手写单例模式

1.懒汉式

public class Singleton{
	private static Singleton instance;
	private Singleton(){}
	public static synchronized Singleton getInstance(){
		if(instance == null){
			instance = new Singleton();
		}
		return instance;
	}
}

2.双重检验锁

public class Singleton{
	private static volatile Singleton instance;		//声明成 volatile   
	private Singleton(){}
	public static Singleton getInstance(){
		if(instance == null){
			synchornized(Singleton.class);
			if(instance == null){
				instance = new Singleton();
			}
		}
		return instance;
	}
}

3.饿汉式

public class Singleton{
 	//类加载时就初始化
	private static final Singleton instance = new Singleton();
	private Singleton(){}
	public static Singleton getInstance(){
		return instance;
	}
}

4.静态内部类

public class Singleton{
	private static class SingletonHolder(){
		private static final Singleton instance = new Singleton();
	}
	private Singleton(){}
	public static final Singleton getInstance(){
		return SingletonHolder.instance;
	}
}

5.枚举enum

public class Singleton{
	private Singleton(){}
	/**
	*枚举类型是线程安全的，并且只会装载一次
	*/
	public enum SingletonEnum{
		INSTANCE;
		private final Singleton instance;
		SingletonEnum(){
			instance = new Singleton();
		}
		private Singleton getInstance(){
			return instance;
		}
	}
	
	public static Singleton getInstance(){
		return SingletonEnum.INSTANCE.getInstance();
	}
}