public class Singleton {
    //私有的 靜態的 本類屬性
    private volatile static Singleton _instance;
    //私有化構造器
    private Singleton() {}
     /*
      * 1st version: creates multiple instance if two thread access
      * this method simultaneouslyX
      */
     public static Singleton getInstance() {
         
 
if (_instance == null) {
             _instance = new Singleton();
         }
         return _instance;
     }
  
    /*
     * 2nd version : this definitely thread-safe and only
     * creates one instance of Singleton on concurrent environment
     * but unnecessarily expensive due to cost of synchronization
     * at every call.
     
 
*/
  
    public static synchronized Singleton getInstanceTS() {
        if (_instance == null) {
            _instance = new Singleton();
        }
        return _instance;
    }
  
    /*
     * 3rd version : An implementation of double checked locking of Singleton.
     * Intention is to minimize cost of synchronization and  improve performance,
     * by only locking critical section of code, the code which creates instance of Singleton class.
     * By the way this is still broken, if we don‘t make _instance volatile, as another thread can
     * see a half initialized instance of Singleton.
     
 
*/
    //雙重檢查鎖：檢查了2次；使用了一個鎖
    //此處需要volatile修飾屬性保證它的內存可見性？？
     public static Singleton getInstanceDC() {
         if (_instance == null) {//第一次檢查
             synchronized (Singleton.class) {
                 if (_instance == null) { //第二次檢查   //線程1創建完對象後，線程會判斷一次就不會創建對象了。解決了首次創建對象的唯一性問題。
                     _instance = new Singleton();
                 }
             }
         }
         return _instance;
     }
 }

單例模式中 的 雙重檢查鎖 概念與用法