102 lines
3.3 KiB
Markdown
102 lines
3.3 KiB
Markdown
# Introduction to Android
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**Linux Kernel**: threading, low-level memory management, driver
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**Hardware Abstraction Layer:** Libraries for hardware modules, because different devices have different hardware
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**Android Runtime:** A virtual machine
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**Native C/C++ Libraries:** Libraries for rendering & fundamental core functionalities
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**Java API Framework:** Programming interface
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**System Apps:** Normal apps that can be customised.
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#### Android-Specific Mods
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Android is based on Linux with some key differences:
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* Wake locks - keep the phone awake
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* Binder - inter-process communication; this is so no app can communicate with others, otherwise banking apps could be compromised
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* `ashmem` - shared memory
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* oom - kills processes when memory is low
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* alarm manager - wakes up the phone when necessary (raise to wake)
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#### Android Apps
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* Applications are sandboxed
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* A security mechanism for separately running applications and data
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* Android application sandbox
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* Linux is a multi-user system - this isn't needed
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* Makes use of Linux permissions and security
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* No root access
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#### System Boot-up Process
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1. **Boot ROM/Bootloader**: Load bootloader into RAM, detect external RAM, set up network, memory, etc.
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2. **Kernel**: Set up cache, protected memory, scheduling and load drivers
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3. **Init**: Mount directories like `/sys`, `/dev` and run `rc` scripts.
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4. **Zygote & VM**: Enables code sharing across the Android VM for quick startup of separate VMs for different apps
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5. **System service**: Power manager, activity manager, telephony registry, package manager.
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##### Zygote
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* Initialise a process that has all core libraries linked in.
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* Load all `*.java` and `*.android` classes at boot.
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* Initially create a single Android VM process
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* When the user runs an application
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* Creates a copy of itself in a separate address space
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* Does not copy memory; instead, refers to original memory until modified
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#### Android Compilation
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* Applications are written in Java and run on Google's own VM - Dalvik/Android Runtime
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* `.apk` is analogous to `.exe`
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##### Dalvik
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* Dalvik architecture is register-based rather than stack-based.
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* This makes it optimised to use less space
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* Executes its own Dalvik bytecode rather than Java bytecode
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##### Android Programming Model
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* Traditional OS applications
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* Single entry point - `main()`
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* OS loads the program into a process and executes
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* Java Applications
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* A Java VM is instantiated
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* Loads all classes used by the application
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* Executes `main()`
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* Component-based model
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* Multiple entry points - think sharing a photo through WhatsApp; WhatsApp loads in a different way.
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* Not all entry points are for the user
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###### 4 Main Components
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Communicating via specific interfaces
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* Inter-process - between
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* Intra-process - within
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* Bound at runtime
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* Each with a specific life cycle
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* Dynamically loaded and unloaded as necessary
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1. **Activities**
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* UI components
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2. **Services**
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* Mechanism for doing something long-running in the background (front-end / back-end)
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3. **Broadcast Receivers**
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* Respond to broadcast messages from the OS / other apps
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4. **Content Providers**
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* Make data available for use by other apps
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