单例

DCLSingleton

缺点：可读性差

class Singleton {
    private volatile static Singleton instance;
    private Singleton(){}
    public static Singleton getInstance() {
    	// 检查是否需要「阻塞」
        if ( instance == null ) {
            synchronized (Singleton.class) {
            	// 检查是否需要「新建实例」
                if ( instance == null ) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

（Singleton）Holder

缺点：能够被反射破坏

public class Singleton {
    /**
     * 类级的内部类，也就是静态的成员式内部类，该内部类的实例与外部类的实例
     * 没有绑定关系，而且只有被调用到才会装载，从而实现了延迟加载
     */
    private static class SingletonHolder{
        /**
         * 静态初始化器，由JVM来保证线程安全
         */
        private static Singleton instance = new Singleton();
    }
    /**
     * 私有化构造方法
     */
    private Singleton(){
    }
    public static  Singleton getInstance(){
        return SingletonHolder.instance;
    }
}

破坏方法

public class ReflectHungry {
    public static void main(String[] args) {
        try {
            Class<?> clazz = HungrySingleton.class;
            Constructor<?> constructor = clazz.getDeclaredConstructor(null);
            constructor.setAccessible(true);
            Object o1 = constructor.newInstance();
            Object o2 = constructor.newInstance();
            System.out.println(o1);
            System.out.println(o2);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}