单例模式的几种写法

//饿汉式
class Singleton{
	private final static Singleton singleton =new Singleton();
	private Singleton() {}
	public static Singleton getSingleton() {
		return singleton;
	}
}

//懒汉式  (线程不安全，不推荐使用)
class Singleton{
	private static Singleton singleton;
	private Singleton() {};
	public static Singleton getSingleton() {
		if(singleton==null) {
			singleton=new Singleton();
		}
		return singleton;
	}
}

//懒汉式，双重检查
class Singleton{
	private static volatile Singleton singleton;
	private Singleton() {}
	public static Singleton getSingleton(){
		if(singleton==null) {
			synchronized (Singleton.class) {
				if(singleton==null) {
					singleton=new Singleton();
					return singleton;
				}
			}
		}
		return singleton;
	}
}

//静态内部类
class Singleton{
	private  Singleton () {}
	private static class SingletonInstance{
		private final static  Singleton singleton = new Singleton();
	}
	public static Singleton getSingleton() {
		return SingletonInstance.singleton;
	}
}

//枚举
class Singleton{
	
}
enum Type{
	INSTANCE;
	private Singleton instance ;
	private Type() {
		// TODO Auto-generated constructor stub
		instance=new Singleton();
	}
	public Singleton getSingleton() {
		return instance;
	}
	
}