1. Android Programming Lecture 8
Testing notes
Multi-threading

2. Timesharing Operating Systems
All modern operating systems are capable of sharing processor(s) between multiple users and processes
Process scheduling algorithms
There are often lots (hundreds) of processes running
Many are managed by the system
Some are user executed

3. Process
A process is typically defined as execution of a single program on a computing device
Processes are scheduled by the OS
Processes are owned by a user or the OS (root)
Processes may have a long lifetime (a web server) or a short lifetime (one run of a short program)
Processes do not share memory between each other
Most OS support inter-process communication

4. Threads A thread is a component of a process
Usually considered the smallest unit that can be scheduled by an OS
Can share memory and instructions with other threads in the same process
Most OS support inter-thread communication

5. Process V.S. Thread

6. Timesharing Operating Systems
On a single processor system,
“Round robin” scheduling system switches between threads and processes fast enough that it appears that multiple programs are running
Only one is every truly executing at a time
On multi-core or multi-processor systems,
Threads or processes can be allocated across the processors
True parallel processing
“Round-robin” scheduling still occurs

7. Android
Android starts a new Linux process for the application with a single thread of execution
All components of the same application run in the same process and thread
This thread is called the “main” thread or “UI” thread
Any updates to the UI/display must be accomplished on the main thread

8. Slow Android App
If a component of the work takes a long time, the rest of the work will be “blocked”
For example, a long time to access data across the network prevents responding to any GUI events
In the Android OS, if a GUI doesn’t respond to an input event in < five seconds, then it is considered unresponsive and the OS will try to kill it!

9. Multi-threading Put non-UI work on a separate thread
For example, download an image, connect to a remote server, open a web browser…
Android
App UI
Main Thread
Java Threads BG
Background Thread

10. What technique shall I use for my background thread execution?UI
Android
App
Java Threads BG
What technique shall I use for my background thread execution?
Linux
Process UI BG
Application
Start
Application
End
Time

11. Asynchronous Techniques
Thread
Executor
HandlerThread
AsyncTask
Service
IntentService
AsyncQueryHandler
Loader

12. Handler – Loop - Message
Message Queue
UI Thread
Message 1
Thread 1
sendMessage()
Message 2
Looper
Handler
Message 3
Message 4
Thread 2

13. Handler
UI Thread
Message Queue
Message 1
@Override protected void onCreate(Bundle savedInstanceState) { …
final Handler handler = new Handler() { public void handleMessage(Message msg) { if (msg.what == 0x123) { // Do the things when the message
} } };
final Timer timer = new Timer(); timer.schedule(new TimerTask() { @Override public void run() {
// Open a new thread }
}, 0, 100);
Thread 1
Looper
UI Thread
Thread 1
@Override public void run() {
// To be executed in the thread … }
handler.sendEmptyMessage(0x123);

14. Timer Thread A thread manages a Runnable object Timer class
The java.util.Timer class provides facility for threads to schedule tasks for future execution in a background thread.
This class schedules tasks for one-time execution, or for repeated execution at regular intervals.
The Timer class in the java.util package schedules instances of a class called TimerTask
final Timer timer = new Timer(); timer.schedule(new TimerTask() { @Override public void run() {
// To be executed in the thread }
}, 0, 100);
Timer Class Reference:
Runnable Class Reference:

15. Handler
Allow you to send and process Message and Runnable objects associated with a thread’s Message Queue
Handler instance is associated with a single thread and that thread’s message queue
When you create a new Handler, it is bound to the thread and message queue of the thread that is creating it
It will deliver Message and Runnable to that message queue and execute them as they come out of the message queue
Need only one Handler object per activity
Handler
Message Queue
Message 1
Looper

16. Handler Loop Create and Start 1 Message Queue Add Process 3 2 1 2 3
onCreate(){ … } 1
Handler h = new Handler() {
@Override
public void handleMessage(Message msg) {
//Process message } }; 2
h.sendEmptyMessage(msg); 3

17. Message Class
Defines a message containing a description and arbitrary data object that can be sent to a Handler.
A message contains two extra int fields and an extra object field
what: User-defined message code so that the recipient can identify what this message is about.
obj: An arbitrary object to send to the recipient.
While the constructor of Message is public, the best way to get one of these is to call Message.obtain() or one of the Handler.obtainMessage() methods, which will pull them from a pool of recycled objects.

18. Handler
UI Thread
Message Queue
Message 1
@Override protected void onCreate(Bundle savedInstanceState) { …
final Handler handler = new Handler() { public void handleMessage(Message msg) { if (msg.what == 0x123) { // Do the things when the message
} } };
final Timer timer = new Timer(); timer.schedule(new TimerTask() { @Override public void run() {
// Open a new thread }
Thread 1
Looper
UI Thread
Thread 1
@Override public void run() {
// To be executed in the thread …
handler.sendEmptyMessage(0x123); }

19. Handler Methods to Send Message
sendEmptyMessage(int what)
Sends a Message containing only the what value.
sendEmptyMessageDelayed(int what, long delayMillis)
Sends a Message containing only the what value, to be delivered at a specific time.
sendEmptyMessageAtTime(int what, long uptimeMillis)
Sends a Message containing only the what value, to be delivered after the specified amount of time elapses.
sendMessage(Message msg)
Pushes a message onto the end of the message queue after all pending messages before the current time.
sendMessageAtFrontOfQueue(Message msg)
Enqueue a message at the front of the message queue, to be processed on the next iteration of the message loop.
sendMessageAtTime(Message msg, long uptimeMillis)
Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis.

20. References: Android Programming: The Big Nerd Ranch Guide
Chapter 24 (Safari Books Online)