Revisiting some popular design patterns in Java: singleton, template method, visitor.

1. Design Patterns
Reconsidered
Alex Miller
@puredanger

2. What is a Design
Pattern?
“Each pattern describes a problem which
occurs over and over again in our
environment and then describes the core of
the solution to that problem, in such a way
that you can use this solution a million
times over, without ever doing it the same
way twice.”
- Christopher Alexander

4. The Patterns Backlash
• Copy/paste
• Design by template
• Cookbook / recipe approach
“The Design Patterns solution is to turn the
programmer into a fancy macro processor”
- M. J. Dominus, “Design Patterns” Aren't

5. The Patterns Backlash
• Design patterns aren’t patterns
• Just workaround for missing language
features
“At code level, most design patterns are
code smells.”
- Stuart Halloway

6. The Patterns Backlash
• Overuse
“Beginning developers never met a pattern
or an object they didn’t like. Encouraging
them to experiment with patterns is like
throwing gasoline on a fire.”
- Jeff Atwood, Coding Horror

7. ...Reconsidered
Template Visitor
Method
Proxy
Singleton

8. Singleton

9. There can be only one...
Singleton
- INSTANCE
+ getInstance() : Singleton
- Singleton()
+ foo() : Object

10. Classic Singleton

11. Classic Singleton
public final class Singleton {

12. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =

13. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();

14. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}

15. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {

16. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;

17. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;
}

18. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;
}
public Object read() {

19. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;
}
public Object read() {
// nasty database call

20. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;
}
public Object read() {
// nasty database call
}

21. Classic Singleton
public final class Singleton {
private static Singleton INSTANCE =
new Singleton();
private Singleton() {}
public static Singleton instance() {
return INSTANCE;
}
public Object read() {
// nasty database call
}
}

22. Things go horribly wrong
public class Victim {
public void something() {
Object foo = Singleton.instance().read();
// etc...
}
}

23. Things go horribly wrong
public class Victim {
public void something() {
Object foo = Singleton.instance().read();
// etc...
}
}
public class TestVictim {
public void testSomething() {
// Holy crap, how do I
// mock the db call in Singleton?
}
}

24. Create
Hidden
Singletons
Coupling!

25. Interfaces to the rescue
public interface Singleton {
Object read();
}
public class SingletonImpl
implements Singleton {
public Object read() {
// nasty database call
return null;
}
}

26. Dependency injection,
you’re my hero!
public class Victim {
private final Singleton singleton;
public Victim(Singleton singleton) {
this.singleton = singleton;
}
public void something() {
Object foo = singleton.read();
// etc...
}
}

27. Now we can test
public class TestVictim {
public void testSomething() {
Singleton s = new MockSingleton();
Victim victim = new Victim(s);
victim.something();
// assertions
}
}

28. Push construction up
public class Component {
private final Victim victim;
public Component() {
victim = new Victim(
new SingletonImpl() );
}
}

29. Up
Bubble
Singletons

30. Singleton

31. What have we learned?
• Interfaces and dependency injection
• Reduce hidden coupling
• Allow testability
• Allow subclassing
• Make construction and use flexible
• If need only one, control by configuration
• Guice
• Spring

32. ...Reconsidered
Template Visitor
Method
Proxy
Singleton

33. Template Method

34. Template Method
public void algorithm() {
TemplateAlgorithm step1();
+ algorithm() step2();
# step1() step3();
# step2() }
# step3()
ConcreteAlgorithm1 ConcreteAlgorithm2
# step1() # step2()
# step2()
# step3()

35. Spring MVC
Controllers
Controller (interface)
AbstractController
AbstractUrlViewController
UrlFilenameViewController
BaseCommandController
AbstractCommandController
AbstractFormController
AbstractWizardFormController
SimpleFormController
CancellableFormController
MultiActionController
ParameterizableViewController

37. “What we’ve got
here is a failure to
communicate....”

