What process happens when there is a system optimization?

What is Android doing when it says “optimizing apps/system”?

Basically the android system will be creating an optimised version of each application. This process makes each app start as fast as possible with the new Android version.

While it says ‘optimising,’ operating system generates ‘odex’ files
for your apps from scratch. As a simple explanation, odex files can
help the system boot faster and reduces the time needed for launching
applications.

Apk files and odex files

The longer story is that when an APK file is added to your Android
system, it’s not actually stored that way. It’s converted to something
called an “odex” file.

The Android OS does not store apps as is after installation (i.e., one single APK file). An optimized version of the app is stored in the Dalvik cache — called the odex file.”

What Is an Odex File?

In Android file system, applications come in packages with the extension .apk. These application packages, or APKs contain certain .odex (Optimized Dalvik EXecutable) files whose supposed function is to save space. These odex files are actually collections of parts of an application that are optimized before booting. Doing so speeds up the boot process, as it preloads part of an application. On the other hand, it also makes hacking those applications difficult because a part of the coding has already been extracted to another location before execution.

Deodex

Deodexing is basically repackaging of these APKs in a certain way, such that they are reassembled into classes.dex files (Dalvik EXecutable).

By doing that, all pieces of an application package are put together
back in one place, thus eliminating the worry of a modified APK
conflicting with some separate odexed parts.

In summary, Deodexed ROMs (or APKs) have all their application packages put back together in one place, allowing for easy modification such as theming. Since no pieces of code are coming from any external location, custom ROMs or APKs are always deodexed to ensure integrity.

There are several explanations on how this works:

In Android, applications come in packages with the extension .apk.
These application packages, or APKs contain certain .odex files whose
supposed function is to save space. These ‘odex’ files are actually
collections of parts of an application that are optimized before
booting. Doing so speeds up the boot process, as it preloads part of
an application.

Android OS uses a Java-based virtual machine for running applications,
called the Dalvik Virtual Machine. A deodexed, or .dex file contains
the cache used by this virtual machine (referred to as Dalvik-cache)
for a program, and it is stored inside the APK. An .odex file, on the
other hand, is an optimized version of this same .dex file that is
stored next to the APK as opposed to inside it. Android applies this
technique by default to all the system applications. Now, when an
Android-based system is booting, the davlik cache for the Davlik VM is
built using these .odex files, allowing the OS to learn in advance
what applications will be loaded, and thus speeds up the booting
process. By deodexing these APKs, a developer actually puts the .odex
files back inside their respective APK packages. Since all code is now
contained within the APK itself, it becomes possible to modify any
application package without conflicting with the operating system’s
execution environment.

How to do the process manually;

If you want to manually optimise your apks, there are some options:

• You could try using using applications like Lucky patcher


• Alternatively with Windows you can zip-align them with
  Apk decomplier (apk sign tool)

Acknowledgements

• What Is Odex And Deodex In Android


• What is Android doing when, after an upgrade, it tells you it's "optimizing" the apps?


• http://alvinalexander.com/android/what-android-doing-optimizing-apps-after-system-upgrade-restart