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 every thread in the program cannot initialize its own instance of the singleton class.

Example Implementations

Java

public class LoggerSingleton {

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

}

Ruby

require 'singleton'

class LoggerSingleton

 include Singleton 

end

Test singletons

Following a code to test singleton class in Java:

import org.apache.log4j.Logger; import junit.framework.Assert; import junit.framework.TestCase; public class SingletonTest extends TestCase {

  private LoggerSingleton sone = null, stwo = null;
  private static Logger logger = Logger.getRootLogger();
  public SingletonTest(String name) {
     super(name);
  }
  public void setUp() {
     logger.info("getting singleton...");
     sone = LoggerSingleton.getInstance();
     logger.info("...got singleton: " + sone);
     logger.info("getting singleton...");
     stwo = LoggerSingleton.getInstance();
     logger.info("...got singleton: " + stwo);
  }
  public void testUnique() {
     logger.info("checking singletons for equality");
     Assert.assertEquals(true, sone == stwo);
  }

}

in ruby;

def test_singleton

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

 OnlyOne.instance.modify_state
 OnlyOne.instance.reset

end

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.