读《研磨设计模式》-代码笔记-单例模式_JAVA_编程开发_程序员俱乐部

public class Singleton {
}

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

/*
 * 双重锁模式 
 * 1.比起单纯的"public static synchronized LazySecureSingleton getInstance()"要快
 * 2.但volatile的使用需要小心。已经建议不使用volatile关键字了
 */
class LazySecureSingleton{
	private volatile static LazySecureSingleton instance;
	public static LazySecureSingleton getInstance() {
		if (instance == null) {
			synchronized (LazySecureSingleton.class){
				if (instance == null) {
					instance = new LazySecureSingleton();
				}
			}
		}
		return instance;
	}
	private LazySecureSingleton(){}
}

class EagerSingleton{
	private static EagerSingleton instance = new EagerSingleton();
	public static EagerSingleton getInstance() {
		return instance;
	}
	private EagerSingleton(){}
}

/*
 * 使用静态内部类
 * 利用“静态内部类只有在被使用时才装载”的特性
 */
class InnerStaticClassSingleton{
	private static class SingletonHolder {
		private static InnerStaticClassSingleton instance = new InnerStaticClassSingleton();
	}
	public static InnerStaticClassSingleton getInstance() {
		return SingletonHolder.instance;
	}
	private InnerStaticClassSingleton(){}
}

/*
 * 用枚举来实现单例 <Effective Java>里面认为这是实现单例的最好方法
 */
enum EnumSingleton{
	INSTANCE;
}