设计模式-单例模式

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;
       }
   }