DEX means Android app's (APK) executable bytecode files in the form of Dalvik Executable (DEX) files, which contain the compiled code used to run your app.
The Dalvik Executable specification limits the total number of methods that can be referenced within a single DEX file to 65,536 (64K)—including Android framework methods, library methods, and methods in your own code.
To overcome this limit requires configure your app build process to generate more than one DEX file, known as a Multidex.
Dex is the name of the file format and encoding to which Android Java code is compiled. Early versions of Android would load and execute
dex binaries directly in a virtual machine named Dalvik. More recent versions of Android use the Android Runtime (ART), which treats
dex files as an intermediate representation and performs further compilations on it prior to running the application.
Dex is a very old file format, in terms of the lifespan of smartphones, and was designed for devices whose main memory was measured in tens of megabytes. The design limitations of those days have remained with us to this day.
dex file format encodes a limit to the number of methods that can be referenced in a single binary. Because the portion of the file format that stores the number of references is two bytes long, the maximum number of method references is
0xFFFF, or 65535. If an application contains more than that number of method references, it will fail to compile.
Google has provided a way around this problem, called Multidex. It has compile-time and run-time components. As its name implies, at compile-time it will divide code between one or more
dex files. At runtime, it will teach the default
ClassLoader how to look up classes from these files.
This approach works well on newer devices, but has some substantial drawbacks. It can increase application startup time dramatically, and on older devices can cause
Application Not Responding failures.
Multidex, while effective, should be avoided if possible.
Before configuring your app to enable use of 64K or more method references, you should take steps to reduce the total number of references called by your app code, including methods defined by your app code or included libraries. The following strategies can help you avoid hitting the dex reference limit:
The first point requires diligence and discipline on the part of the developer. When incorporating third-party libraries, one must consider the size of the library. For example, two popular JSON libraries are Jackson and Gson. Functionally they are quite similar, but Gson tends to see greater use in Android. One reason is that Jackson weighs in around 9,000 methods, whereas Gson contributes 1,900.
There are several tools available to help developers keep track of the size of their application: