The Singleton design pattern

The singleton pattern is used for ensuring that no more than one instance of a class type exists in an application system. The single instance can be created at the time of usage or at class loading. The former is known as lazy initialization and the later is eager initialization. The key point of the singleton pattern is that the constructor must be either private or protected, which means that the user can not directly call the constructor.

1. Lazy Initialization

public class Test {
      private static Test test = null;

      //synchronized to ensure that no more than one instance is created at the same time
      public static synchronized Test getInstance() {
            if (test == null) {
                  test = new Test();
            }
            return test;
      }

      private Test() {
            //constructing code
      }

      public static void main(String[] args) {
             Test test = getInstance();
       }
}

2. Eager Initialization

public class Test {
      private static Test test = new Test();

      public static Test getInstance() {
            return test;
      }

       private Test() {
            //constructing code
      }
}

If the constructor throws exception or the initiation needs other processing, a static block can be used to instantiate the class.

public class Test {
     private Test() throws Exception {
            //constructing code
      }

      private static Test test;

      static {
            try {
                  test = new Test();
            } catch (Exception e) {
                  //code to precess the exception
             }
      }

      public static Test getInstance() {
            return test;
      }
}

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