An Introduction to the Android Framework -- a core architecture view from apps to the kernel --

Note: The Copyrights of the referenced materials and figures go to their original authors. As a result, the slides are for non-commercial reference only. For the contents created in this document, the Copyright belongs to William W.-Y. Liang. © 2005-2016 All Rights Reserved. NTU CSIE Open Source System Software 2016.03.29 An Introduction to the Android Framework -- a core architecture view from apps to the kernel -- William W.-Y. Liang (梁文耀), Ph. D. http://www.ntut.edu.tw/~wyliang for 台大資工系開源系統軟體課程 hosted by Prof. Shih-Hao Hung

© 2016 William W.-Y. Liang, All Rights Reserved. Advantage of the Android Operating System High application portability Consistent user experience Component-based design Suitable for resource-limited handheld devices Consider the performance issue Acceptable effort to port to different hardware 3 NTU OSSSP 2016: Android Framework Think: Architecture design and considerations • Multi-layer design • Programming languages • License issues

© 2016 William W.-Y. Liang, All Rights Reserved. Android/Linux Android differs from the typical GNU/Linux in that it adopts only the Linux kernel, not everything. The first process ‘init’ transfers to Android’s own middleware and application environment. BSD libc is used instead of glibc or uClibc. As a result, it is called Android/Linux, not GNU/Linux. 4 NTU OSSSP 2016: Android Framework Think: how different they are?

© 2016 William W.-Y. Liang, All Rights Reserved. Important Subjects for the Core Architecture Android task and process Activity and service The binder IPC Thread operations Concurrency control and thread safety Android framework components User-space device control Android Java native interface for C/C++ Hardware abstraction layer Linux device driver and device manipulation 8 NTU OSSSP 2016: Android Framework Think: Android boot flow, system services, and subsystems

© 2016 William W.-Y. Liang, All Rights Reserved. Activity -- the Primary Framework Component Displays a user interface component and responds to system/user. When an application has a user interface, it contains one or more “Activities”. Each “Activity” can be invoked by the same or other apps. The new Java class must extend the framework “Activity” class. Created “Activity” must be defined into the application’s Manifest.xml. An existing “Activity” can be replaced with a new one that fulfill the same contract (intent). 9 NTU OSSSP 2016: Android Framework Source from “Deep inside Android,” Gilles Printemps, Esmertec © Think: activity stack and process

© 2016 William W.-Y. Liang, All Rights Reserved. Intents and Intent Filters Provide a late runtime (asynchronous) binding between the code in different applications (for activities, services, and broadcast receivers) Intents: Simple message objects that represent an intention to do something Intent Filters: A declaration of capacity and interest in offering assistance to those in need 11 NTU OSSSP 2016: Android Framework Slide from “Deep inside Android,” Gilles Printemps, Esmertec © Think: The design philosophy

© 2016 William W.-Y. Liang, All Rights Reserved. Life Cycle of an Activity 12 NTU OSSSP 2016: Android Framework Source from http://developer.android.com/ Think: 1. The design philosophy 2. How to deal with data loss?

© 2016 William W.-Y. Liang, All Rights Reserved. The Service Framework Component Similar to activities, but without UI For long-running background tasks Extended from the Service class It's possible to connect to (bind to) an ongoing service. The connected service can be communicated through the Binder interface. 13 NTU OSSSP 2016: Android Framework Source from “Deep inside Android,” Gilles Printemps, Esmertec © Think: service vs. activity

© 2016 William W.-Y. Liang, All Rights Reserved. Binder IPC Problems to solve Applications and Services may run in separate processes but must communicate and share data. IPC can introduce significant processing overhead and security holes. Solution Driver to facilitate inter-process communication (IPC) High performance through shared memory Reference counting, and mapping of object references across processes Synchronous calls between processes 16 NTU OSSSP 2016: Android Framework Source from "Android Anatomy and Physiology," Patrick Brady © https://kushinbunte.wordpress.com/2015/01/29/binder- communication-mechanism-of-android-processes/ Think: IPC overheads

© 2016 William W.-Y. Liang, All Rights Reserved. AIDL – a Wrapper of the Binder IPC 17 NTU OSSSP 2016: Android Framework AIDL: Android Interface Definition Language Used to generate code in a remote procedure call (RPC) form that marshals the Binder IPC communication details. Think: how it works?

