1. Design Patterns in Operating Systems

2. A Word on Patterns
These routinely appear as solutions to common classes of problems
They can help in numerous ways
Well known resuable solution with documentation
Pitfalls known in advance
Performance and testing known & done in advance
Generally offer levels of abstraction to cope with complexity by decoupling events and actions

3. Patterns in Operating Systems
There are many, and we’ll examine a few closely
The Observer, The Decorator, The Active Object, The Façade, The Singleton, The Visitor, The Factory

4. Singletons Used to share access to a singular object
Usually, it’s an error to create this object twice
Examples: A teller’s transaction log, a kernel’s memory manager thread, rollbacks
Can be used (somewhat sloppily) to provide global access to a shared resource
What kinds of issues arise in a SMP environment?
For more information, see problem 6.21 in the text

5. UML Singleton ------------------------------------------
Data: (1) private object instance
Methods: private Singleton()
public getInstance()

6. The Action in a Singleton
public Object getInstance() {
if(myInstance == null) { //lazy instantiation
myInstance = new Object(); }
return myInstance;

7. Singletons and SMP Architectures
Race Condition
public Object getInstance() {
if(myInstance == null) {
myInstance = new Object(); }
return myInstance;

8. Double-Checked Lock
Somewhat similar in spirit to Petersons, but more general
Check for null
Acquire my lock
Check again for null, and if null instantiate

9. DC Action in Java public Object getInstance() {
if(myInstance == null) {
mutex.acquire();
If(myInstance == null) {
myInstance = new Object(); }
mutex.release();
return myInstance;

10. DC Action in Java public Object getInstance() {
if(myInstance == null) {
mutex.acquire();
If(myInstance == null) {
myInstance = new Object(); }
mutex.release();
return myInstance;
Two parallel threads may both discover myInstance to be null
But only one will acquire the lock
So the thread in first will build the (1) object
The second thread blocks here and when released, notices myInstance is no longer null

11. Rewrite using synchronized blocks
public Object getInstance() {
if(myInstance == null) {
mutex.acquire();
If(myInstance == null) {
myInstance = new Object(); }
mutex.release();
return myInstance;

12. Rewrite using synchronized blocks
public synchronized Object getInstance() {
//if(myInstance == null) { //no need now for this
//mutex.acquire();
If(myInstance == null) {
myInstance = new Object(); }
//mutex.release();
return myInstance;

13. Double-Checked Locking
More information on our message board and at:

14. Spinlocks Versus Blocking
TestAndSet usually is a spinlock
Semaphores frequently are implemented as blocking
When is one more appropriate?
Which is heavyweight?
Would we ever not want to spin?

15. Socket Programming -> Too Low Level?
What’s the solution?
Make a class that wraps the low level details and provides a clean-and-simple interface
You’ve just made a Façade or Wrapper Class

16. Façade or Wrapper Pattern
Wrap an OO interface around existing code
Or cross-platform library code
Goals: Ease of use, simplicity, known pitfalls
Example: rendering in Vista is done using DirectX, but those wanting to create windows & GUIs can still use createWindowEx()

17. GUI’s Further: The Decorator
Defn: Allows users to wrap objects in containers that provide additional functionality.
Example: new JScrollPane( new JTextArea())
new JBorder( new JTextArea())

18. The Decorator Pattern Observed

19. The Observer (from Wiki)
“The observer pattern (a subset of the asynchronous publish/subscribe pattern) is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems.”

20. More on Observer See java.util.Observer //listeners
And java.util.Observable //the trigger
Lets see some code…

21. The Infamous Factory
Defers Object instantiation to concrete subclasses
Usually defined as an interface
Lets make one for Windows

22. The Factory Pattern Observed
public interface Factory {
public Object create(); }