14 Singleton Design Pattern

Singleton

An example of where it is used

An example of where the singleton pattern is used is in the Spring Framework. If you add this to your code and run your application http://localhost:8080 you can detect that the StudentsController class is only instanciated once.

    private int time = 1;
    
    public StudentsController(){

        System.out.println("I was called " + time + " time(s)");
        time++;
    }

Example 1

    public class Singleton {

        private static Singleton singleton = new Singleton();

        private Singleton() { }
        
        public static Singleton getInstance( ) {
            return singleton;
        }
 
        protected static void singletonMethod( ) {
            System.out.println("I am a singleton method");
        }
    }
    
    
    public class Main {
        public static void main(String[] args) {
            Singleton.getInstance().demoMethod();
        }
    }

Example 2 (a classic Singleton design pattern)

   public class ClassicSingleton {

        private static ClassicSingleton instance = null;
        private ClassicSingleton() {
            
        }

        public static ClassicSingleton getInstance() {
            if(instance == null) {
                instance = new ClassicSingleton();
            }
            return instance;
        }
   }

Proof

        public static void main(String[] args) {

            ClassicSingleton singleton = ClassicSingleton.getInstance();
            System.out.println(singleton);

            ClassicSingleton singleton2 = ClassicSingleton.getInstance();
            System.out.println(singleton2);

            Normal normal = new Normal();
            System.out.println(normal);

            Normal normal2 = new Normal();
            System.out.println(normal2);
        
       }

Literature

• Java - How to Use Singleton Class

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