© 2016 William W.-Y. Liang, All Rights Reserved. Thread Issues for Android The Andoid UI toolkit is not thread-safe. Do not manipulate your UI from a worker thread Two rules to Android's single UI thread model: Do not access the UI toolkit from outside the UI thread Do not block the UI thread 18 NTU OSSSP 2016: Android Framework Problematic Example public void onClick(View v) { new Thread(new Runnable() { public void run() { Bitmap b = loadImageFromNetwork("http://example.com/image.png"); mImageView.setImageBitmap(b); } }).start(); } Think: 1. How to solve the problem 2. The ANR issue

© 2016 William W.-Y. Liang, All Rights Reserved. The Message Queue Message Queue Looper Handler 19 NTU OSSSP 2016: Android Framework class LooperThread extends Thread { public Handler mHandler; public void run() { Looper.prepare(); mHandler = new Handler() { public void handleMessage(Message msg) { // process incoming messages here } }; Looper.loop(); } } Think: how the framework works?

© 2016 William W.-Y. Liang, All Rights Reserved. Process Lifecycle In Android, the system needs to remove old processes when memory runs low. To determine which processes to keep and which to kill There are five levels, listed in order of importance: Foreground process Visible process Service process Background process Empty process 20 NTU OSSSP 2016: Android Framework Think: when it changes and how it affects?

© 2016 William W.-Y. Liang, All Rights Reserved. The Linux Kernel Android relies on Linux kernel 2.6 and above for core system services, e.g. memory management, process management, network stack, and driver model, security, etc. The kernel also acts as an abstraction layer between the hardware and the rest of the software stack. 21 NTU OSSSP 2016: Android Framework

© 2016 William W.-Y. Liang, All Rights Reserved. Features that Android added in Kernel Binder: Binder driver provides high performance inter-process communication (IPC) through shared memory. Ashmem: The Android shared memory driver, which create a memory region to be shared between processes. Pmem and ION: Used to allocate and manage a physically contiguous memory for user space driver. Power: The android power management driver Alarm: It’s a driver which provides a real time clock. 22 NTU OSSSP 2016: Android Framework Think: Linux mainline kernel vs. Android kernel

© 2016 William W.-Y. Liang, All Rights Reserved. User-space Device Control Typical device drivers exist in the Kernel However, some of the device control logic can be performed from the user space. Two methods to do so Communicate with the kernel-space drivers through the device files or other system interfaces such as sysfs Direct hardware access through memory mapped I/O, by mmap 23 NTU OSSSP 2016: Android Framework Think: performance issues for the above methods

© 2016 William W.-Y. Liang, All Rights Reserved. Android System Services Core Services Activity Manager Package Manager Window Manager Resource Manager Content Providers View System Hardware Services Telephony Service Location Service Bluetooth Service WiFi Service USB Service Sensor Service 26 NTU OSSSP 2016: Android Framework Source from “Deep inside Android,” Gilles Printemps, Esmertec © Think: compare with the concept used in micro-kernel

© 2016 William W.-Y. Liang, All Rights Reserved. Android Native Libraries C Library Android includes a set of C/C++ libraries used by various components of the Android system. Bionic C Library: A BSD-derived implementation of the standard C library, tuned for embedded Linux-based devices Example Media Libraries SQLite Webview SGL FreeType 3D libraries Many more … 28 NTU OSSSP 2016: Android Framework Think: 1. The Dynamic Shared Library 2. Performance Issue

© 2016 William W.-Y. Liang, All Rights Reserved. Cross the Language: Java, C/C+, and JNI JNI: Java Native Interface Standard interface between Java and C/C++ 29 NTU OSSSP 2016: Android Framework class HelloWorld { public native void helloworld(); static { System.loadLibrary("hello"); } public static void main(String[] args) { new HelloWorld().helloworld(); } } Think: 1. Bi-directional invocation 2. Dalvik VM, JIT, and ART

© 2016 William W.-Y. Liang, All Rights Reserved. The Android Hardware Abstraction Layer User space C/C++ library layer Defines the interface that Android requires hardware “drivers” to implement Separates the Android platform logic from the hardware interface Kernel drivers are GPL which may expose the proprietary IP. 31 NTU OSSSP 2016: Android Framework Source from "Android Anatomy and Physiology," Patrick Brady ©

Note: The Copyrights of the referenced materials and figures go to their original authors. As a result, the slides are for non-commercial reference only. For the contents created in this document, the Copyright belongs to William W.-Y. Liang. © 2005-2016 All Rights Reserved. Q & A william.wyliang@gmail.com http://www.ntut.edu.tw/~wyliang http://www.facebook.com/william.wyliang

An Introduction to the Android Framework -- a core architecture view from apps to the kernel --

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An Introduction to the Android Framework -- a core architecture view from apps to the kernel --