Android Platform and Architecture

Reference: Griffiths and Griffiths, Head First Android Development, 2nd Ed., O'Reilly, 2017.

Based on Linux kernel.
Contains Android specific drivers and libraries.
Has accomodations for resource constrained devices, such as slower CPUs, limited memory, low battery life.
Hardware Abstraction Layer (HAL) makes hardware available to higher level APIs.
The Android Ecosystem

Before 2013, Android apps ran on the Oracle Java Virtual Machine (JVM).
On the JVM, Java source code files are compiled into bytecode executables:
```
.java → .class
```
Since Kitkat (October, 2013), Java apps run in the Android Runtime (ART) virtual machine.
The Android Runtime (ART) has its own copy of Java.
The core Java packages in ART are the same as the packages in JVM version, for example, java.util.
Native C/C++ libraries are used for graphics, network access, security.
Compiling an Android Java file is a two step process:
(1) Java source code files are compiled into bytecode files,
(2) Bytecode .class files are compiled into .dex files that run on the ART:
```
.java → .class → classes.dex
```
Instead of using the standard Java bytecode format for executable files, Android uses the Dalvik executable file format.
Dalvik executable files have the file extension .dex (Dalvik Executable).
All .class files are compiled into one .dex file classes.dex.
In the Android Studio project files, classes.dex is located here:
```
app/build/immediates/dex/debug/out
```
For a time, Google was developing a compiler named JACK and a linker named JILL that converted .java source code files directly into .dex files.
However, some Java tools work directly on .class files.
In 2017, Google decided to compile .class files into the .dex file classes.dex. This way, new Java classes will be automatically supported by Android, as long as the JVM .class files don't change.

Executable .dex files and resource files are packaged into an .apk file. APK is derived from Android Package.
Using the Android Studio Device File Explorer, the .apk file base.apk for the app myapp is located here:
```
data/app/it372/smiths/myapp/
```
Double click on base.apk to see the contents of this compressed file. It includes classes.dex.
To distribute Android apps in the Google Play Store, they must be signed.
To create a signed APK file:
Select Build >> Generate Signed Bundle / APK...
To run an APK file, Android uses Zygote, which means a half-started process. This process is copied (forked) to produce a new process, which is overlayed with the users process code.
Forking a process is faster than creating a new process from scratch.

Details of Dalvik Executable Format
At the beginning of a .dex file, this magic number is included so that the .dex file is recognized as such:
```
Hex:   64 65 78 0a 30 33 39 00
ASCII:  d  e  x \n  0  3  9 \0 
```
MUTF-8 (Modified UTF-8) Encoding is used to encode Unicode files.
Standard Java only uses encodings for one- and two-byte Unicode characters.
MUTF-8 recognizes one-, two-, and three-byte encodings.
See this Wikipedia article for information about UTF-8.
Exercise: Show how to encode this string in UTF-8:
```
Greek letters: αβγ
```
Use this ASCII Table and Hex Digits Table.
Here are the details of encoding this string in UTF-8: UTF-8 Example.
Set up an Android textview with text set to the string in the preceding exercise.
See this link for more information about MUTF-8 encoding:
https://source.android.com/devices/tech/dalvik/dex-format