1. Multithreaded Programming in Java
Multimedia Technology Programming
Medialogy, Semester 7
Aalborg University, Aalborg
2010
David Meredith

2. Sources
Chapter 14 of The Java Programming Language (Fourth Edition) by Ken Arnold, James Gosling and David Holmes (Addison-Wesley, 2006)

3. Single-threaded programming
single sequence or thread of instructions, executed one after the other
only one instruction being carried out at any given instant in time
the single thread in a single-threaded program is called the main thread

4. SingleThreadedProgram.java

5. Multithreaded programming
Most of the programs you use every day are multithreaded programs because they have to do two or more things at the same time
For example
word processor has to listen for user input, update the screen, save periodic backups, look-up typed words in a dictionary as they are typed, etc.
web crawler crawls many different web pages simultaneously, creating a record for each one in the search engine’s database

6. Multithreaded programming
Multithreaded program consists of more than one independent thread (sequence of instructions), running at the same time or concurrently
hence sometimes called concurrent programming
Two or more threads can access the same data
this is good in a web crawler where many different crawling threads can update the same central database
HOWEVER this can cause problems such as the race condition

7. Race Condition
Suppose a is the joint bank account of Hr. and Fru Jensen and it starts off with a balance of 1000kr
Hr. Jensen goes to the branch near his work and deposits 100 kr
Fru Jensen goes to the branch near their home and withdraws 50 kr
The final balance should be 1050 kr...but is it?
It depends on whether the account is locked while a transaction is taking place

8. Creating Threads in Java
Create a new thread of control in Java by instantiating a Thread object:
Thread worker = new Thread();
Can then set its initial priority, name, etc.:
worker.setPriority(worker.getPriority()+1);
worker.setName(“myThread”);
When ready call the Thread’s start method to set it off
worker.start();
This starts a new thread of control based on data in Thread object
VM invokes new Thread’s run method which makes the Thread active
Thread’s run method does nothing, but can override it in a subclass of Thread

9. PingPong.java

10. Creating threads by specializing Thread
In PingPong, we defined a subclass of Thread called PingPongThread and redefined the run method in this subclass to specify precisely what PingPongThreads should do
But what if we had wanted PingPongThread to inherit from a class other than Thread?
We couldn’t, because it has to inherit from Thread
There’s a better way...

11. Using the Runnable interface
A Thread abstracts the concept of a worker
i.e., something that carries out some task
When you subtype Thread, the task carried out by the new subtype is defined in its run() method
Instead, can define a new class that implements the Runnable interface
Runnable interface abstracts the concept of a task
Runnable interface declares a method public void run();
A class that implements Runnable must implement this run() method
A Thread (worker) can be assigned a Runnable object (task) to run
The Thread’s run() method calls the run() method of the Runnable object assigned to it when the Thread is started (with the start() method)

12. PingPong2.java

13. Thread runs Runnable objects and implements Runnable itself

14. Interference and Critical Regions
Interference is when interleaved operations from different threads on shared data could corrupt the data
The code segments that contain the interfering actions are called critical sections or critical regions

15. Synchronization and Locks
Prevent interference by synchronizing access to critical regions
A Thread should have to acquire a lock on a shared object before processing it
Here, would have to acquire a lock on the bank account, a, before changing it
Thread releases lock on an object when finished with it
A synchronized method or statement acquires a lock on an object before it executes and releases the lock once it has finished

16. Synchronized Methods
HrOgFruJensen.java

17. Synchronized Statements
HrOgFruJensen2.java