CERN – European Organization for Nuclear Research
GS Department – Administrative Information Services
Design Patterns in Groovy
Nicolas Décrevel
Advanced Information Systems
CERN – Geneva, Switzerland
Design Patterns
Someone has already solved your
problem!
 A design pattern is a general repeatable
solution to a commonly occurring
problem in software design
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Design Patterns
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Creational patterns
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Abstract Factory Design Pattern
Builder Design Pattern
Factory Method Design Pattern
Object Pool Design Pattern
Prototype Design Pattern
Singleton Design Pattern
Structural patterns
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Adapter Design Pattern
Bridge Design Pattern
Composite Design Pattern
Decorator Design Pattern
Facade Design Pattern
Flyweight Design Pattern
Private Class Data
Proxy Design Pattern
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Behavioral patterns
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Chain of Responsibility
Command Design Pattern
Interpreter Design Pattern
Iterator Design Pattern
Mediator Design Pattern
Memento Design Pattern
Null Object Design Pattern
Observer Design Pattern
State Design Pattern
Strategy Design Pattern
Template Method Design Pattern
Visitor Design Pattern
Problem
Can my SquareObject fit the RoundHole?
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Adapter Pattern
Allows objects satisfying one interface
to be used where another type of
interface is expected
 The SquareObject should be used where
we expect a RoundObject
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Adapter Pattern in Java (1)
1. Create an interface
2. Refer to it in your code
3. Adapt the behaviour
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Adapter Pattern in Java (2)
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Adapter Pattern in Groovy (1)
No need of an interface,
as long as the object has a radius property
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Adapter Pattern in Groovy (2)
We can even inherit,
as not type is defined in objectFits
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Adapter Pattern in Groovy (3)
If an interface were to exist
We could create a closure
And use it like this
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Adapter Pattern
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Two flavours of the pattern:
– The delegation flavour
– The inheritance flavour
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Not a lot of difference between Java and
Groovy
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Problem
What if Bernard get fired but
stays in the database ?
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Null Object Pattern
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The intent of a Null Object is to
encapsulate the absence of an object by
providing a substitutable alternative that
offers suitable default do nothing
behaviour
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Null Object Pattern in Java
NO!
YES!
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Null Object Pattern in Groovy
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Null Object Pattern in Groovy?
Due to syntax simplification, the Null
Object Pattern may be less useful in
Groovy than in Java for simple cases
 It will still be useful when the object to
nullify is complex and has multiple
collaboration with the client
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Problem
Let’s extend my simple Logger behaviour
Add time stamp
Add upper message
Add both
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Decorator Pattern
Attach additional responsibilities to an
object dynamically
 Provide a flexible alternative to
subclassing for extending functionality
 Doesn’t modify source code
 Decorators can be combined in flexible
ways
 Avoid class explosion
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Decorator Pattern in Java
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Decorator Pattern in Groovy (1)
Use @Delegate annotation
to delegate method calls
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Decorator Pattern in Groovy (2)
A touch of dynamic behavior
All String arguments will
be lowered
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Decorator Pattern in Groovy (3)
Is my logger slow?
Let’s trace it
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Decorator Pattern in Groovy (4)
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Decorator or Observer Pattern?
Using interceptor as decorator looks like
the observer pattern
 Except that the observation code is part
of its own class (no need to change the
observed class)
 You can use a
MultiInterceptorProxyMetaClass to pass
multiple observer
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– http://groovy.codehaus.org/JN3515-Interception
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Problem
I want to reuse my algorithm with
two other encodings
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Template Method Pattern
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Template Method lets subclasses
redefine certain steps of an algorithm
without changing the algorithm’s
structure
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Template Method Pattern in Java
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Template Method Pattern in Groovy
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Template Method Pattern Problem
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We still have to create two classes
– ReversingRot13 extends TwoPhaseEncryption
– Base64Reversing extends TwoPhaseEncryption
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What if we want to inverse the execution of the
phases?
– We have to create two other classes
• Rot13Reversing extends TwoPhaseEncryption
• ReversionBase64 extends TwoPhaseEncryption
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What if we want to use Base64 with Rot13 ?
– new classes, etc, etc
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Is there an other way to write TwoPhaseEncryption?
– Extract the parts which are changing : the encryptions
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Strategy Pattern
Define a family of algorithms,
encapsulate each one, and make them
interchangeable
 Strategy lets the algorithm vary
independently from the clients that use it
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Strategy Pattern in Java
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Strategy Pattern in Java
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Strategy Pattern in Groovy
Let’s use some closure
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Strategy Pattern
This pattern is really used to have
first class functions
 Groovy already has first class functions
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Is-A vs Has-A
We have seen two ways of implementing
a solution using either inheritance (Is-A)
or delegation (Has-A)
 One should prefer the Has-A version of
an algorithm which will be more
resistant to changes
 Remember the @Delegate
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Grails Framework
A lot of the Design Patterns are getting
useless with usage of complex
Frameworks like Spring or Grails
 These Frameworks already implement a
lot of these Design Patterns to simplify
coding
 Examples: Template, Strategy, Proxy,
Builder, Abstract Factory, Singleton,
Observer, etc
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Conclusion
All Design Patterns can be coded the
same way in Groovy as in Java
 Some Patterns have been designed
because of restriction of the language
 Groovy, allowing first class function and
weak typing, highlights similarities of
different patterns
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Useful urls
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Design patterns in Groovy
– http://groovy.codehaus.org/Design+Patterns+with+Groovy
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Design patterns in Java
– http://sourcemaking.com/design_patterns
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Thank You
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