单例模式典型创建方法（三种）

单线程

class Singleton {

	

	private Singleton() {};

	private static Singleton instance = null;

	

	public static Singleton createInstance() {

		if (instance == null) {

			instance = new Singleton();

		}

		return instance;

	}

	

}

 

public class HelloWorldjava {



	public static void main(String[] args) {

		Singleton s1 = Singleton.createInstance();

		Singleton s2 = Singleton.createInstance();

		

		if (s1 == s2) {

			System.out.println("只有一个实例被创建！");

		}

	}

}

预先加载

class Singleton {

	

	private Singleton() {};

	private static Singleton instance = new Singleton();

	

	public static Singleton createInstance() {

		return instance;

	}

	

}





public class HelloWorldjava {



	public static void main(String[] args) {

		Singleton s1 = Singleton.createInstance();

		Singleton s2 = Singleton.createInstance();

		

		if (s1 == s2) {

			System.out.println("只有一个实例被创建！");

		}

	}

}

按需创建

class Singleton {

	

	private Singleton() {};

	

	private static class Helper {

		static Singleton instance = new Singleton(); 

	}

	

	public static Singleton createInstance() {

		return Helper.instance;

	}

	

}



public class HelloWorldjava {



	public static void main(String[] args) {

		Singleton s1 = Singleton.createInstance();

		Singleton s2 = Singleton.createInstance();

		

		if (s1 == s2) {

			System.out.println("只有一个实例被创建！");

		}

	}

}