单例

• 双重锁检测

public class SingleInstance{
   //volatile:禁止指令重排序；可见性保证
  private static volatile SingleInstance instance  = null;
  private SingleInstance(){ }
  public static SingleInstance getInstance(){
    if(instance == null){      // 性能考虑，细化synchronized同步块粒度
        synchronized(Singleton.class){
            if(instance == null){   // 防止多次初始化
              instance = new SingleInstance();
            }
        }
      }
return instance;
}
}

new SingleInstance()指令
1.加载到内存，分配内存地址
2.执行对象初始化
3.赋值
volatile 保证了2->3的顺序

• 破坏单例

1. 反射

  Singleton singleton = Singleton.getInstance();
    try {
      Constructor<Singleton> constructor = Singleton.class.getDeclaredConstructor();
      constructor.setAccessible(true);
      Singleton reflectSingleton = constructor.newInstance();
      System.out.println(reflectSingleton == singleton);
    } catch (Exception e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

阻止：
getInstance中加判断，采用一个全局变量，初始化过就不再初始化。

2. 继承Clonable接口，重写clone方法

  @Override
  protected Object clone() throws CloneNotSupportedException {
    // TODO Auto-generated method stub
    return super.clone();
  }

破坏：clone方法不调用super.clone() 直接返回instance单例对象
⚠️ clone是浅拷贝，也就是说对象的引用对象不会被拷贝。需要深拷贝的话，引用对象也要实现clone方法，该对象引用的其他对象也clone一份

3. 序列化 implements Serializable
   FileOutputStream.writeObject
   FileInputStream.readObject
   阻止：implements Serializable，重写 readResolve

  protected Object readResolve() throws ObjectStreamException{
    return mInstance;
  }