單例模式在設計模式中比較常見，在多執行緒訪問的時候容易出現執行緒不安全的隱患

懶漢式（延遲載入）/餓漢式（立即載入）

直接給出雙檢查鎖單例模式

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

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