After I installed MultiDex I noticed,first ever launch of the app takes extra 4-5 seconds. However after a few researches, I noticed that the app size inside the phone settings(app manager) went from 7 MB to 19 MB and if I clear data, app goes back to 7 MB. But every time that I launch the app for the first time, app size increases to more than double.
Now my question is, what happens that makes the app size increase so much?
So far I have found a few topics on slackoverflow about MultiDex but none talks about what really happen with the code, and what kind of data MultiDex saves/caches.
Multi-Dexing is enabled in your gradle and extended in your Application class.
This is used when you use over 64,000 methods.
https://developer.android.com/studio/build/multidex
I would say probably 90% of the time if you are hitting multi dex needs, you have likely not properly managed your dependencies. I'm NOT saying every time. However, typically the issue is people bring in entire Google dependencies instead of just the ones you need. For example the Google Play Services. If you include this, it will instantly force you into multi-dexing. However, this does come with a performance hit. You now have multiple dex files to load. There is some pre-dexing of course for things that will not change such as 3rd party dependencies to help your speed a bit on building and deploying. However, having multiple lookup tables comes with it's speed consequences. For example, if you included.
com.google.android.gms
has about 44,000 methods alone in it, You should specify which one you want like
com.google.android.gms:play-services-location:16.0.0
for example. So before you go down the road of using Multi-Dex, ensure you have properly cleaned up your unused dependencies, and that you are properly managing your transitive dependency tree. Also don't forget to use ProGuard or the new D8 minification process as that may also help you, although may require you to run in Debug as well if you have that heavy of dependencies.
If you have done all that and you still need to use Multi-Dex (and I have run into this at larger companies that force tons of bloat libraries on you) then you go for it.
Now as for what is happening, well Dex stands for Dalvik Executable. It is the process of packaging the code into Dalvik bytes for execution. This is limited to 65,536 methods. They say 64k in the documentation, but everywhere I've read shows 65k+. Many of Google's libraries already contain 17k methods which puts you 1/4 of the way there right out the gate.
I believe the issue has something to do with the header allocation of 2 bytes per method signature and the lookup table. they are limited on number of unique IDs they can create. So it requires you to create multiple dex files with multiple lookup tables for the method signatures. So the short answer is, it makes multiple Dalvik Executable files to ensure unique method signatures are properly found and executed on the Dalvic Virtual Machine.
Other important things to note, is that prior to Android API 21, the Virtual Machine only supports 1 dex file. Therefore you need to do multi-dex install on your application onCreate to get the rest brought in properly. However, if you are using proguard, your additional dex files could have been removed so you may need to address a MultDexProguard file as well.
Now, it's important to realize that Android completely redid their Virtual Machine and no longer relies on Dex for their modern OS virtual machines. So then the next question is "should you still use it"?
Well if you are still needing to support pre-Lollipop, then you are better off leaving your multi-dex in place. Otherwise if you are Lollipop and up. Android uses ART (Android Runtime) and does not have this limitation. Honestly the population that has pre-Lollipop is so small that it is not worth supporting in my personal opinion, but it depends on your product and your needs.
Hope that helps shed some light on things here.
Happy Coding