CSC/ECE 517 Fall 2007/wiki1b 6 c1

Singleton Pattern

The Singleton pattern is a design pattern used to insure the existence of a single instance of a class in the run time. It constructs the object once and returns a reference to this object every time thereafter. In a multithreading environment, special care should be taken, as it will be illustrated later in this page.

Implementation

The Singleton pattern is implemented by
- Create a class with a method which will create a new instance of the class (If one doesn't already exist)
- The constructor of this class is turned to private or protected
- Another public function is used to call this constructor.
- If the object is not initialized yet (object == null) it calls the constructor and returns a reference else it will just return a reference.

In multithreading environment, a synchronized command should be used so not every thread in the program can initialize its own instance of the singleton class.

Example Implementations

To demonstrate further let us consider programs that implement a logger in Java and then in Ruby.

Java

public class LoggerSingleton {
  private static LoggerSingleton instance = null;
  private LoggerSingleton() {
     // Exists only to defeat instantiation.
  }
  public static LoggerSingleton getInstance() {
     if(instance == null) {
        instance = new LoggerSingleton();
     }
     return instance;
  }
  // add other attributes
}

Ruby

require 'singleton' 
class LoggerSingleton
  include Singleton 
end 

Test singletons

Following a code to test singleton class in Java:

import junit.framework.Assert; 
import junit.framework.TestCase; 
public class TestSingleton extends TestCase {
 private LoggerSingleton logger1 = null, logger2 = null;
 public TestSingleton(String name) {
   super(name);
 }
 public void setUp() {
       logger1 = LoggerSingleton.getInstance();
       logger2 = LoggerSingleton.getInstance();
  }
 public void testUnique() {
       System.out.println(logger1);
       System.out.println(logger2);
       Assert.assertEquals(true, logger1 == logger2);
      }
   }

in ruby;

def test_singleton
 class << OnlyOne.instance
   def reset
     # reset state
   end
 end
 OnlyOne.instance.modify_state
 OnlyOne.instance.reset
end

Java versus Ruby

Java and Ruby have their similarities and their differences so lets compare the two languages.

Ruby	Java
Interpreted	Compiled to byte code
Dynamically typed	Statically typed
Purely object oriented	Distinction between primitives and object types
Unbounded polymorphism	Inheritance and Interfaces
Multiple inheritance through mixins	Interfaces
Syntactic regular-expression support	Not present (can do the same thing in Java by creating a pattern with java.util.regex, etc.)
Syntactic support for arrays and hashes	Arrays and hashes (HashMap, Hashtable) present in Collections; no special syntax
nil is an object => no null-pointer exceptions	null means no reference to object
Everything is a message	Method invocations are compiled, not treated as messages at run time.
Possible to capture calls to non-existent methods using method_missing	Not possible
All classes are open to extension	Classes cannot be extended at run time
Dynamic evaluation of code using eval	Not possible easily
Reflection is easy	Reflection is much more verbose
Blocks are closures	Anonymous inner functions are closures but less powerful

Conclusion

Java doesn’t contain singleton pattern in the language itself, although implementation for the pattern is still possible. While in Ruby the singleton pattern is in the language itself, which became possible since Ruby is a dynamic language through singleton module.