java單例模式幾種常見實現方式

1. 什麼是單例模式
   因進程需要，有時我們只需要某個類同時保留一個對象，不希望有更多對象，此時，我們則應考慮單例模式的設計。

2. 單例模式的特點
   1、單例模式只能有一個實例。
   2、單例類必須創建自己的唯一實例
   3、單例類必須向其他對象提供這一實例。

3. 單例模式的實現

   1. 懶漢式(線程不安全)

   public class SingletonDemo {
       private static SingletonDemo instance;
       private SingletonDemo(){
    
       }
       public static SingletonDemo getInstance(){
           if(instance==null){
               instance=new SingletonDemo();
           }
           return instance;
       }
   }
   

   2. 懶漢式,線程安全

   public class SingletonDemo {
       private static SingletonDemo instance;
       private SingletonDemo(){
    
       }
       public static synchronized SingletonDemo getInstance(){
           if(instance==null){
               instance=new SingletonDemo();
           }
           return instance;
       }
   }
   

   這種模式在多線程的時候效率比較低,生產上不推薦.

   3. 餓漢式,線程安全

   public class SingletonDemo {
       private static SingletonDemo instance=new SingletonDemo();
       private SingletonDemo(){
    
       }
       public static SingletonDemo getInstance(){
           return instance;
       }
   }
   

   4. 靜態內部類(線程安全)

   public class SingletonDemo {
       private static class SingletonHolder{
           private static SingletonDemo instance=new SingletonDemo();
       }
       private SingletonDemo(){
           System.out.println("Singleton has loaded");
       }
       public static SingletonDemo getInstance(){
           return SingletonHolder.instance;
       }
   }
   

   5. 枚舉方法(線程安全)

   enum SingletonDemo{
       INSTANCE;
       public void otherMethods(){
           System.out.println("Something");
       }
   }
   

   6. 雙重校驗(線程安全)

   public class SingletonDemo {
       private static SingletonDemo instance;
       private SingletonDemo(){
           System.out.println("Singleton has loaded");
       }
       public static SingletonDemo getInstance(){
           if(instance==null){
               synchronized (SingletonDemo.class){
                   if(instance==null){
                       instance=new SingletonDemo();
                   }
               }
           }
           return instance;
       }
   }