38. Refactoring to steps
Step1Strategy
+ step1()
TemplateAlgorithm
- Step1Strategy
Step2Strategy
- Step2Strategy
+ step2()
- Step3Strategy
+ algorithm()
Step3Strategy
+ step3()
public void algorithm() {
step1Strategy.step1();
step2Strategy.step2();
step3Strategy.step3();
}

39. Sharing context
Step1Strategy
+ step1(Context ctx)
TemplateAlgorithm
- Step1Strategy
Step2Strategy
- Step2Strategy
+ step2(Context ctx)
- Step3Strategy
+ algorithm()
Step3Strategy
+ step3(Context ctx)
public void algorithm() {
Context
Context context = new Context();
step1Strategy.step1(context);
step2Strategy.step2(context);
step3Strategy.step3(context);
}

40. What have we learned?
• Prefer composition to inheritance
• Allows greater reuse
• Communicates intent better
• Easier to understand and maintain
• More robust as it evolves
• Inheritance is a very strong form of coupling
• Especially in a single-inheritance language

41. ...Reconsidered
Template Visitor
Method
Proxy
Singleton

42. Composite hierarchy
Node
operation1()
operation2()
ConcreteNode1 CompositeNode
operation1() operation1()
operation2() operation2()

43. Visitor Pattern
Node
accept(NodeVisitor v)
ConcreteNode1 CompositeNode
accept(NodeVisitor v) accept(NodeVisitor v)
NodeVisitor
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
...
ConcreteVisitor1 ConcreteVisitor2
visit(ConcreteNode1 n) visit(ConcreteNode1 n)
visit(ConcreteNode2 n) visit(ConcreteNode2 n)
... ...

44. N
av
iga
tio
n

45. Internal Navigation
public class CompositeNode
implements Visitable {
private List<Node> nodes;
public void accept(NodeVisitor v) {
v.visit(this);
for(Node n : nodes) {
v.visit(n);
}
}
}

46. Navigation oracle
public class CompositeNode {
private List<Node> nodes;
public void accept(NodeVisitor v) {
v.visit(this);
List<Node> children =
Navigation.getChildren(this);
for(Node n : children) {
n.acceptVisitor(this);
}
}
}

47. Navigation Visitor
NodeVisitor
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
...
ConcreteVisitor1 ConcreteVisitor2
visit(ConcreteNode1 n) visit(ConcreteNode1 n)
visit(ConcreteNode2 n) visit(ConcreteNode2 n)
... ...
NavigationVisitor
NavigationVisitor(NodeVisitor v)
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
...

48. Evo
lutio
n

49. Node
accept(NodeVisitor v)
ConcreteNode1 CompositeNode
accept(NodeVisitor v) accept(NodeVisitor v)
NewNode
accept(NodeVisitor v)
NodeVisitor
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
visit(NewNode n)
...
ConcreteVisitor1 ConcreteVisitor2
visit(ConcreteNode1 n) visit(ConcreteNode1 n)
visit(ConcreteNode2 n) visit(ConcreteNode2 n)
visit(NewNode n) visit(NewNode n)
... ...

50. NodeVisitor
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
visit(NewNode n)
...
BaseVisitor
visit(ConcreteNode1 n)
visit(ConcreteNode2 n)
visit(NewNode n)
...
ConcreteVisitor1 ConcreteVisitor2
visit(ConcreteNode1 n) visit(ConcreteNode1 n)
visit(ConcreteNode2 n) visit(ConcreteNode2 n)
visit(NewNode n) visit(NewNode n)
... ...

51. Visitor Types
• “Collector” visitor - accumulate state
• “Finder” visitor - search and return
• “Event” visitor - stateless, fire events
• “Transform” visitor - modify while walking
• “Validation” visitor - validate and report

52. Visitor abort
public class FindVisitor
implements ConcreteVisitor {
private final int seek;
private Node match;
public FindVisitor(int seek) {
this.seek = seek;
}
public Node getMatch() {
return this.match;
}
public void visit(ConcreteNode1 n) {
if( this.match == null &&
n.getValue() == seek) {
this.match = n;

53. Exceptions
public class ComputeVisitor
implements ConcreteVisitor {
public void visit(ConcreteNode1 n) {
try {
// blah blah
} catch(BadException e) {
// what now?
}
}
}

54. What have we learned?
• Expression problem is tough
• Abstract base classes help plan for evolution
• Generics add precision and flexibility by
revealing hidden coupling

55. Questions?
http://tech.puredanger.com/
presentations/design-
patterns-reconsidered