单例模式之饿汉方式实现与静态内部类实现的区别

package priv.jack.dp.demo.singleton;

/**
 * @author Jack
 * 饿汉模式单例
 * 线程安全
 * 不支持懒加载，容易产生垃圾对象
 * 优点：没有加锁，执行效率会提高。
 */
public class HungrySingleton {

    private static HungrySingleton INSTANCE = new HungrySingleton() ;
    
    private HungrySingleton() {
        System.out.println("init...");
    }
    
    public void hello() {
        System.out.println("hello  world!");
    }
    
    public static HungrySingleton getInstance() {
        return INSTANCE ;
    }
    
    public static void main(String [] args) {
        System.out.println("main start...");
        HungrySingleton.getInstance().hello(); 
    }
}

package priv.jack.dp.demo.singleton;

/**
 * @author Jack
 * 支持懒加载，支持多线程
 * 对象在提一次使用时才被创建
 */
public class StaticInnerSingleton {

    private static class StaticInnerSingletonHolder{
        private static final StaticInnerSingleton INSTANCE = new StaticInnerSingleton() ;
    }
    
    private StaticInnerSingleton() {
        System.out.println("init...");
    }
    
    public static final  StaticInnerSingleton getInstantce() {
        return StaticInnerSingletonHolder.INSTANCE ;
    }
    
    public void hello() {
        System.out.println("hello  world!");
    }
    
    
    public static void main(String [] args) {
        System.out.println("main start...");
        StaticInnerSingleton.getInstantce().hello(); 
    }
    
}

init...
main start...
hello  world!

main start...
init...
hello  world!