设计模式 单例模式

//jvm管理 线程安全的
//lazy initialization holder class

//内部类给单例作延时加载

public class Singleton {
	private static final class Holder {
		private static final Singleton instance = new Singleton();
	}

	private Singleton() {
	}

	public Singleton getInstance() {
		return Holder.instance;

	}
}

// 枚举--单例模式
enum EnumSingleton {
	INSTANCE; // 枚举类型构造函数必须为 private 并且元素是静态的 定义完 枚举就可以使用
	private EnumSingleton() {}

	public Singleton build() {
		final Singleton instance = new Singleton();
		return instance;
	}
}
//外部调用
Singleton instance = EnumSingleton.INSTANCE.build();  

// 懒汉模式
public class Singleton {
	// initailzed during class loading
	private static final Singleton INSTANCE = new Singleton();

	// to prevent creating another instance of Singleton
	private Singleton() {
	}

	public static Singleton getSingleton() {
		return INSTANCE;
	}
}
//