Java设计模式之单例模式

单例模式分为： 饿汉式单例 和 懒汉式单例 、登记式单例
我们这只讲前两种单例！
一、饿汉式单例

package single;


public class Hungry {

    private Hungry() {}

    private static final Hungry single = new Hungry();
    //静态工厂方法
    public static Hungry getInstance() {
        return single;
    }

}

二、懒汉式单例

package single;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;


public class Lazy {
    private Lazy() {
//        synchronized (Lazy.class){
            if(single!=null){
              throw new RuntimeException("不要试图反射破坏异常");
            }
//        }
        System.out.println(Thread.currentThread().getName()+"OK");
    }

    private volatile static Lazy single=null;   //volatile避免指令重排

    //DCL  double check lazy 双重检查锁
    public  static Lazy getInstance() {
        if (single == null) {
            synchronized (Lazy.class){
                if (single == null) {
                    single = new Lazy();   //不是原子性操作
                    
//                    System.out.println(single);
                }
            }
        }
        return single;
    }

  // 多线程并发
  public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
//1.
//      for (int i = 0; i < 10; i++) {
//                  new Thread(()->{
//                  Lazy.getInstance();
//                  }).start();
//      }

//2.
//      Lazy s =  Lazy.getInstance();
//      System.out.println(s);
//      Lazy a = Lazy.getInstance();
//      System.out.println(a);

//3.反射
      Lazy instance = Lazy.getInstance();
      Constructor declaredConstructor =  Lazy.class.getDeclaredConstructor(null);
      declaredConstructor.setAccessible(true);
      Lazy instance2 =declaredConstructor.newInstance();

      System.out.println(instance);
      System.out.println(instance2);


  }

}

三、总结