单例模式

1.单例模式一：

package designModel;

public class Singleton1 {
	private Singleton1() {};
	private static Singleton1 instance = new Singleton1();
	public static Singleton1 getInstance() {
		return instance;
	}
}

优点：效率高
缺点：没有实现懒加载
2.单例模式二：

package designModel;

public class Singleton2 {
	private Singleton2() {};
	private static Singleton2 instance = null;
	public static Singleton2 getInstance() {
		if(instance == null) {
			synchronized(instance) {
				if(instance == null) {
					instance = new Singleton2();
				}
			}
		}
		return instance;
	}
}

优点：实现了懒加载
缺点：使用了同步块，效率低。
3.单例模式三：

package designModel;

public class Singleton3 {
	private Singleton3(){};
	private static class SingletonHolder{
		private static Singleton3 instance = new Singleton3();
	}
	public static Singleton3 getInstance() {
		return SingletonHolder.instance;
	}
}

结合了前俩种模式的优点、