Explains foundations of proxy and composite patterns

1. Design Patterns
Proxy & Composite
@sarat, architect
Experion Technologies

2. composite pattern

3. is a structural pattern

4. Structural design patterns ease the
design by identifying a simple way
to realize relationships between
entities

5. Compose objects into tree structures to represent
part-whole hierarchies.
!
describes that a group of objects are to be treated
in the same way as a single instance of an object

6. Leaf (primitive element)
• represents leaf objects in the composition.
• A leaf has no children.
• defines behavior for primitive objects in the
composition

7. Composite
• defines behavior for components having children.
• stores child components.
• implements child-related operations in the
Component interface.

8. Client
!
• manipulates objects in the composition through the
Component interface.

10. /** "Component" */
interface Graphic {
//Prints the graphic.
public void print();
}

11. class CompositeGraphic implements Graphic {
//Collection of child graphics.
private List<Graphic> childGraphics = new ArrayList<Graphic>();

12. class CompositeGraphic implements Graphic {
//Collection of child graphics.
private List<Graphic> childGraphics = new ArrayList<Graphic>();
!
//Prints the graphic.
public void print() {
for (Graphic graphic : childGraphics) {
graphic.print();
}
}

13. class CompositeGraphic implements Graphic {
//Collection of child graphics.
private List<Graphic> childGraphics = new ArrayList<Graphic>();
!
//Prints the graphic.
public void print() {
for (Graphic graphic : childGraphics) {
graphic.print();
}
}
!
//Adds the graphic to the composition.
public void add(Graphic graphic) {
childGraphics.add(graphic);
}
//Removes the graphic from the composition.
public void remove(Graphic graphic) {
childGraphics.remove(graphic);
}
}

14. /** "Leaf" */
class Ellipse implements Graphic {
//Prints the graphic.
public void print() {
System.out.println("Ellipse");
}
}

15. /** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
Ellipse ellipse1 = new Ellipse();
Ellipse ellipse2 = new Ellipse();
Ellipse ellipse3 = new Ellipse();
Ellipse ellipse4 = new Ellipse();

16. /** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
Ellipse ellipse1 = new Ellipse();
Ellipse ellipse2 = new Ellipse();
Ellipse ellipse3 = new Ellipse();
Ellipse ellipse4 = new Ellipse();
!
//Initialize three composite graphics
CompositeGraphic graphic = new CompositeGraphic();
CompositeGraphic graphic1 = new CompositeGraphic();
CompositeGraphic graphic2 = new CompositeGraphic();

17. /** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
Ellipse ellipse1 = new Ellipse();
Ellipse ellipse2 = new Ellipse();
Ellipse ellipse3 = new Ellipse();
Ellipse ellipse4 = new Ellipse();
!
//Initialize three composite graphics
CompositeGraphic graphic = new CompositeGraphic();
CompositeGraphic graphic1 = new CompositeGraphic();
CompositeGraphic graphic2 = new CompositeGraphic();
!
//Composes the graphics
graphic1.add(ellipse1);
graphic1.add(ellipse2);
graphic1.add(ellipse3);
graphic2.add(ellipse4);

18. /** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
…
!
//Initialize three composite graphics
…
!
//Composes the graphics
graphic1.add(ellipse1);
graphic1.add(ellipse2);
graphic1.add(ellipse3);
graphic2.add(ellipse4);
graphic.add(graphic1);
graphic.add(graphic2);
!
graphic.add(graphic1);
graphic.add(graphic2);

19. /** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
…
!
//Initialize three composite graphics
…
!
//Composes the graphics
…
…
!
…
!
//Prints the complete graphic (4 times "Ellipse").
graphic.print();
}

20. Questions?

21. What’s proxy pattern?

22. it makes the clients of a component
communicate with a representative
rather than to the component Itself.

23. enables enhanced efficiency,
easier access and protection
from unauthorized access.

24. examples?

25. a network proxy

26. large object in memory

27. reference counting
pointer objects

28. Context
• A client needs access to the services of
another component
• Direct access is technically possible, but
may not be the best approach

29. Problem
• Often it’s inappropriate to access the
component directly
• Direct and unrestricted access can be
insecure and inefficient

30. Key considerations

31. run-time efficient

32. cost effective

33. safe for both client and
component

34. transparent interfaces

35. should be similar to
component’s interface

36. aware of performance
penalties

37. solution

38. communicate to
representative — proxy

39. proxy performs
pre & post processing (e.g
access control)

40. structure

41. uses proxy to fulfill it’s
task
Client

42. a common interface for
proxy and original
AbstractOriginal

43. provides interface of the
original to clients
proxy

44. ensures safe, efficient and
correct access to the
original
proxy

45. collaborates with
original
proxy

46. implements a particular
service
Original

48. dynamics

50. implementation
considerations

51. migrate all client
responsibilities to proxy

52. remove all direct
relationship with client
and original

53. variants

54. To shield network addresses and inter-process communication
protocols from clients
Remote Proxy

55. Access authorization
e.g network authorization to use Internet
Protection Proxy

56. Multiple local components can share results from a remote proxy
e.g. Cache server
Cache Proxy

57. Provide synchronized access to services in a multi-access or multi-
threaded environment
Synchronization Proxy

58. Prevents accidental deletion of components or collects usage
statistics
e.g. reference counting objects
Counting Proxy

59. Protects local clients from outside world
Firewall proxy

60. benefits

61. enhanced efficiency and
lower cost
e.g. cache proxy

62. decouple clients from
real objects

63. separate housekeeping
from functionality

64. liabilities

65. less efficiency due to
indirection

66. overkill using sophisticated strategies
!
e.g. caching server in a dynamic
environment

67. Questions?