设计模式-单例模式

Posted 2022-07-12 bosslv

1. 饿汉式单例

   // 缺点: 在类不使用时也会加载，浪费内存
   class Singleton 
       private static final Singleton INSTANCE = new Singleton();
   
       private Singleton() 
   
       public static Singleton getInstance() 
           return INSTANCE;
       
       

2. 同步单例

   // 缺点：在类创建成功后其实不需要同步，性能低
   class Singleton 
       private static Singleton instance;
   
       private Singleton() 
   
       public static synchronized Singleton getInstance() 
           if (instance == null) 
               instance = new Singleton();
           
           return instance;
       
   

3. 双重检查锁单例

   // 缺点：写法复杂，仍需要同步
   class Singleton 
       private static volatile Singleton instance;
   
       private Singleton() 
   
       public static Singleton getInstance() 
           if (instance == null) 
               synchronized (Singleton.class) 
                   if (instance == null) 
                       instance = new Singleton();
                   
               
           
           return instance;
       
       

4. 静态内部类单例

   // 优点: 1.延迟初始化，不占用内存；2.无需同步，通过jvm类加载保证线程安全  
   class Singleton 
   
       private Singleton() 
   
       private static class SingletonHolder 
           private static final Singleton INSTANCE = new Singleton();
       
   
       public static Singleton getInstance() 
           return SingletonHolder.INSTANCE;
       
   

5. 枚举类单例

   // 优点: 写法简单，序列化安全，避免反射攻击
   // 没有懒加载
   public enum Singleton 
   
       INSTANCE;
   
       public static Singleton getInstance() 
           return INSTANCE;