單例模式

單執行緒

餓漢式：

  public Class Singleton{
  //類被載入的時候就例項化一個物件
        private static Singleton singleton1 = new Singleton();
  //私有的構造方法
        private Singleton(){}
  //以自己例項為返回值的靜態的公有方法
        public static Singleton getSingleton(){
              return singleton1;
        }
  }

懶漢式：

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              if(singleton2 == null){
                    singleton2 = new Singleton();
              }
              return singleton2;
        }
  }
 

多執行緒

餓漢式天生執行緒安全

懶漢式保證執行緒安全的四種方法：

• synchronized方法

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static synchronized Singleton getSingleton(){
              if(singleton2 == null){
                    singleton2 = new Singleton();
              }
              return singleton2;
        }
  }
   
  

• synchronized程式碼塊

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              synchronized（Singleton.class）{
                    if(singleton2 == null){
                          singleton2 = new Singleton();
                    }
                    
              }
              return singleton2;
        }
  }
   
  

• 內部類

  public Class Singleton{
        private Singleton(){}
        private static Singleton Holder(){
              private static singleton2 = new Singleton() ;
        }
        public static Singleton getSingleton(){
              return Holder.singleton2;
        }
  }
  

• 雙重延遲檢測

  public Class Singleton{
        private static volatile Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              if(singleton2 == null){
              //只在第一次new例項的時候同步
                    synchronized（Singleton.class）{
                          singleton2 = new Singleton();
                    }
                    
              }
              return singleton2;
        }
  }