efficient for running on Android devices. It stores information about classes, methods, fields, strings, types, and debugging data in a format optimized for the runtime.

During the build process, Java and Kotlin sources are compiled into Java bytecode and then translated into one or more dex files. By default, a typical app produces a single classes.dex, which holds most of the code. If an application exceeds the 65,536 method limit, or for other architectural reasons, the build system can generate additional dex files such as classes2.dex, classes3.dex, and so on, a concept known as MultiDex. Modern Android projects using App Bundles may deliver multiple dex sets or split APKs as part of the installation process.

Tools and analysis: developers and analysts can inspect or reverse engineer dex files using tools like dex2jar, jadx, or ApkTool, which can decompile or translate the contents into more readable forms. Such examination is common in debugging, security research, or malware analysis. The dex file is loaded by the Android runtime (ART or Dalvik) when the app is installed and run, with additional dex files loaded as needed by the class loader.

Terminology: the file is typically referred to as classes.dex in the APK, though discussions may refer to the main dex file in a broader sense. The dex file itself is not executed directly by developers; it is interpreted by the runtime environment on the device.