单例模式学习

特点:延迟加载,等需要用到的时候再实例化(不能防止别人运用反射的方法破坏单例的设计)

常规实现如下:

public class LazySingleton implements Serializable{
	//私有实例
	// volatile 防止java指令重排
	private volatile static LazySingleton shareInstance;
	
	//私有构造函数
	private LazySingleton{
	}
	
	//公开的获取实例方法
	public static getShareInstance(){
		if(shareInstance == null){
			synchronized(LazySingleton.class){
				if(shareInstance == null){
					shareInstance = new LazySingleton();
				}
			}
		}
		return shareInstance;
	}
	
	//添加此方法阻止序列化时重新生成对象
	private Object readResolve(){
	      return doubleLock;
	}
}

使用静态内部类实现如下:

特点:看起来像饿汉模式,实际上是懒汉模式,因为静态类部类ShareInstanceHolder是在被调用的时候才开始加载的,而且利用java的类加载机制保证了线程安全

public class InnerClassSingleton{

	//私有的静态内部类
	private static class ShareInstanceHolder{
		public static InnerClassSingleton shareInstance = new InnerClassSingleton();
	}
	
	private InnerClassSingleton{
		//防止别人运用反射破坏单例模式
		if(ShareInstanceHolder.shareInstance != null){
			throw new RuntimeException("实例已存在");
		}
	}
	
	public static getShareInstance(){
		return ShareInstanceHolder.shareInstance;
	}

}

特点:类加载时直接实例化

public class HungrySingleton{

	private static HungrySingleton shareInstance = new HungrySingleton();
	
	private HungrySingleton{
	}
	
	public HungrySingleton getShareInstance(){
		return shareInstance;
	}

}